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Assessment of ecosystem services and perceived benefits of street trees : a case study of Coyoacan, Mexico… Navarro Perez de Leon, Nuria Monica 2017

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  ASSESSMENT OF ECOSYSTEM SERVICES AND PERCEIVED BENEFITS OF STREET TREES. A CASE STUDY IN COYOACAN, MEXICO CITY   by Nuria Monica Navarro Perez de Leon B.A., Universidad Nacional Autonoma de Mexico, 2013 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Forestry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) December 2017 © Nuria Monica Navarro Perez de Leon, 2017     ii  Abstract  Mexico City, one of the biggest and most polluted cities in the world, is facing a gradual disappearance of its green areas, especially the loss of street trees. In the current situation, street trees are subject to constant removal, poor management and vandalism, and few studies have been done about their role and effectiveness in air pollution removal and carbon sequestration. For this study, the ecosystem services of street trees were quantified using the software i-Tree Eco along with a social survey of how people perceived them. The study site is Coyoacan, one of the greenest districts in Mexico City, comprised of 95 neighbourhoods, of which 12 were sampled. A random stratified sample was carried out to estimate the number of street trees in Coyoacan and the number of street units (blocks) to be sampled in each of the 12 neighbourhoods, in order to reach a sample size of 500 trees. The surveys were performed following a pass-by method. In general street trees provide important quantities of ecosystem services at the district level; but the predominance of small trees produces lower ecosystem service values compared to other cities with similar populations. The situation can be improved if the survival and healthy growth of those small trees can be guaranteed through better management practices. At the neighbourhood level we encountered an uneven distribution in the number of street trees and in the proportion of large trees over small trees, affecting the quantity of ecosystem services which a neighbourhood receives. These findings suggest that to increase the provision of ecosystem services at a city or district level, resources should be directed at a local level, looking to provide equality in the distribution of ecosystem services among neighbourhoods rather than allocating resources through random tree plantations across the city. Finally, people show high appreciation for trees and knowledge about a wide range of   iii  benefits, suggesting a tree caring culture that can be strengthened through social involvement programs for the care and management of street trees.     iv  Lay Summary  The goals of this study were to estimate the number of street trees in Coyoacan, Mexico City and quantify the ecosystem services they are providing, and, importantly, to raise awareness that proper management that guarantees survival and healthy growth of trees can considerably increase the benefits provided by trees. The study suggests that instead of referring to green areas, it might be better to refer to the urban forest as a unifying concept that assists with management and resource allocation, while also providing a healthy environment. Finally, the study suggests that efforts to increase tree coverage should be made at the local level, and aspire to create an equal distribution of ecosystem services among neighbourhoods.       v  Preface This thesis represents original, unpublished work by Nuria M. Navarro Perez de Leon. I was responsible for developing the research questions, collecting and analyzing the data, and writing the manuscript. I was guided by my supervisor, Dr. Cecil Konijnendijk, in developing my research questions and analyzing my data. The fieldwork for this study took place at Coyoacan district, Mexico City, where I coordinated teams of three people to gather tree data required by i-Tree Eco 6.  The calculations to quantify carbon sequestration, carbon storage, avoided runoff and pollution removal, as well as a description of the anatomy of the urban forest was made by the i-Tree support team.  Angel Merlo and Lorena Perez de Leon helped to count trees for the pre-sampling. Juan Antonio Corrales, Cecilia Gutierrez, Daniela Briseño, Cristina Ayala, Maria Raquel Perez de Leon, Ricardo Navarro, Abisay Ortiz, Carla Vazquez, Ariel Acosta, Diana Vazquez, Sandra Petrone and Sofia Murga helped to collect tree data.  PhD candidate Lorien Nesbitt and Dr. Tahia Devisscher recommended using the software NVIVO to analyze the surveys' open questions, while Dr. Cecil Konijnendijk recommended using IBM SPSS Statistics to analyze closed ended questions.     Drs. Cecil Konijnendijk, Cindy Prescott and Stephen Sheppard contributed to the thesis revision process. I am also grateful for external examiner Dr Justin Morgenroth’s comments and suggestions. The study design and preliminary results (up to April 2016) have previously been presented at the following conference:   vi  Navarro Perez de Leon, N. M and Konijnendijk, C.C. 2017.The underestimated value of Mexican street trees. Lecture presented at Green Infrastructure: Nature based solutions for sustainable and resilient cities, Orvieto, Italy.  The UBC Behavioural Research Ethics Board approved on June 2nd, 2016 the project “Carbon storage and sequestration in street trees in Coyoacan district, Mexico City” with certification number H16-01210 to conduct a passers-by survey in three popular sites in Coyoacan district, Mexico City.     vii  Table of Contents Abstract ................................................................................................................................. ii Lay Summary ....................................................................................................................... iv Preface ................................................................................................................................... v Table of Contents ................................................................................................................ vii List of Tables ......................................................................................................................... x List of Figures ..................................................................................................................... xii List of Equations ................................................................................................................ xiv List of Abbreviations .......................................................................................................... xv Acknowledgements ............................................................................................................ xvi Dedication .......................................................................................................................... xvii Chapter 1: Introduction ....................................................................................................... 1 1.1 Project overview ........................................................................................................... 1 1.2 Literature review ........................................................................................................... 3 1.2.1 Historic overview ................................................................................................... 3 1.2.2 Urban forestry: environmental legal framework in Mexico................................. 12 1.2.2.1 Legal framework ............................................................................................... 14 1.2.3 Coyoacan district: actions at the local level ......................................................... 22 1.3 Research objectives ..................................................................................................... 24 Chapter 2: Methodology .................................................................................................... 26 2.1 Study area ................................................................................................................... 26 2.1.1 Mexico City .......................................................................................................... 26 2.1.2 Coyoacan .............................................................................................................. 26 2.2 Assessment of the street trees using the i-Tree Eco 6 model ...................................... 31   viii  2.2.1 i-Tree Eco 6 model ............................................................................................... 32 2.2.2 Tree sampling method .......................................................................................... 34 2.3 Social survey ............................................................................................................... 47 2.1.5 Data analysis ............................................................................................................ 49 2.1.5.1 Estimated values of tree benefits for the twelve neighbourhoods ..................... 49 2.1.5.2 Ecosystem services ............................................................................................ 49 2.1.5.3 Street tree per capita and difference according to street layout ........................ 49 2.1.5.4 Survey data ........................................................................................................ 50 Chapter 3. Case study results ............................................................................................ 54 3.1 Urban forest structure ................................................................................................. 54 3.1.1. Species composition ............................................................................................ 54 3.2 Estimated street tree population .................................................................................. 55 3.3 Ecosystem services provided by street trees ............................................................... 56 3.3.1. Carbon storage and sequestration ........................................................................ 58 3.3.2 Avoided surface runoff......................................................................................... 59 3.3.3 Air pollution removal ........................................................................................... 60 3.4 Street tree per capita ................................................................................................... 62 3.5 Street layout ................................................................................................................ 64 3.6 Survey results .............................................................................................................. 64 Chapter 4: Discussion ......................................................................................................... 75 4.1 Urban forest structure ................................................................................................. 75 4.1.1 Species composition ............................................................................................. 75 4.2 Ecosystem services ..................................................................................................... 77 4.2.1 Carbon storage and sequestration ......................................................................... 77   ix  4.2.2 Avoided surface runoff......................................................................................... 78 4.2.3 Air pollution removal ........................................................................................... 79 4.3 People’s tree perceptions and preferences .................................................................. 79 4.4 Unexpected findings ................................................................................................... 82 4.5 Study limitations ......................................................................................................... 84 Chapter 5: Conclusions ...................................................................................................... 86 References............................................................................................................................ 88 Appendix 1: Street tree population of Coyoacan district ............................................... 99 Appendix 2. Trees that require maintenance tasks ....................................................... 103 Appendix 3. Site conditions ............................................................................................. 107     List of Tables   Table 1.1. Green areas classification. .................................................................................. 20  Table 2.1. Zonation of the sample neighbourhoods. ............................................................ 35 Table 2.2. Street unit selection ............................................................................................. 38 Table 2.3. Process to obtain the street segment length. ....................................................... 39 Table 2.4. Estimated number of street trees per street unit. ................................................. 42 Table 2.5. Estimated number of street trees in the twelve neighborhoods. ......................... 43 Table 2.6.  Respondent age nominal classes ........................................................................ 51 Table 2.7. Common tree traits liked by the respondents. .................................................... 53  Table 3.1. Percentage distribution of the ten most common dominant tree species in Coyoacan district. ................................................................................................................. 55 Table 3.2. Estimated number of street trees per zone segment. ........................................... 56 Table 3.3. Benefits provided by trees. ................................................................................. 57 Table 3.4. Tree species mentioned by respondents as the "favourite" trees. ....................... 66 Table 3.5.  Most liked tree traits mentioned by respondents. .............................................. 67 Table 3.6. Tree species respondents mentioned as most disliked trees. .............................. 68 Table 3.7.  Most disliked tree traits mentioned by the respondents. .................................... 68 Table 3.8. Tree benefits mentioned by respondents. ........................................................... 69 Table 3.9. Tree disservices mentioned by respondents........................................................ 70 Table 3.10. Preferred tree location in the city and reasons. ................................................. 70   xi  Table 3.11. Most important management actions mentioned by the respondents. .............. 71 Table 3.12. Statistics results of the non-parametric test chi-squared ................................... 74  Table A1-1. Estimated number of trees in each of the 95 neighbourhoods in Coyoacan.... 99 Table A.2-1. General maintenance task found during fieldwork.  .................................... 103 Table A.3-1. Sidewalk conflict. ......................................................................................... 107 Table A.3-2. Trees conflicting with utility lines. ............................................................... 109     xii  List of Figures Figure 1.1. Five lakes that comprised the Basin of Mexico Source: Wikipedia, 2017. ........ 4 Figure 1.2. Ecological settlement of the territory of Mexico City....................................... 18  Figure 2.1. Map of the 95 neighborhoods in Coyoacan. ..................................................... 29 Figure 2.2. Marginalization indicators chart........................................................................ 30 Figure 2.3. Map of sampled neighbourhoods. ..................................................................... 36 Figure 2.4.  Example of a block. .......................................................................................... 37 Figure 2.5. Example of a street segment.............................................................................. 37 Figure 2.6. i-Tree Eco Complete Inventory Data Sheet. ..................................................... 46 Figure 2.7. Questionnaire on how people perceive street trees in Coyoacan. ..................... 48  Figure 3.1. Distribution of the ten most common species among the neighbourhoods....... 54 Figure 3.2. Distribution of the total carbon storage among the neighborhoods. ................. 58 Figure 3.3. Distribution of the gross carbon sequestration among the neighborhoods. ...... 59 Figure 3.4. Distribution of the total avoided runoff according to the total leaf area of each zone segment. ....................................................................................................................... 60 Figure 3.5. The amount of pollutants removal according to the total leaf surface area of each zone segment (neighborhood). ..................................................................................... 61 Figure 3.6. Total leaf area distribution among zone segments. ........................................... 61 Figure 3.7. Annual pollution removal (kilograms). ............................................................. 62 Figure 3.8. Comparison between the surface area (ha) and the population density among zone segments. ...................................................................................................................... 63   xiii  Figure 3.9. Changes in street trees per capita according to the population density. ............ 63 Figure 3.10. Difference in tree density according to street layout. ..................................... 64 Figure 3.11. Age group distribution of the 186 respondents. .............................................. 65 Figure 3.12. Gender distribution among age groups. .......................................................... 65 Figure 3.13. Percentage of respondents with residency in Coyoacan ................................. 66 Figure 3.14.  Dwelling preference according to tree location. ............................................ 72 Figure 3.15. Relative number of trees in your neighbourhood. ........................................... 73 Figure 3.16. Relative number of street trees in people neighborhoods. .............................. 73  Figure A.2-1. Distribution of the maintenance task among the zone segments. ............... 103 Figure A.2-2. Trees infected with mistletoe. ..................................................................... 104 Figure A.2-3. Poor pruning practices. ............................................................................... 105 Figure A.2-4. Dead-standing trees.. ................................................................................... 106  Figure A.3-1. Distribution of the sidewalk conflict among the neighborhoods. ............... 107 Figure A.3-2. Examples of trees with sidewalk conflicts. ................................................. 108 Figure A.3-3. Inappropriate tree pit size. ........................................................................... 108 Figure A.3-4. Distribution of the utility conflict among neighborhoods. .......................... 109 Figure A.3-5. Examples of trees with utility line conflict and with no potential conflict..110      xiv  List of Equations  Equation 2.1. Formula for weighted average ...................................................................... 39 Equation 2.2  Average number of street trees per street unit in each zone segment ........... 41 Equation 2.3 Total number of street trees in the twelve neighbourhoods ........................... 41 Equation 2.4 Percentage (wi) of the total city street tree population that is located in each zone segment ........................................................................................................................ 44 Equation 2.5 Number of street units to be sample within each zone segment to reach a sample size of 400 trees ........................................................................................................ 44 Equation 2.6 Overall tree density per street unit in Coyoacan district ................................ 44                xv  List of Abbreviations  For their acronym in Spanish  CPEUM = Political Constitution of the United Mexican States  ENCC = National Climate Change Strategy  LADF = Environmental Law of the Federal District  LGCC = General Law on Climate Change LGEEPA = General Law of Ecological Equilibrium and Protection to the Environment  LMACCDS = Law on Mitigation and Adaptation to Climate Change and Sustainable Development  SEDEMA = Secretary of Environment  PAOT = Environmental and Territorial Planning Department   Neighbourhoods abbreviations  BT = Bosques de Tetlameya CA = Copilco el Alto CN = Canal Nacional CSF = Cuadrante de San Francisco DC = Del Carmen HM = High marginalization IC = Insurgentes Cuicuilco LC = Los Cedros LM = Low marginalization MM = Medium marginalization OU = Oxtopulco Universidad PC = Pedregal de Carrasco PSA = Parque San Andrés PT = Petrolera Taxqueña SDC = San Diego Churusbusco                  xvi   Acknowledgements  I would first like to thank my former supervisor, Dr.Cindy Prescott, for giving me the opportunity to pursue this project, for her initial guidance and for directing my project to the field of urban forestry under the supervision of Dr. Cecil Konijnendijk van den Bosch.  Similarly, I would like to thank Dr.Cecil Konijnendijk van den Bosch, for accepting me as his student, for helping me shape this project, for his guidance, and for all his support along the way. I have learned so much from you. I would also like to acknowledge my committee, which consisted of Dr. Konijnendijk, Dr. Cindy Prescott and Dr. Stephen Sheppard.   To Dr. Alicia Chacalo from the Metropolitan Autonomous University and Dr. Hector Benavides from the National Institute of Forestry, Agricultural and Livestock Research for their help on the identification of street trees species. To the i-Tree support team for their guidance on the sampling method and the data analysis. Special thanks to Angel Merlo, Lorena Perez de Leon, Juan Antonio Corrales, Cecilia Gutierrez, Daniela Briseño, Cristina Ayala, Maria Raquel Perez de Leon, Ricardo Navarro, Abisay Ortiz, Carla Vazquez, Ariel Acosta, Diana Vazquez, Sandra Petrone and Sofia Murga for their valuable field assistance.   Several other members of the UBC faculty and staff, thank you for all the help and support provided along the way and for making my time as a graduate student a wonderful experience. Thank you Dr. Tahia Devisscher and PhD candidate Lorien Nesbitt for their guidance and help to clarify doubts about statistics.   I would like to extend special thanks to my peers Tim Philpott, Catch Catomeris, Camille Defrenne, Morgane Maillard for listening to all my doubts, for giving me suggestions and most of all support.   Funding for this project was made possible by CONACYT sponsorship and the UBC Faculty of Forestry. Faculty of Forestry funding consisted of one graduate teacher assistantship and three internal awards (two Peter Rennie Memorial Award and Donald S McPhee Fellowship), plus the travel funding support from the grants of Dr. Cindy Prescott and Dr. Cecil Konijnendijk. I am incredibly grateful for all the support during my degree.  Thanks to my family and friends whose support and love inspire me every day. Special thanks to Carlos, my partner in crime, thank you for your words, for your love and for believing in me every second.         xvii    Dedication        To my parents and grandparents                     Chapter 1: Introduction 1.1 Project overview The ecosystem services that trees provide to cities and how these services  contribute to improving the quality life of their citizens have been widely study (Escobedo, Seitz, & Zipperer, 2009; Nowak, 2002; Nowak, Crane, & Stevens, 2006; Selmi et al., 2016; Soares et al., 2011; Xiao & McPherson, 2002; Xiao, McPherson, Simpson, & Ustin, 1998; Yang, McBride, Zhou, & Sun, 2004; Zhao, Tang, & Chen, 2016). Consequently, there has been a general interest in making cities greener and more sustainable as a strategy to combat climate change (Rosenzweig et al., 2010). However, for various reasons this shift has not been easy to achieve for all cities. Take for example Mexico City: although Mexico has signed all international agreements to combat climate change (including the recent Paris Agreement), the reality seems very different from what may be expected from the capital of the country. Starting with the ineffective programs to combat the city’s poor air quality, for more than 20 years there have constantly been recorded higher pollutant values than allowed, causing severe problems to human health, and especially to children’s health (Calderón-Garcidueñas et al., 2016; Calderón-Garcidueñas, Franco-Lira, et al., 2015; Calderón-Garcidueñas, Kulesza, Doty, D’Angiulli, & Torres-Jardón, 2015; Romieu et al., 1996). This situation is enhanced by the little interest given to the protection of the city’s urban tree cover which is subject to neglect and rapid loss due to urban development. Former studies have reported that the state of the green areas and street trees of Mexico City varies from average to poor (Benavides-Meza & Segura-Bailon, 1996; Chacalo Hilú, Grabinsky, & Aldama, 1996; Falcon Lara, 1994; Rojo Negrete, 2006; Waluyo Moreno,   2  2013). In this context, it is not surprising that the shift towards a greener and more sustainable future seems highly difficult to achieve. Under this premise, the present study was intended to provide more in-depth information about Mexico City’s urban forest and its benefits. This was done by quantifying the benefits of street trees, the most vulnerable component of the urban forest, in one of the city’s districts, in order to raise awareness of their value and the importance of their establishment, protection and management.  Apart from assessing part of Mexico City’s urban forest and its benefits, this thesis also includes a social survey of people’s perceptions, with the aim to learn more about people’s familiarity with tree species, the benefits they receive from the trees and their respective valuation, and their attitudes about increasing the tree cover of the city. The author considered it important that the study should not only focus on describing the current environmental situation of Mexico City’s urban forest, but also to address subjects such as cultural and legal aspects that may be influencing the current conditions of the trees in Mexico City. For this reason, the literature review is intended to introduce the reader to the historical context of the urban forest and the legal framework that sustains it, and to provide an understanding of the complex environmental crisis that Mexico City is suffering. The cultural and legal aspects presented in this thesis deserve a study of their own due to their complexity; however, the author’s intentions were to underline their importance with respect to the current management of the urban forest, as well as propose new avenues through which the urban forest of Mexico can be studied.   3  This thesis complements previous studies in Mexico City, but also intends to raise the recognition of street trees as important assets. Thus, the aim should be to increase the city’s tree cover, as long as proper management programs are implemented, and social engagement is promoted. 1.2 Literature review 1.2.1 Historic overview The urban forest in Mexico City is as old as the city itself, i.e. close to 700 years (as the city was founded in 1325 A.D.). During its history, the development and accelerated growth of the city reduced the surrounding forests at an alarming rate, losing hundreds of hectares. Together with the desiccation of the lakes1 in the area where the city was settled, this deforestation sank Mexico City into an environmental crisis that persists today (Ezcurra, 1992; Ezcurra et al., 1991; Izazola, 2001; Schteingart, 1987; Simonian, 1999).  In my opinion, this long existing environmental crisis is strongly linked to the shift from an ecocentric to an anthropocentric worldview (Gagnon Thompson & Barton, 1994). This shift follows a series of actions and decisions taken over time that have made Mexico City unable to thrive as a sustainable and green city.  As a review on the entire history of the environmental crisis of the city exceeds the scope of this thesis, the author chose three periods to exemplify the worldview shift and some actions that may explain the impossibility thus far shown to face the environmental crisis, not to mention the effects of climate change.                                                             1 Mexico City is located in the Basin of Mexico which prior to the Spanish arrival consisted on a system of five interconnected lakes. In the north were the lakes of Zumpango and Xaltocan, in the center was lake Texcoco, and in the south were Xochimilco and Chalco lakes (Candiani, 2014).   4  For a more thorough revision of the history of the urban forest of Mexico City, the reader is referred to the works of Simonian (1999) and Martinez Gonzalez (2008).    1.2.1.1 The major role of nature on Aztec everyday life    Early Mesoamerican cultures believed they originated from the trees and their lineages were connected through their roots. This belief was professed with great respect, keeping a close relationship with the environment (Heyden, 1993). Therefore, trees were treated as deities, guardians, and suppliers of sacred materials and food (Heyden, 1993).  Figure 1.1. Five lakes that comprised the Basin of Mexico. Source: Wikipedia, 2017.    Figure 1.2. Ecological settlement of the territory of Mexico City. Modified from PAOT, 2010.Figure 3.1. Five lakes that comprised the Basin of Mexico Source: Wikipedia, 2017.    Figure 1.2. Ecological settlement of the territory of Mexico City. Modified from PAOT, 2010.Figure 4.1. Five lakes that comprised the Basin of Mexico Source: Wikipedia, 2017.    5  For the Aztecs, representing a culture which settled where Mexico City is today, cutting a tree required performing a ceremony to ask the god Quetzalcoatl, protector of the natural world, for permission to enter the forest and cut a tree, and promising that the place where the stem would be set would be venerated (Heyden, 1993; Martínez González, 2008). The resin of Liquidambar sp.  and pine trees was highly praised, as it was used as incense known as copal for all religious and political ceremonies (Peterson & Townsend Peterson, 1992). As with trees, other elements of the natural world were also venerated. Flowers and quetzal feathers were considered very valuable and were used for all religious ceremonies and as common tributes (de Rojas, 2012; Peterson & Townsend Peterson, 1992).  The Aztecs possessed a wide knowledge of their lacustrine environment, as well as the use of medicinal plants to treat various diseases (de Rojas, 2012). They built aqueducts to bring water to the city, and constructed a dike to separate the water from the lake of Texcoco from the water of the city (Ezcurra, 1992; Martínez González, 2008; Simonian, 1999). They also developed a sustainable agriculture method called chinampas2 and the main transportation system was an extended channel network. The dike and the chinampas also fulfilled the function of controlling the water levels of the lake during the rainy season (de Rojas, 2012; Ezcurra, 1992; Martínez González, 2008; Simonian, 1999). Transportation was mainly done by canoes.                                                            2 Chinampas were artificial islets for agriculture, made of interlayers of aquatic plant remains and sediment from the bottom of the lake until reaching 60 cm above the water mirror. This height was ideal to avoid waterlogging and to allow roots to uptake water from the soil without having to rely on irrigation or rains (Ikkonen et al., 2012; Reyes-Ortigoza y García-Calderón, 2004; Uribe, 2009, cited in Navarro, 2013).   6  Inside the city there were numerous gardens that were described in Spanish chronicles as terrestrial paradises and which could be divided into five types (Folguera Morales, 2004):  1) Ludic gardens, exclusive for Aztec kings, and which displayed only aromatic trees, flowers, and some medicinal plants. The grandeur of the gardens represented and enhanced the power and greatness of their masters.  2) Botanical gardens reserved for the priests and considered places of knowledge and experiment; these were mostly composed of medicinal plants.  3) Forests, used to obtain resources, but also recreation areas where the Aztecs kings went hunting.  4) Orchards meant for merchants and slaves.  5) Chinampas located next to the houses of the commoners and merchants, which consisted of polyculture plantations of edible, medicinal and ornamental plants (Ezcurra, 1992; Folguera Morales, 2004). The Aztecs praised the beauty of nature and honored plants and animals, but the increase in population, which by the end of the 14th century had reached almost one million inhabitants (Martínez González, 2008), put pressure on the environment, and signs of deforestation started to appear (Ezcurra, 1992; Peterson & Townsend Peterson, 1992). Some rulers, like King of Texcoco, Nezahualcoyotl (reign 1418 to 1472) worried about this situation and established the first laws that restricted logging areas (Simonian, 1999).     Before the Spanish arrival in 1521, the territory already showed signs of deterioration, although the Aztecs, like other indigenous cultures, carefully managed their resources as a conscientious effort to reduce the impact (Simonian, 1999).    7   The Spanish arrival brought the fall of the Aztec Empire and one of the worst periods of environmental deterioration (Ezcurra et al., 1991). Unlike the Aztecs, the Spanish settlers did not share the belief of seeing nature as a deity, rather they pictured it as a resource that could and should be exploited (Simonian, 1999). In addition, the Spanish Conquest suppressed the indigenous peoples and with them, all of their knowledge about the lacustrine environment in which the city was situated. 1.2.1.2 The establishment of a new worldview The Spanish Colonization represents a break point in the history of Mexico and has been studied by several disciplines, including history, anthropology, sociology, and archaeology. However, more studies are needed to explore how the shock of two cultures affected the conservation and protection of the natural resources of Mexico City. In my opinion it was during the Spanish Colonization that two attitudes emerged, namely the breach of laws and the use of short-term solutions, as well as two ideologies: the disregard for previous knowledge and conservation interposes with development. Unfortunately, these all prevail today in the collective imagination of Mexican society, something which has prevented a shift in the course of decisions towards the protection of the urban forest of Mexico City, and the overall sustainable development of the country. Disregard for previous ecologic knowledge and breach of laws for attempting against private interests By the time the Spanish arrived at the New World in 1519, Spain was already familiar with the consequences of deforestation in the Iberian Peninsula (Simonian, 1999). To prevent this, the Spanish Crown promulgated Forest Conservation laws for all of its   8  colonies.  However, the contrasting and exuberant nature of the Mexican landscape created the misconception that forests and other resources in the New World were inexhaustible, and settlers did not adhere to the terms of the laws as they considered them too inflexible to the local circumstances and to their own needs (Simonian, 1999). The capital of New Spain was built over the territory of the fallen empire, but unlike the Aztecs, the Spanish were unfamiliar with the lacustrine nature of the site, and during its construction they disregarded and ignored the techniques used by the Aztecs to control the city’s water level (Ezcurra, 1992; Martínez González, 2008; Simonian, 1999). In their eagerness to build a Spanish-style city, they built high streets, filled in the canals and extracted large volumes of wood from the surrounding forests for construction (Ezcurra, 1992; Martínez González, 2008). The abrupt change in the landscape brought along intense floods that caused many deaths and massive destruction of the city. Five of these floods were registered between the years 1553 to 1629 (Ezcurra, 1992; Martínez González, 2008). Conservation interposed with development and the use of short-term solutions rather than long-term solutions In 1608, the suggested solution to the flooding problem was to open the Mexican Basin on the north side towards the Atlantic Ocean to divert the water that reached the Basin (Ezcurra, 1992; Martínez González, 2008). This decision is now considered one of the biggest ecological disasters, as it did not solve the flooding problem, and caused the lakes to become shallow and salty, as a result of the deforestation and erosion (Ezcurra, 1992). By 1769 discussions were held again, but this time resulted in a plan to dry the lakes completely. The scientist Jose Antonio de Alzate (1739-1799) opposed to the idea and proposed an alternative plan that consisted of keeping the lakes and building a regulatory   9  channel to control the water level of Lake Texcoco. His plan was discarded (Ezcurra, 1992; Martínez González, 2008). The works to dry out the lakes continued until the 19th century, and the negative effects soon became evident: the soil was covered with saltpeter crusts, important transport routes were lost, and the water supply to the city became a problem. As a result, great works to extract ground water started, but caused the city to sink nine meters between 1910 and 1988  (Ezcurra, 1992; Martínez González, 2008).    These attitudes and ideologies require deeper evaluation and study, but the main purpose for suggesting and exemplifying them was to transmit to the reader a possible cultural explanation of a repetitive pattern, without listing all the events and bad decisions that have occurred up to the present day.  1.2.1.3 A little support results in great achievements  In each historical period it is possible to find persons like Jose Antonio Alzate, or a small  group of people that can be called ‘conservationists’ (Simonian, 1999) that represent an opposing force, and who follow ideals more in line with the principles of conservation and protection of the forest resources. This opposing force, although not always successful, managed to get attention and the support from some representatives of the Mexican government (Martínez González, 2008; Simonian, 1999), which enabled important victories in the fight over the protection of the natural resources, and contributed to the adoption of the first forest laws (Simonian, 1999). During the 20th century, Miguel Angel de Quevedo emerged as the leading promoter of forest conservation. He was a maritime and hydrological engineer who studied in France. His foreign education allowed him to understand the close relationship between forests and   10  the water regime, as well as the consequences of deforestation in any place (Martínez González, 2008; Simonian, 1999).  In 1901, Miguel Angel de Quevedo presented at the II National Congress on Climate and Meteorology held in Mexico how the destruction of forests negatively affects water supplies. He called for more strict laws for the conservation of forests (Martínez González, 2008; Simonian, 1999). In 1904, he created the Central Forest Board as part of the Secretary of Public Works, which is in charge of promoting the benefits of forests in Mexico (Martínez González, 2008; Simonian, 1999). In 1907, de Quevedo undertook a trip to various regions of Europe to learn about techniques applied for the recovery of the forests in other countries, and for the establishment of forest protection zones in different cities. During the same trip, he visited Algeria from where he brought a large number of seeds of various species: acacias, eucalyptus, casuarinas, pines and tamarix (Martínez González, 2008). In France, he met with the head of the French Forestry Service, Lucien Daubree (Simonian, 1999), who provided financial and personal assistance to support reforestation works in the Valley of Mexico and Veracruz (Martínez González, 2008; Simonian, 1999). The French assistance helped de Quevedo found the Forest Education School in 1908. This school started to offer courses in arboriculture and forestry, and trained the first Mexican forest technicians (Martínez González, 2008; Simonian, 1999).  Miguel Angel de Quevedo conceived a plan to reforest Mexico City. According to this plan, the city would have harboured four forest reserves, six nurseries and a system of nine suburban parks united by a wide arborized avenue (Martínez González, 2008). The plan had the support of two presidents, Porfirio Diaz (1876-1911) and Francisco I. Madero   11  (1910-1913), and de Quevedo could undertake the task of expanding the public spaces of the capital and to the new neighbourhoods. New neighbourhoods like Roma, Condesa, Doctores and Del Valle were built based on a design of wide avenues and extensive parks and gardens (Martínez González, 2008). According to regulations, the new neighbourhoods needed to include 10% parks and gardens (Meza Aguilar & Moncada Maya, 2010), and the streets needed to be 5 meters wide and lined with trees. Finally, the new neighbourhoods were connected with the old city through beautiful plantations of street trees; the idea was that this design would serve as an example for future real estate development in the city (Martínez González, 2008). The outbreak of the Mexican Revolution (1910-1920) and the assassination of President Francisco I. Madero in 1913, endangered or weakened many of de Quevedo's achievements and plans, e.g. through the closure of the Forest Education School (1911) and the urbanization of several parks and nurseries, and the green belt was never established (Martínez González, 2008; Simonian, 1999). In addition, the new president in power, Victoriano Huerta (1913-1914), unlike his predecessors, showed no interest in the work of de Quevedo, and strongly disagreed with him, forcing him into exile in Europe (Martínez González, 2008; Simonian, 1999). The forest conservation movement had its greatest boom during the 1920s and 1930s. For example, President Lazaro Cardenas (1934 to1940) showed great concern for the conservation of natural resources, and wanted to stimulate scientific research. To this end, he called back de Quevedo and named him head of the first autonomous conservation agency. From his new position, de Quevedo established a system of national parks (Bustillos-Roqueñi & Benavides-Zapién, 2000), forest reserves, protected forest areas and   12  nurseries (Martínez González, 2008; Simonian, 1999). The government also established Tree Day to promote the importance and care of trees for the benefit of the inhabitants. Moreover, it started publishing the magazine Protection of Nature directed to all parts of the population to promote the natural beauties of the Nation (Simonian, 1999). The work of de Quevedo from 1900 to 1946 represents an important era for the country’s conservation movement, the development of the forest and environmental legislation, as well as for the promotion and planning of urban green areas in Mexico City and the rest of the country (Bustillos-Roqueñi & Benavides-Zapién, 2000). The brief historic review presented in this thesis does not mean to simplify the complexity behind each period, and especially where wars were involved, or give more importance to some periods over the others. Rather it tries to acknowledge the weight of historic development (Miller, Hauer, & Werner, 2015; R. A. Rowntree, 1984) on urban forests, and the impact of changing cultural values on the current existence and management of the urban forest in Mexico City.   1.2.2 Urban forestry: environmental legal framework in Mexico The environmental legal framework at national, regional, and local level sustains the management programs for the urban forests of Mexico. Moreover, the state of the urban forest reflects the efficiency with which those programs are carried out. In this section the legal framework for the concept of urban forest, and the current environmental legislation in Mexico regarding urban forests are briefly presented. The focus then shifts to introduce the management programs for urban trees in the Coyoacan district, the case study area selected for this thesis.     13  1.2.2.1 The concept of ‘urban forest’ In a first assessment, it was found that the concept of urban forest as such is not used in the environmental legislation. According to Benavides (1989 in Benavides Meza 2015) the urban forest should be considered the wooded mass within the limits of a city which is composed of street trees3 and urban green areas4. Meanwhile, Miller (2015) defines the urban forest as the sum of all woody and associated vegetation in and around dense human settlements, ranging from small communities in rural settings to metropolitan regions. More specifically, the urban forest is the sum of street trees, residential trees, park trees and greenbelt vegetation. It includes trees on unused public and private land, trees in transportation and utility corridors, and forest on watershed lands. Mexico does not have an integral concept such as that of ‘urban forest’, but rather a series of independent concepts such as environment, natural resources, green areas, protected natural areas and areas of environmental value. Instead of leading to an integrative approach to urban forests, this array of concepts creates conflicts when it comes to green-space management, first, because they are not equally important and second, because the definitions are entangled. For example, in the case of Mexico City, the concept ‘urban forest’ is introduced in the Environmental Law of the Federal District (presented in the following sections) and its defined is rather unclear:  Urban forests are area of environmental value located in urban land, in which tree and shrub species predominate, where other wildlife species associated with and representative of biodiversity are distributed , as well as introduced                                                           3 Street trees: located along sidewalks, avenues, and transportation corridors (Benavides, 1989). 4 Urban Green areas: Parks, gardens, roundabouts, open spaces with vegetation like cemeteries, road rights, riverbanks, and streams that surround the city. Similarly, the great natural or introduced wooded masses within the boundaries of the city (Benavides, 1989).   14  species to improve its environmental, aesthetic, scientific, educational, recreational, historic or  touristic value, or other similar areas of general interest, whose extension and characteristics contribute to maintaining the quality of the environment in the Federal District. Article 90 Bis 1, Environmental Law of the Federal District. In my opinion, the law’s previous concept only defines urban forest as an area of environmental value which makes it more valuable than other types of green areas (Table 1.1), and thus their management is different. However, the urban forest is still diminished to a type of green area, and what is more, the listed characteristics that comprise an urban forest are confusing, causing trouble at the moment of wanting to determine what is an urban forest and what is not.  This concept is nothing like the one of Benavides (1989 in Benavides Meza, 2015) or Miller (2015). First, both concepts promote an ecosystem-based vision encompassing both the natural and urban elements that are present in the cities. Second, all components of the urban forest, however small, have the same importance and value, so urban woodland or park are equally important because they contribute to maintaining the quality of the environment. A more integral concept could facilitate the elaboration of integrative policies for the management and administration of urban forests at the local, state and national levels. 1.2.2.1 Legal framework  National level In Mexico, the legal foundations to protect the environment are based on the 4th article of the Political Constitution of the United Mexican States (CPEUM, 2017), which establishes that everyone has the right to an adequate environment for their development and well-  15  being (CPEUM, 2017). This article gained weight with the following international agreements signed by Mexico: 1) The Montreal Protocol (1987), which recognizes the need to take measures to protect human health and the environment through the elimination of substances used in human activities that deplete the ozone layer; and 2) The Kyoto Protocol (1988), which aims to promote appropriate reforms in the relevant sectors to promote policies and measures that limit or reduce greenhouse-gas emissions. Referring again to the Political Constitution of the United Mexican States, the 73rd article, section XXIX-G, grants the power to the Congress to issue laws that establish the concurrence of the Federal Government, the governments of the federative entities, the Municipalities and the territorial boundaries of Mexico City, within the scope of their respective competence, and in relation to environmental protection and the preservation and restoration of the ecological balance. These two articles and the international agreements gave rise to the current General Law of Ecological Equilibrium and Protection to the Environment (LGEEPA, 1988) that came into force in 1988. This law aims for the preservation and restoration of the ecological balance, as well as protecting the environment, promoting sustainable development, and establishing the basis for preventing and controlling air, water, and soil pollution (LGEEPA, 1988). In 2012, the General Law on Climate Change was published, which, like the LGEEPA, seeks to guarantee the right of Mexicans to a healthy environment, but this law focuses on regulating emissions of greenhouse gases and compounds through the elaboration of strategies, programs and comprehensive projects for mitigation and adaptation to climate change (LGCC, 2012).   16  These laws are included in the National Climate Change Strategy of 2013 which aims to reduce emissions by 30% in 2020 and 50% by 2050. The National Strategy has among its pillars to conserve and use sustainable ecosystems and maintain the environmental services they provide and reduce emissions of short-lived climate pollutants and promote health and welfare co-benefits (ENCC, 2013). To achieve this, there is the Special Climate Change Program 2014-2018 which has a sectoral program on Environment and Natural Resources 2013-2018 with the following objectives: 1) Promote and facilitate sustainable and low carbon growth with equity and socially inclusive; 2) Increase resilience to climate change and reduce emissions of greenhouse-gas compounds; 3) Cease and reverse the loss of natural capital and pollution of water, air and soil; 4) Develop, promote, and implement policy, information, research, education, training, participation, and human rights instruments to strengthen environmental governance (Plan Nacional de Desarrollo, 2013). The above can be considered the legal foundations and strategic planning at the national level in relation to the issue at hand.                                        Mexico City: environmental legislation  Before 2016, the Federal District, capital of Mexico, did not have its own Constitution like the rest of the states of the country. As a consequence, its laws were governed by the Political Constitution of the United Mexican States (CPEUM, 2017). In 2016, the Federal District was renamed Mexico City, as it is internationally known, and on the 5th of February of 2017 the Political Constitution of Mexico City was published, and will come into force until 2018. This represents a great change in the administration of Mexico City, causing changes in the laws that govern it.  This Constitution should be taken into account in future   17  references, but this thesis presents the legal framework in environmental matters that Mexico City is currently following.  According to the ecological settlement of the territory, Mexico City is divided into urban land and conservation land (Figure 1.2). Both classifications were established by sections I and II respectively of the 30th article of the Law of Urban Development of the Federal District. Urban land includes green areas5 within the administrative boundaries of the urban area, while conservation land is composed of areas of environmental value6 and protected natural areas7 (LADF, 2000).   This division intends to facilitate the planning and regulation of productive activities in the territory as well as the conservation of natural resources and the improvement on the quality of life of the citizens  (SEDEMA, 2017).                                                              5 Green Area: Any surface covered with natural or introduced vegetation to be located in the Federal District (Art. 5 Ley Ambiental del Distrito Federal, 2000). 6 Areas of environmental value: Green areas where the original environments have been modified by anthropogenic activities and which require restoration or preservation, depending on the fact that they still maintain certain biophysical and scenic characteristics, which allow them to contribute in maintaining the environmental quality of the city (Art. 5 Ley Ambiental del Distrito Federal, 2000).  7 Natural Protected Areas: The natural physical spaces where the original environments have not been sufficiently altered by anthropogenic activities, or that need to be preserved and restored, due to their structure and function for the recharge of the aquifer and the preservation of biodiversity. These are areas that, due to their ecogeographic characteristics, the content of environmental and cultural species, goods, and services that they provide to the population, make it essential to preserve them (Art. 5 Ley Ambiental del Distrito Federal, 2000).   18    Mexico City is subject to the Environmental Law of the Federal District (LADF) that came into force in 2000, and defines the principles to formulate and evaluate the environmental policy. This law includes eight objectives. For this study, we only refer to the sections of the fourth that refers to establishment and regulation of green areas, areas of environmental value and protected natural areas of the Federal District (…) and the fifth, Figure 1.2. Ecological settlement of the territory of Mexico City. Modified from PAOT, 2010.  Figure 2.1.  Map of the 95 neighborhoods in Coyoacan. The area in green color is the National Autonomous University of Mexico campus and was not included in the study. Figure 1.7. Ecological settlement of the territory of Mexico City. Modified from PAOT, 2010.  Figure 2.1.  Map of the 95 neighborhoods in Coyoacan. The area in green color is the National Autonomous University of Mexico campus and was not included in the study. Map taken and modify from Secretary of Housing and Urban Development (SEDUVI).   Figure 1.8. Ecological settlement of the territory of Mexico City. Modified from PAOT, 2010.  Figure 2.1.  Map of the 95 neighborhoods in Coyoacan. The area in green color is the National Autonomous University of Mexico campus and was not included in the study. Figure 1.9. Ecological settlement of the territory of Mexico City. Modified from PAOT, 2010.    19  that aims to apply measures to prevent and control air, water and soil pollution in the Federal District (...). In order to meet these objectives, the 8th article of the LADF establishes that it is up to the Head of Government (...) to establish the environmental fund (...) for research, study and attention of those matters that in environmental matters are considered of interest for Mexico City. According to the 69th article, the resources of the fund will be used mainly for the management of areas of environmental value and protected natural areas. This means the fund is mainly targeted to the conservation land, and not so much to the green areas within the urban land. The 10th article of the same law mentions that each of the districts of the Federal District have the responsibility to “V. Implement conservation and restoration actions (...) as well as the environmental protection (...)”. Moreover, as stated in clause VIII, Districts should earmark a percentage of their annual budget that guarantees the maintenance, protection, preservation, and monitoring of green areas within their boundaries. The 20th article establishes once again that the inhabitants of Mexico City have the right to enjoy a healthy environment and that it is the authorities that will take the necessary measures to preserve that right. Likewise, every inhabitant of Mexico City has the power to demand respect for this right and the fulfillment of the obligations by the authorities of Mexico City. The Environmental Law of the Federal District devotes articles 87 to 90 entirely to green areas. The 87th article again addresses the responsibility of each district for the construction, rehabilitation, administration, preservation, protection, restoration, afforestation, reforestation, development, and monitoring of green areas (Table 1.1). It states, “The districts will seek the increase of green areas of their competence, in balanced proportion with the uses of land different from green areas, open and gardened spaces (...)   20  and incorporate them into the district urban development programs”.  Table 1.1. Green areas classification.  Green areas according to the 87th article on the Environmental Law of the Federal District.  Types of green areas Parks and gardens Garden or wooded squares Garden pots Area with any vegetation cover on public roads  ‘Alamedas’ (poplar stands) and groves Promontories, hills, natural pastures and rural forested areas, agro-industrial areas or areas that provide ecotourism services Aquifer recharge areas Other similar areas  The 88th Bis article states that the Secretary of Environment of Mexico City (SEDEMA) and the districts may establish agreements with the neighbors of the green areas of their competence, to participate in their maintenance, improvement, restoration, promotion, and conservation, as well as in the execution of reforestation, recreational and cultural actions, providing in-kind support mechanisms, and when necessary promoting involvement in the surveillance of such areas. The 88th Bis 4 article states the SEDEMA has the responsibility of creating the General Inventory of Green Areas of the Federal District, in order to know, protect and preserve those areas, as well as to propose to the Secretary of Urban Development and Housing and other districts, according to their competence, the increase of such green spaces in areas where this is required. For their part, districts will “carry the inventory of   21  green areas of their competence in their own territory (...), providing annual updates (...) that will form part of the Environmental Information System of the Federal District”. Finally, the 90th article refers to the damage of green areas and that those responsible will have to repair the damages “(...) restoring the affected area or carrying out compensatory actions”. In 2011, the Law on Mitigation and Adaptation to Climate Change and Sustainable Development was published for Mexico City, which according to the 2nd article includes establishment of public policies to promote the mitigation of greenhouse gases, and adaptation to climate change. Both former mentioned laws are the foundations of the General Program for Development of the Federal District, the Green Plan, the Climate Action Program and the Local Climate Action Strategy of Mexico City, which in general terms seek to increase the green areas of Mexico City. Without going further into each of these plans, the baseline of the legal framework of Mexico City shows to be extensive and in theory very promising. However, since the publication of the environmental laws implementation has been insufficient, nonexistent, ignored or in many cases unfinished. As an example, the General Inventory of Green Areas of the Federal District carried out in 2010 by PAOT (Environmental and Territorial Planning Department) has not been updated since then. Moreover, not all the districts have inventories, even though they should in accordance with the 88th article BIS 4 of the Environmental Law. Another example is the implementation of the program No Driving Today, which seeks to reduce the number of vehicles in circulation to reduce pollutant emissions. This program has proved ineffective due to permits granted for major roadworks that favor the use of the automobile in detriment of the urban forest. For example, the headline of a news story on May 24, 2015 published in Sin Embargo newspaper writes: “El   22  DF pierde en 15 años 56 mil árboles por obras” (The Federal District loses in 15 years 56 thousand trees per road works). One of the problems is that the law allows trees to be removed if they obstruct construction plans, and replanting is not mandatory, and there is no data to corroborate if replantation occurred.  This situation generates serious doubts about the implementation of the environmental legal framework of Mexico City. 1.2.3 Coyoacan district: actions at the local level   On June 19, 2017, the author conducted an interview with the Head of the Departmental Unit of Parks and Gardens and the Field Coordinator of Coyoacan district, with the purpose of investigating more about the district’s management programs. The following information is derived from this interview, as there is very little written information available.  The Departmental Unit of Parks and Gardens (DUPG) has an overall budget which covers salaries, equipment and material. The former distribution does not consider a percentage destined for tree maintenance. The DUPG is responsible for Coyoacan’s 152 parks and all secondary roads, while SEDEMA is responsible for the primary roads. The aforementioned unit has a team of 430 workers, which includes 25 certified tree climbers, 20 drivers, 50 ‘macheteros’ (‘cutters’), 47 gardeners, and the remainder are administrative staff. The official management program for the green areas consists of grass mowing and weed removal, whereas tree pruning, tree removal and transplanting of trees are subject to the requests of the citizens. The DUPG has on electronic record that in 2016, 3,030 trees were pruned and 418 were cut down. However, according to the people interviewed, there is a 9-year lag in requests for pruning, tree removal and tree transplantation, and half of the   23  vehicle fleet is not functioning. Moreover, Coyoacan has no inventory of street trees, parks and/or gardens, and the DUPG do not have a pest control program. Reforestation and replanting are done in response to requests of citizens who can specify the species to plant (as long as these are available in the district’s nursery). According to the interviewees, in contrast to pruning and removal, they do not keep a reliable record of reforestation or replantation. Also, the departmental unit in question does not maintain direct contact with SEDEMA or with specialized institutions such as the Mexican Association of Arboriculture (AMA). The latter has the most up-to-date standards on arboriculture and urban forestry to promote the care of trees in streets, parks and other green areas (AMA, 2017). According to the information provided it has been almost three years since operative staff received new training. The current situation of the Departmental Unit of Parks and Gardens suggests that most of the budget is allocated to administration, while a smaller amount is allocated to operations. Thus, it is not surprising that the demands of green area management far exceeds the available specialized staff. Moreover, this calls for more specialized staff like tree health inspectors, arborists, specialists in geographic information systems, and better equipment and technologies such as computers, GPS, clinometers, and the repair of the existing vehicle fleet, to mention a few. In general terms, the actions taken by the local government of Coyoacan have not fulfilled the objectives of the City’s plans or what the laws dictate. One of the reasons for this is that the Departmental Unit is operating without sufficient financial resources. For example, cities like Santa Monica and Modesto in California spend 56% and 46% respectively of their urban forest management budget on annual tree programs and great part of that budget is allocated to tree pruning (Xiao, 2002). In the city of Strasbourg,   24  France management programs dictate the regular pruning of street trees to maintain the geometric design as well as for safety reasons (Selmi et al., 2016). The city of Lisbon (100 km2), which is almost twice as big as Coyoacan (54 km2), spends approximately $1.9 million on tree management, administration, and related activities (Soares et al., 2011). It is fair to note that Coyoacan is not a city as in these examples, but a major difference can be noted in terms of prioritizing structural and proactive urban tree management.   1.3 Research objectives  The first objective of this study was to assess and quantify the ecosystem services provided by the trees in the Coyoacan district, in Mexico City, by focusing on twelve neighbourhoods. These twelve neighbourhoods work as a case study of the current structure and function of the urban forest in Mexico City. The second objective was to learn about people’s appreciation and valuation of street trees. This study addressed two research questions related to this topic.  1. What are the estimated values of the ecosystem services provided by the street trees in Coyoacan?    The street trees population was estimated for twelve neighbourhoods in Coyoacan using stratified random sampling. Following the same method, trees were measured to quantify tree benefits using the i-Tree Eco 6 software. It was expected that neighbourhoods with a greater number of trees would also have a higher amount of ecosystem services than neighbourhoods with fewer trees.     25  2. What are people’s perceptions towards street trees in Coyoacan?  People’s perceptions were analyzed using passer-by surveys and descriptive statistics to learn about the most liked and disliked trees, most well-known tree benefits, associated tree disservices, preferred locations for tree planting, and numbers of street trees in residents’ neighbourhoods. It was expected that residents in neighbourhoods with few trees appreciate trees less than do residents from neighbourhoods with many trees.     Chapter 2: Methodology 2.1 Study area  2.1.1 Mexico City Mexico City is in the southern part of Mexico (19° 20’ N, 99° 22’ W). The city has a surface of 1495 km2 and has an average altitude of 2,240 m.a.s.l. It is surrounded by mountains and the local weather is considered semiarid with a medium temperature of 16° C. The annual precipitation ranges from 700 to 816 mm, with most falling during the rainy season (May to October). Mexico City is the political and economic center of the country and it is divided into sixteen districts or ‘delegations’ (Escobedo & Chacalo, 2008; INEGI, 2004; Rojo Negrete, 2006).  2.1.2 Coyoacan  2.1.2.1 Geographical aspects The present study was carried out in Coyoacan, one of the 16 districts of Mexico City. Coyoacan is located at the geographic center of the city (19° 22' N, 99° 15’ W) covering a total surface of 54.12 km2 and representing 3.6% of the entire city  (INEGI, 2004; Delegacion Coyoacan, 2016). It borders five other districts (figure 1.2): Benito Juarez to the north, Iztapalapa the northeast, Alvaro Obregon the northwest, Tlapan the south, and Xochimilco the southeast (INEGI, 2004).  Coyacan sits on the Neovolcanic Axis within the subprovince Lagos and Lagunas de Anahuac. Forty-seven percent of the district sits over the basaltic plateau ‘Malpais,’ while 39% is situated on a lake plain and the remaining 14% is located in an alluvial plain (INEGI, 2004).    27  The district presents two types of soil: litosol olivine basalt of volcanic origin and feozem characteristic of the lacustrine regions, also known as transition zone (Delegacion Coyoacan, 2016; Programa Delegacional de Desarrollo Urbano de Coyoacan, 2010).  According to the Köppen classification, Coyoacan’s northeast, central and south areas have a temperate sub-humid climate with summer rains of medium humidity C(w1), while the west area presents a temperate sub-humid climate with summer rains of lower humidity C(w0). The average annual temperature is 17°C. Annual precipitation is 808.8 mm, with the rainiest months being May to October and the driest period extending from December to February (Delegacion Coyoacan, 2016; INEGI, 2004).  2.1.2.2 Population and urban development The last official census (2005), registered a population of 628,063 inhabitants in Coyoacan, equivalent to 7.2% of the total population of Mexico City (INEGI, 2004). Land use is predominantly residential, covering 57.46% of the territory with individual houses, condos, mid-high buildings, and housing complexes. Another 32.4% of the district is covered by open spaces, green areas and the university campus. Public or commercial services represent 3.79%, industry another 3.14% and mixed land uses occupy the last 3.11%. By 2000 land use has gradually shifted towards a mix of commercial/residential land use ( Programa Delegacional de Desarrollo Urbano de Coyoacan, 2010).  During the 1930’s Coyoacan remained as a semi-rural area comprised of extensive farms or ‘haciendas’, communal land or ‘ejidos’, and small towns also called ‘pueblos’ (Delegacion Coyoacan, 2016). It was not until 1940 that urban development began. Urbanisation started on the north side where various haciendas were divided to create neighbourhoods with an organized design, but gradually development became more   28  disorganized and unplanned, especially towards the south, in an area known as ‘Pedregales’ (Delegacion Coyoacan, 2016). During the 1950s and 1960s, Coyoacan was considered a potential site for large housing complexes that could concentrate greater numbers of people, but their construction did not follow proper planning and organization, resulting in densely populated areas with few open spaces. For example, in 1958, the construction of Ciudad Universitaria, a name granted to the campus of the National Autonomous University of Mexico, on the southwest side of Coyoacan brought along the development of important roads and the emergence of new neighbourhoods with vast housing complexes (Programa Delegacional de Desarrollo Urbano de Coyoacan, 2010).   During the 1970s the city was subjected to mass migration from the rural areas that increased the population and rapidly exceeded the capacity of housing and services availability. By that time, Coyoacan functioned as an important ‘receiver’ of people: the still spacious lands available were in various cases taken over by the new residents. As a result, soon the landscape was full of disorganized, anarchic constructions that subsequently became densely populated neighbourhoods that did not always have access to services such as a functioning sewage system (Delegacion Coyoacan, 2016).   Between 1970 and 1980, another district, Iztapalapa, was rapidly expanding and fostered growth towards the east side of Coyoacan, an area known as ‘Culhuacanes’. Once again the growth was disorganized and densely populated (Programa Delegacional de Desarrollo Urbano de Coyoacan, 2010). In 2005, the Secretary of Housing and Urban Development (SEDUVI) along with the Mexican Postal Service (SEPOMEX), the Treasury, the General Direction of Territorial Regularization and Coyoacan’s local government developed a single map that indicates the 29  limits of each of the neighbourhoods in the district (Figure 2.1). Currently, Coyoacan has 82 neighbourhoods, 9 ‘barrios’ and 4 ‘pueblos’; in total 95. For the purpose of this thesis hereafter all of these classifications are going to be referred as neighbourhoods (Delegacion Coyoacan, 2016).  Figure 2.1. Map of the 95 neighborhoods in Coyoacan. The area without a colored layer is the National Autonomous University of Mexico campus and was not included in the study. Map taken and modified from http://eldefe.com/mapa-colonias-delegacion-coyoacan/ on 12th March 2016.    Figure 2.3.  Example of a block. In this street unit trees were observed on the inside perimeter of the selected block unit in rectangular street layout 30  2.1.2.3 Marginalization index distribution The national marginalization index (MI) intends to reflect the shortcomings of Mexico’s population at national, estate, municipal and local level(CONAPO, 2012a). It should be understood as an indicator that identifies when part of the society does not have sufficient development opportunities, nor the capacity to find them (CONAPO, 2012b). For this purpose, socioeconomic dimensions like education, population distribution, housing and monetary income (Figure 2.2) function as strong indicators of the stage of marginalization (CONAPO, 2012a).  Figure 2.2. Marginalization indicators chart.  Modified from CONAPO, 2010a.   31  Overall, at a municipal level Coyoacan, presents a low marginalization index (CONAPO, 2012a). However, at the local level, i.e. that of the neighbourhoods, we encountered a significant proportion of neighbourhoods with either very high, high or medium marginalization levels. According to the Social Development Information System (SIDESO) Coyoacan has 3 neighbourhoods with a very high MI, 12 with a high MI, 7 with a medium MI, 25 with low and 48 with very low MI. This means that 23.15% of the occupants of some of the neighbourhoods present or are subject to great inequality in terms of development and the benefits that come with it (CONAPO, 2012a).  2.1.2.4 Green areas Coyoacan is one of the districts in Mexico City with the highest share of green areas (27.50%). These green areas include either urban woodland or grass/shrubs. The urban woodland represents 20.90%, being the greatest represented by the Pedregal de San Angel Ecological Reserve, a UNESCO World Heritage Site, located at the university campus and the other one the Zacatepetl Hill urban forest and the Coyoacan Nursery (both under protection decrees). Grass/shrubs represent only 6.60% and this includes grass courts for sport activities.  2.2 Assessment of the street trees using the i-Tree Eco 6 model   For this thesis I chose to study street trees, since they are the component of the urban forest that do not receive much attention, turning them into easy targets to be removed when new developments happen, especially road works. The absence of an updated inventory that tells how many trees are in Coyoacan, or in the entire city, makes street-tree management and protection even more difficult.   32  I apply the method developed by Jaenson et al. (1992) to estimate the tree population of the twelve neighbourhoods and Coyoacan district and also, to randomly select trees to be sampled to enter the tree data in the i-Tree Eco6 model.  The following section gives an overview of the i-Tree Eco6 model and describes in detail the method developed by Jaenson et al. (1992). 2.2.1 i-Tree Eco 6 model For studies outside United States, Canada, Australia and United Kingdom, i-Tree eco V6 is not adapted to analyze the data automatically, and thus tree data for this project was analyzed by the i-Tree team, a process which took approximately three months (November 2016-January 2017).  The i-Tree Eco V6 model uses standardized field data, air pollution, and meteorological data to quantify the urban forest structure and other ecosystem services or tree benefits as carbon storage and sequestration, avoided surface runoff, and air pollution removal ( i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017). The following are general descriptions of the methods i-Tree uses for data analysis; for a thorough understanding of these methods, the i-Tree website has available the corresponding studies.  Structure of the urban forest: The model quantifies the general structure of the urban forest based on field data and arranges it according to its DBH class and tree species. Using the field data the model also genereates information about species composition, tree density, tree health, leaf area and leaf and tree biomass. To obtain the leaf area of individual trees the model uses a regression equation with the obtained measurements of crown   33  dimensions and the percentage of crown canopy missing in regression equations (i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017; Nowak & Crane, 2000).   Air pollution removal: The pollution removal is calculated for ozone (O3), sulphur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO) and particulate matter less than 2.5 microns (PM2.5). To derive air-pollution removal estimates the model uses the calculations for hourly tree-canopy resistance for ozone, sulphur and nitrogen dioxides based on a hybrid of big-leaf and multi-layer canopy deposition models (Baldocchi, 1988; Baldocchi et al., 1987, in i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017). The model also assumes a 50% resuspension rate of particles back to the atmosphere (Zinke, 1967 in i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017). Carbon storage and sequestration: carbon storage is the amount of carbon bound up in the above-ground and below-ground parts of woody vegetation ( i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017). To calculate current carbon storage the biomass for each tree was obtained using biomass equations from the literature and measured data. According to McPherson, Nowak, & Rowntree (1994) the biomass in open-grown and maintained trees tends to be less than the predicted by forest-derived biomass equations. To adjust for this difference, biomass results for open-grown urban trees were multiplied by 0.8. Finally, tree dry-weight biomass was converted to stored carbon by multiplying by 0.5. Carbon sequestration occurs when carbon dioxide from the air is removed by plants. i-Tree Eco V6 model estimates the gross amount of carbon sequestered annually. It uses the existing tree diameter (year x) and add the average diameter growth from the appropriate   34  genera and diameter class and tree condition to estimate tree diameter and carbon storage in year x+1 ( i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017).  Avoided runoff: The model estimates the annual avoided surface runoff based on rainfall interception by vegetation. For this analysis the model only took into account precipitation intercepted by leaves and no other parts of the trees, like bark and branches. For the annual estimations, the model considers the difference between annual runoff with and without vegetation ( i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017). 2.2.2 Tree sampling method   For this thesis, it was decided to sample 12 neighbourhoods to provide an overview of the district as a whole. A total of 497 street trees were sampled within these twelve neighbourhoods to obtain an insight into the district’s street tree population. Coyoacan does not have a complete inventory of street trees and the partial one done by Chacalo et al. (1996) for Mexico City has not been updated since then. Coyoacan was thus considered an area with no previous inventory. In these circumstances, we followed the sampling method developed by Jaenson et al. (1992) which was designed exclusively to sample street-tree population without previous inventories. Following this method a suitable random selection of a small fraction of the population of interest can provide highly accurate estimates of the total number of trees in a city.   2.2.2.1 Zoning The method of Jaenson et al. (1992) suggests stratifying a city’s area into zone types. This stratification divides the district into regions where the sample areas are spread appropriately across the entity. The zone types are determined by the major land use in an   35  area (like residential or commercial neighbourhoods), the physical street layout, or the time of development. The three common zone types are rectilinear residential (RR)8, curvilinear residential (CR)9, and downtown (DT)10.  Jaenson’s method was developed for entire cities, while Coyoacan is not a city, but a district. Therefore, and considering that the predominant land use is residential, the zoning was not done according to Jaenson et al. (1992), but to the marginalization index map elaborated by CONAPO 2012. For practical reasons, the five marginalization indexes (Figure 2.2) were compiled into three: high, medium, and low. Table 2 shows the number of neighbourhoods corresponding to each marginalization index and the proportional number of neighbourhoods that need to be sampled. Table 2.1. Zonation of the sample neighbourhoods. This zonation was made according the marginalization index registered by the CONAPO 2012.  Marginalization index Total number of neighbourhoods Sampled neighbourhoods Low 73 8 Medium 7 2 High 15 2                                                           8 Rectilinear residential (RR) neighborhoods usually contain the majority of street trees found in the city. These areas, consisting primarily of rectangular blocks, are often the neighborhoods that were first developed around the core of the city and usually contain sidewalks and street tree planting areas (Jaenson et al, 1992).  9 Curvilinear Residential (CR) zones, were usually developed later within a city, and the layout of these neighborhoods often does not include sidewalks and specified tree pits. The streets in CR zones usually do not form a grid-like pattern (Jaenson, Bassuk, Schwager, & Headley, 1992).   10 Downtown (DT) or central business district. These areas are substantially different from residential areas. The streets in DT zones usually form a grid-like pattern (Jaenson et al., 1992).    36   Some neighbourhoods with high marginalization indexes are well known for their high levels of crime and insecurity (Servín Vega, 2012). Twenty neighbourhoods showed high insecurity levels and had to be discarded during the random selection because of the risks for the researchers associated with sampling. Thus, although all neighbourhoods were considered during the random selection, if an insecure neighbourhood was picked, it was discarded, and another area was picked. The National Autonomous University of Mexico (UNAM) is considered a single neighbourhood (Figure 2.1) and the management of green areas enters in its jurisdiction, thus, it was also left out of the study.  The sampled neighbourhoods (Figure 2.3), each one with defined political boundaries, represent independent zone segments that displayed either a rectilinear or curvilinear street layout. According to Jaenson et al. (1992), a zone segment is a contiguous region of a single zone type containing between 20 and 500 sampling units (city blocks).            Figure 2.3. Map of sampled neighbourhoods. The boundaries of Coyoacan district are delineated by the yellow line. Neighbourhoods with low marginalization index are in blue. Neighbourhoods with medium and high marginalization index are in red and green respectively.   37  2.2.2.2 Establishment of a uniform sampling unit Sampling units represent the chosen areas where the data collection would occur. To facilitate data collection, each zone segment (neighbourhood) was divided into sampling units called street units. There are two types of street units. The first type is the inside perimeter of a block (figure 2.4), common in zone segments with rectilinear street layouts (RS). The advantage of sampling blocks is that you can tally and sample all the trees around the block without counting the same tree twice.   The second type is that of a street segment (figure 2.5) common in zone segments with curvilinear street layouts (CS), meaning areas with no blocks or grid-like patterns. A street segment is obtained by estimating the average block perimeter (A) and divided by two (A/2) (Jaenson, Bassuk, Schwager, & Headley, 1992). Figure 2.4.  Example of a block. In this street unit trees were observed on the inside perimeter, indicated by the arrows, of the selected block unit in rectangular street layout neighborhoods. Modified from Jaenson et al. (1992). Figure 2.5. Example of a street segment. In this street unit trees were observed on both sides of the street, as indicated by the arrows. Modified from Jaenson et al. (1992).  Table 2.3. Process to obtain the street segment length. Zone segments (neighborhoods) with rectilinear streetsFigure 2.4. Example of a street segment. In this street unit trees were observed on both sides. Modified from Jaenson et al. (1992).   38  2.2.2.3 Preliminary sample Jaenson et al. (1992) suggest doing a pre-sample to estimate the average block perimeter length of RS zone segments and to estimate the number of street trees in every zone segment.  Average block perimeter  Five of the twelve randomly selected units had a RS layout (Table 2.3), so it was necessary to calculate the street segment length (A/2) for the other seven CS zone segments. Using images from Google Maps (2015) every block of each RS zone segments was enumerated from 1 to Bi, with Bi referring to the number of blocks in a particular zone segment (i). Parks and green areas were not enumerated, since I focused only on street trees. Then, 4 to 10 blocks were randomly selected according to the conditions established on Table 2.2.    Table 2.2. Street unit selection. Suggested number of street units (blocks and street segments) for the pre-sampling and sampling phases. Source: Jaenson et al.,1992).  Number of blocks in zone segment i (Bi)  Number of blocks in pre-sample i (bi)  20 – 50 blocks                                                    20% of Bi (rounded to nearest integer) 50 – 500 blocks                                                  10 blocks  The perimeter of every randomly selected block was measured using the ruler tool in Google Earth Pro (2015). Table 4 shows the average block perimeter for every zone segment. Then, using a weighted average A (equation 1) the average block perimeter length overall for the RS zone segments was estimated. Finally, the estimated average block   39  perimeter A was divided by 2 to estimate the street segment length for use in the rest of the seven zone segments.  Equation 2.1. Formula for weighted average A. Taken from Jaenson et al. (1992). 𝐴 =∑𝑖 = 1(𝑏𝑙𝑜𝑐𝑘 𝑐𝑜𝑢𝑛𝑡)𝑖 𝑥 (𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑏𝑙𝑜𝑐𝑘 𝑝𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟)𝑖∑𝑖 = 1(𝑏𝑙𝑜𝑐𝑘 𝑐𝑜𝑢𝑛𝑡)𝑖                Zone segment (neighbourhood) Total blocks in the neighbourhood Random block number Measured Perimeter (mts) San Diego Churubusco 15 2 559.8 San Diego Churubusco 4 431.0 San Diego Churubusco 6 1193.5 San Diego Churubusco 10 421.7 (Average block perimeter)SDC= 651.5 Parque San Andrés 29 12 747.6 Parque San Andrés 27 567.4 Parque San Andrés 22 362.9 Parque San Andrés 11 361.9 Parque San Andrés 26 536.1 Parque San Andrés 7 480.4 (Average block perimeter)PSA= 509.4 Los Cedros 14 3 268.0 Los Cedros 5 235.0 Los Cedros 8 450.0 Los Cedros 10 739.0 (Average block perimeter)LC= 423.0 Petrolera Taxqueña 10 9 320.5 Petrolera Taxqueña 4 649.8 Petrolera Taxqueña 1 472.9 Petrolera Taxqueña 6 677.2 (Average block perimeter)PT= 530.1 Table 2.3. Process to obtain the street segment length. Zone segments (neighborhoods) with rectilinear streets.  Table 2.3. Process to obtain the street segment length. Zone segments (neighborhoods) with rectilinear streets.  Table 2.4.  Estimate number of street trees per street unit.Table 2.3. Process to obtain the street segment length. Zone segments (neighborhoods) with rectilinear streets.  Table 2.3. Process to obtain the treet segment length. Zone segments (neighborhoods) with rectilinear streets.  Table 2.4. Estimated number of street trees per street unit.Table 2.3. Process to obtain the street segment length. Zone segments (neighborhoods) with rectilinear streets.  Table 2.3. Process to obtain the street segment length. Zone segments (neighborhoods) with rectilinear streets.  Table 2.4.  Estimate number of street trees per street unit.Table 2.3. Process to obtain the street segment length. Zone segments (neighborhoods) with rectilinear streets.  Table 2.3. Process to obtain the street segment length. Zone segments (neighborhoods) with rectilinear streets.   40           Street segments Using Google Earth Pro (2015) the total surface of each CS zone segments was drawn in a polygon and following street segments of 193 meters were marked off and numbered in every CS zone segment. Then, street segments were randomly selected using the method describe in Table 2.2.  Estimation of street tree population in the twelve neighbourhoods Street trees were counted in all the selected street units. Trees situated in a middle strip were considered as street trees if they were within 1.10 meters of the curb. Following Jaenson et al.’s (1992) suggestions, we only counted trees from one side for blocks in RS zone segments (Figure 2.3), and from both sides of the street segment in CS zone segments (Figure 2.4).  The following steps were followed to estimate the street-tree populations of the twelve neighbourhoods (as presented in Tables 2.4 and 2.5). Zone segment (neighbourhood) Total blocks in the neighbourhood Random block number Measured Perimeter (mts) Del Carmen 108 104 440.3 Del Carmen 18 410.8 Del Carmen 75 101.4 Del Carmen 57 478.9 Del Carmen 90 458.3 Del Carmen 58 268.4 Del Carmen 108 532.9 Del Carmen 102 352.0 Del Carmen 72 419.3 Del Carmen 31 399.3 (Average block perimeter)DC= 386.1     Weighted average A (mts) 440.2   Street segment length (A/2)  220.1    220 mts     41  1. Estimate the average number of street trees per street unit in each zone segment (Table 2.4).  Equation 1.2: 𝑁𝑡𝑟𝑒𝑒𝑠 =(𝑡𝑜𝑡𝑎𝑙 𝑡𝑟𝑒𝑒𝑠 𝑐𝑜𝑢𝑛𝑡𝑒𝑑 𝑖𝑛 𝑝𝑟𝑒 − 𝑠𝑎𝑚𝑝𝑙𝑒 𝑜𝑓 𝑠𝑡𝑟𝑒𝑒𝑡 𝑢𝑛𝑖𝑡𝑠 𝑖𝑛 𝑧𝑜𝑛𝑒 𝑠𝑒𝑔𝑚𝑒𝑛𝑡)𝑖(𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑡𝑟𝑒𝑒𝑡 𝑢𝑛𝑖𝑡𝑠 𝑝𝑟𝑒 − 𝑠𝑎𝑚𝑝𝑙𝑒𝑑 𝑖𝑛 𝑧𝑜𝑛𝑒 𝑠𝑒𝑔𝑚𝑒𝑛𝑡)𝑖  2. Estimate the total number of street trees in the twelve neighbourhoods.  Equation 2.2:   (𝑁𝑡𝑟𝑒𝑒𝑠)𝑥 (𝑇𝑜𝑡𝑎𝑙 # 𝑜𝑓 𝑠𝑡𝑟𝑒𝑒𝑡 𝑢𝑛𝑖𝑡𝑠 𝑖𝑛 𝑧𝑜𝑛𝑒 𝑠𝑒𝑔𝑚𝑒𝑛𝑡) 42   Zone type Zone segment Neighbourhood Total # trees counted # street units pre-sampled Estd. Avg. # trees per street unit (density) LM PC Pedregal de Carrasco 84 4 21 LM DC Del Carmen 534 10 53.4 LM PSA Parque San Andrés 387 6 64.5 LM SDC San Diego Churubusco 256 4 64 LM IC Insurgentes Cuicuilco 183 4 45.75 LM OU Oxtopulco Universidad 136 4 34 LM LC Los Cedros 208 4 52 LM PT Petrolera Taxqueña 167 4 41.75 MM CN Canal Nacional 142 4 35.5 MM CSF Cuadrante San Francisco 44 4 11 HM CA Copilco el Alto 124 4 31 HM BT Bosques de Tetlameya 252 4 63 Table 2.4. Estimated number of street trees per street unit.  Table 2.5 Estimated number of street trees in the twelve neighborhoods.   Figure 2.7. Age group distribution of the 186 respondents.Table 2.5 Estimate number of street trees on the twelve neighborhoods.   Figure 3.2. Tendency of carbon storage according to DBH dimensions. The power trendline is the red color, while the blue is a linear trendline.Table 2.5 Estimate number of street trees on the twelve neighborhoods.   Figure 2.7. Age group distribution of the 186 respondents.Table 2.5 Estimate number of street trees on the twelve neighborhoods.Table 2.4. Estimated number of street trees per street unit.   43    Zone type Zone segment Neighbourhood Estd. Avg. # trees per street unit Actual # of street units in zone segment Estd # of trees (Ni) LM PC Pedregal de Carrasco 21 24 504 LM DC Del Carmen 53.4 106 5660.4 LM PSA Parque San Andrés 64.5 29 1870.5 LM SDC San Diego Churubusco 64 15 960 LM IC Insurgentes Cuicuilco 45.75 19 869.25 LM OU Oxtopulco Universidad 34 9 306 LM LC Los Cedros 52 14 728 LM PT Petrolera Taxqueña 41.75 10 417.5 MM CN Canal Nacional 35.5 21 745.5 MM CSF Cuadrante San Francisco 11 19 209 HM CA Copilco el Alto 31 16 496 HM BT Bosques de Tetlameya 63 10 630  Determine the sample size Jaenson et al. (1992) suggest a sample size of 2000 trees for any city. For this thesis however, it was decided to reduce the sample size to 400 trees, primarily because of time and resource limitations. 3. Estimate the percentage (wi) of the total city street-tree population that is located in each zone segment. Table 2.5 Estimated number of street trees in the twelve neighborhoods.   Figure 2.7. Age group distribution of the 186 respondents.Table 2.5 Estimate number of street trees on the twelve neighborhoods.   Figure 3.2. Tendency of carbon storage according to DBH dimensions. The power trendline is the red color, while the blue is a linear trendline.Table 2.5 Estimate number of street trees on the twelve neighborhoods.   Figure 2.7. Age group distribution of the 186 respondents.Table 2.5 Estimate number of street trees on the twelve neighborhoods.   Table 2.5 Estimated number of street trees in the twelve neighborhoods.   Figure 2.7. Age group distribution of the 186 respondents.Table 2.5 Estimate number of street trees on the twelve neighborhoods.   Figure 3.2. Tendency of carbon storage according to DBH dimensions. The power trendline is the red color, while the blue is a linear trendline.Table 2.5 Estimate number of street trees on the twelve neighborhoods.     44  Equation 2.4: 𝑤𝑖 =(𝐸𝑠𝑡𝑑. # 𝑡𝑟𝑒𝑒𝑠)𝑖(𝐸𝑠𝑡𝑑. 𝑡𝑜𝑡𝑎𝑙 # 𝑡𝑟𝑒𝑒𝑠 𝑖𝑛 𝑐𝑖𝑡𝑦) 4. Determine how many street units should be sample within each zone segment to reach a sample size of 400 trees.  Equation 2.5: 400 𝑥 𝑤𝑖𝐸𝑠𝑡𝑑. 𝑎𝑣𝑔. # 𝑡𝑟𝑒𝑒 𝑝𝑒𝑟 𝑠𝑡𝑟𝑒𝑒𝑡 𝑢𝑛𝑖𝑡  Estimation of the total street trees population in Coyoacan To estimate the total population of street trees in Coyoacan all the blocks and street segments of the rest 83 neighbourhoods were counted using Google Earth for street segments and Google maps images for blocks. An overall tree density of 46 street trees per street unit was obtained with equation 2.6 and assigned to each of the 83 neighbourhoods. Then, the tree density was multiplied by the total number of street units in each neighbourhood to estimate the total population.    Equation 2.6:   𝐸𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑡𝑜𝑡𝑎𝑙 𝑠𝑡𝑟𝑒𝑒𝑡 𝑡𝑟𝑒𝑒 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 12 𝑧𝑜𝑛𝑒 𝑠𝑒𝑔𝑚𝑒𝑛𝑡𝑠𝑇𝑜𝑡𝑎𝑙 # 𝑜𝑓 𝑠𝑡𝑟𝑒𝑒𝑡 𝑢𝑛𝑖𝑡𝑠 𝑎𝑙𝑙 𝑧𝑜𝑛𝑒 𝑠𝑒𝑔𝑚𝑒𝑛𝑡𝑠  2.2.2.4 Tree data collection Applying the same random selection process as the pre-sample, new street units were selected for data collection. The materials used were a Nikon Laser clinometer, 50-meter tape measure, i-Tree Eco complete inventory data sheets (Figure 2.6), 60-cm tape   45  measure, wooden stick, camera and plastic (Ziploc®) bags. In every street unit, trees were sampled in a clockwise manner beginning with the northern point. Tree sampling and assessment was carried out from July 2016 to the first week of August 2016. A group of three people collected the tree data following the methods of the i-Tree Eco Field Manual. If a tree species could not be identified, a twig and leaves were collected for later identification. For this study, a column for soil compaction was added to the data sheets to record the level of compaction of the street trees. The size of tree pit was also measured to evaluate how much free space each street tree has to grow. Finally, the only pest registered was the presence of mistletoe (Cladocolea loniceroides and Struthanthus quericola) a common plague in Mexico City. Trees with initial-to-severe infestation or that showed clear signs of being killed by mistletoe where registered   46   Figure 2.6. i-Tree Eco Complete Inventory Data Sheet. Source: itreetools.org (2016).  The orange columns were manually introduced for the study. CREW/DATA COLLECTOR:PROJECT NAME: LOCATION: DBH:Tree IDSTRATAZONEDATE STAT TREE SPECIES ADDRESSLAND USEPHOTOIDHT DBHDBH1DBH2DBH3DBH4DBH5DBH6CROWN COND TOTAL TREE HEIGHTLIVE CROWN HEIGHTCROWN BASE HEIGHTCROWN WIDTH:  TREE PIT MANAGEMENT FIELDS TREE GPS:N-S E-WCROWN %MISSCLE Width LongMAINT TASKSIDE WALK CONFLUTIL/WIRE CONFLSOIL COMPACTIONMISTLETOE'SPRESENCEPUBLCor PRIVLATCOORDYLONGCOORDXCOMMNENTSSTREET TREE/NON-ST  Y/N47  2.3 Social survey   In order to obtain insight into local people’s perceptions and preferences regarding urban trees, a passer-by survey was carried out in three popular areas of the Coyoacan district: Coyoacan centre, “Las Islas” – an open space at the National Autonomus University of Mexico, and outside Cineteca Nacional. These three areas receive significant numbers of people every day, which increased the chances of finding enough people willing to participate in the study. The surveys were conducted on weekdays in August 2016 at mid-day and during evenings. At the sites of the interview, a banner was placed with the following text: “We want to know your opinion about the trees in your neighbourhood.” The people who approached the interviewers received a consent letter specifying the research purpose, the researcher information and the withdrawal procedure followed by a questionnaire of ten questions (Figure 2.7). As only people who came up to the interviewers were included, there is self-selection bias in the study, and it can be expected that there was an over-representation of people with an interest in trees. Also, although the surveys were undertaken at locations in Coyoacan, the passer-by method did not guarantee that all respondents actually lived in Coyoacan district. It is worth mentioning that during the tree data collection a few neighbors approached asking questions about the purpose of the study, and the opportunity was taken to ask them about the survey, which they agreed to fill in.       48  The questionnaire (Figure 2.7) was based on the study of Camacho-Cervantes et al. (2014), but some of the questions were modified to Coyoacan’s situation. A tenth question was added which was not part of the survey of Camacho-Cervantes et al. (2014).                      Figure 2.7. Questionnaire on how people perceive street trees in Coyoacan. Modified from Camacho-Cervantes et al., 2014.     49  2.1.5 Data analysis 2.1.5.1 Estimated values of tree benefits for the twelve neighbourhoods After data collection and submission of the data to the i-Tree Eco 6 technical support team in the US, the team quantified the benefits of each tree and arranged them according to neighbourhood. Next, they added up the values from all the trees that belonged to each zone segment and provided a total value for each zone segment. These final values were used to obtain an average of benefits per tree to then extrapolate for the estimated population on each zone segment. Unfortunately, using the average benefits per tree resulted in losing variability in the process, since an average value was given to all trees regardless of size.  2.1.5.2 Ecosystem services The relationships between the quantity of the ecosystem services provided by each individual tree (carbon storage, gross carbon sequestration, avoided runoff and pollution removal) and tree sizes (DBH and leaf area) were analyzed using scatter plots and trendlines.  The relation between tree benefits and tree size was compiled into charts that combine stacked columns and line charts to identify the differences between neighbourhoods. 2.1.5.3 Street tree per capita and difference according to street layout Tree density (tree/ha) and people density (persons/ha) were used to obtain the street trees per capita for each zone segment. People density was compared with the surface of each zone segment to identify highly populated neighbourhoods. Number of street trees per   50  capita was compared with population density to illustrate the relationship between street tree numbers and population density.  A non-parametric test of Wilcoxon signed-rank test was performed to find if there was a significant difference in the number of trees on rectilinear and curvilinear streets. This test was chosen because the number of samples were less than 30 and the data did not follow a normal distribution. 2.1.5.4 Survey data  A total of 186 surveys were completed. To analyze the close-ended questions, the Statistical Package for Social Sciences (IBM SPSS Statistics 24) was used to run a descriptive statistical analysis to obtain the frequencies and percentages of the entries in relation to the analyzed sample group: age, sex, plant selection site, management actions, preferred house/vegetation combinations, and most similar number of trees in your neighbourhood.  SPSS allows for creating and editing graphs directly from the results table, and thus the survey data in the results section display the format given by the software.   A nonparametric chi-squared (x2) test was used to determine whether two categorical variables are independent or related (Cunningham & Aldrich, 2012). The tests were made to determine if there was a relationship between the respondents’ age and the number of identified benefits, and respondents’ age and preferred tree planting site. The same tests were made once again but using the categorical variable sex instead of age. Another chi-squared (x2) test was carried out to determein if there was a relationship   51  between the relative number of trees in respondents’ neighbourhoods and the number of identified benefits. Respondent ages showed a wide range, so it was necessary to transform them from scale variables into nominal variables to facilitate their analysis (Table 2.6). The ages were arranged in six classes with a width of 14 and accommodated into a string variable named Age Group (Cunningham & Aldrich, 2012). Table 2.6.  Respondent age nominal classes.   Age Group Age A 8 to 22 B 23 to 36 C 37 to 50 D 51 to 64 E 65 to 78 F 79 to 92  For questions one and three (Figure 2.7), it was easier to run a descriptive statistical test with SPSS rather than a qualitative data analysis, because most of the respondents gave the common name of the trees, while a few others provided enough information to determine the tree species even when no tree name was listed. Also, 20 respondents out of the 186 gave either broad answers like all of them, native trees or fruit trees or did not provide enough information to identify the trees or left the space in blank. For these cases, it was assumed that they did not have favourite or least-favourite trees, and the reply was considered as not answered.  The program NVIVO 11, a computer-assisted qualitative data-analysis software, was used to analyze the remaining four open questions (www.qsrinternational.com). Open questions have the quality to store an unlimited number of answers for a same question, and   52  this software allows the researcher to analyze text using word and text queries and a straightforward coding system, and one can import complete interviews, audios, pdfs, or excel sheets to make the analyses (Bazeley & Jackson, 2013).  The answers from the paper-based surveys were translated from Spanish into English and transcribed in full into an excel sheet, and then exported to NVIVO 11. The software has the flexibility to allow you to analyze the survey questions individually or in groups, but for the present study questions were analyzed individually. In a first iteration, word-frequency queries were run for each question to identify frequently occurring answers. NVIVO allows you to adjust the word query to look for exact matches, stemmed words, synonyms, specializations, or generalizations (Bazeley & Jackson, 2013). Using the coding system, new categories were created, the answers were then coded according to the existing category they fell into. For example, for question two (Figure 2.7) respondents had the liberty to write down their favourite tree traits, and some respondents mention one or two while others gave eight favourite traits. Using the word query, it was possible to identify attributes that can be distributed along ten categories (Table 2.7).         53  Table 2.7. Common tree traits liked by the respondents. The bold categories were found using a word query and categories in italics were found using a text query in NVivo.  To categorize and then code attributes that refer to more than one object, for example, color, size, and texture refer to either, leaves, flowers, trunk, or the entire tree, a text query was used. Text query allows the researcher to search for phrases in her sources. So instead of considering each of the aforementioned attributes as a single category, with the text query we built three combined categories (Table 2.7). The text query was also used to determine which attributes belong to a certain tree species. Attributes that were mentioned by 4 or less respondents were entered in the category ‘others.’  Once all the answers are coded into their corresponding categories, NVIVO provides the weighted percentage (%) of the number of times a certain attribute within a category was mentioned by the respondents. The weighted percentages (%) for each category were organized from highest to lowest and listed in tables.        Aesthetically pleasing Leafiness Fruits  Color and shape of the leaves Aroma/odor Height  Color and shape of the tree Others Size and texture of the trunk  Evergreen Refresh Strength  Flowers Shade  54  Chapter 3. Case study results 3.1 Urban forest structure 3.1.1. Species composition We found 36 species within the twelve neighbourhoods; 38.6% of them are native of Mexico while the 61.4% are exotic or introduced species, mainly from Asia. The two most common, Fraxinus uhdei (Mexican ash) is native of Mexico while Ligustrum lucidium (glossy privet) comes from Asia (Figure 3.1).    Figure 3.1. Distribution of the ten most common species among the neighbourhoods. Each stacked column is the 100% of species located in each zone segment. Thus, each color is the x percentage of a species over the total.           0102030405060708090100 BT  CA  CN  CSF  DC  IC  LC  OU  PC  PSA  PT  SDCPercentageZone segmentOthersLiquidambar styracifluaCupressus lusitanicaErythrinaThujaFicus retusaJacaranda mimosifoliaCupressus sempervirensFicus benjaminaLigustrum lucidumFraxinus uhdei  55  Table 3.1. Percentage distribution of the ten most dominant tree species in Coyoacan district.  Tree species Percentage (%) Fraxinus uhdei 27.4 Ligustrum lucidum 17.5 Ficus benjamina 8.5 Cupressus sempervirens 7.4 Jacaranda mimosifolia 5.2 Ficus retusa  5.2 Thuja 4.0 Erythrina 3.8 Cupressus lusitanica 2.4 Liquidambar styraciflua 2.2 Others ≤1.6% 16.3   3.2 Estimated street-tree population Coyocan district presents an estimated population of 125,866 street trees with a mean of 1325 (±1553 standard deviation SD) per zone segment (neighbourhood). This suggests that there is a great variation in tree distribution among the district’s 97 neighbourhoods (Appendix 1). The analysis of the 12 zone segments, hereafter named neighbourhoods, works as a case study to provide important information about the role of street trees in the provision of ecosystem services.    Following the method designed by Jaenson et al. (1992) , in the 12 neighbourhoods (Figure 2.3) the estimated street-tree population is 13,397 with a mean of 1,195 (± 1,466 SD) in each neighbourhood. The variation among segments is important because 9 out of 12 zone segments had fewer than 1,000 street trees, while only three segments surpassed it, especially Del Carmen which had 5,660 street trees (Table 3.2).    56  Table 3.2. Estimated number of street trees per zone segment.           3.3 Ecosystem services provided by street trees Table 3.3 provides a summary of the estimated total benefit values per neighbourhood. It was expected that benefits would increase directly with the number of street trees, thus neighbourhoods with more trees will have higher benefit values. High marginalization (HM) and medium marginalization (MM) neighbourhoods show this trend; however, low marginalization (LM) neighbourhoods with <800 street trees do not. The assumption of “more trees = more benefits” is not always met when looking between neighbourhoods without considering the marginalization index, instead we encountered contradictory situations, such as: Zone type (strata) Zone segment  Estimated number of street trees (Ni) LM Pedregal de Carrasco 504 LM Del Carmen 5660 LM Parque San Andres 1871 LM San Diego Churubusco 960 LM Insurgentes Cuicuilco 869 LM Oxtopulco Universidad 306 LM Los Cedros 728 LM Petrolera Taxqueña 418 MM Canal Nacional 746 MM Cuadrante San Francisco 209 HM Copilco el Alto 496 HM Bosques de Tetlameya 630 Total  13,397    57  Less trees, more benefits:  Petrolera Taxquena (PT), a neighbourhood with low marginalization index, with 78 less trees than Copilco el Alto (CA), a neighbourhood with high marginalization index, provides two to four times more in all benefits.  More trees, less benefits: Pedregal de Carrasco (PC), a neighbourhood with low marginalization index, with 295 more trees than Cuadrante de San Francisco (CSF), a neighbourhood with medium marginalization index, shows lower values of carbon storage and sequestration, but higher avoided runoff, than the neighbourhood with the lowest street-tree population.   Table 3.3. Benefits provided by trees. The values in this table were estimated for the total street-tree population of twelve neighbourhoods in Coyoacan district.  (Strata) Neighbourhood Estimated total number of street trees  Estimated Carbon Storage (Tonne) Estimated Gross Carbon Sequestration (Tonne/Yr) Estimated Avoided Runoff (m3/Yr) Estimated Pollution Removal (Tonne/Yr) (HM) CA 496 41.26 1.80 129.64 0.00 (HM) BT 630 89.54 3.61 242.09 0.10 (MM) CSF 209 24.23 1.33 60.71 0.00 (MM) CN 746 137.90 6.10 326.43 0.23 (LM) PT 418 79.61 2.88 253.24 0.14 (LM) OU 306 45.29 1.53 181.76 0.10 (LM) LC 728 45.60 2.30 47.51 0.00 (LM) PC 504 12.60 1.26 125.37 0.00 (LM) IC 869 65.47 4.62 390.24 0.18 (LM) SDC 960 275.68 7.61 599.55 0.36 (LM) PSA 1871 1104.63 25.98 1935.45 1.04 (LM) DC 5660 2031.13 56.60 5910.78 2.89 Total 13397 3952.94 115.63 10202.75 5.05   58  3.3.1. Carbon storage and sequestration  The differences in carbon storage and carbon sequestration are attributed to the sizes of the trees in each neighbourhood. Figure 3.2 and 3.3 show that those neighbourhoods with a greater percentage of medium (DBH > 30 to 60 cm) and large (> 60 cm) street trees have higher values of carbon storage and carbon sequestration than neighbourhoods comprised mostly of small trees with DBH < 30 cm.  Consider the example formerly used: despite being the neighbourhood with fewest trees, Cuadrante San Francisco (CSF) has 27% medium-sized trees, while Pedregal de Carrasco (PC) has a 100% small trees, resulting in higher values of carbon storage and sequestration at CSF.      051015202530354045500102030405060708090100DC PSA SDC BT CN PT OU IC CSF CA LC PCTOTAL CARBON STORAGE (TONNE)DBH CLASS (%)ZONE SEGMENT< 30 cm > 30 to 60 cm > 60 cm  Carbon Storage Figure 3.2. Distribution of the total carbon storage among the neighborhoods. The stacked columns represent the percentage of each DBH class in every zone segment (neighborhood).   59              3.3.2 Avoided surface runoff The volume of rainfall that is intercepted by the trees and does not reach the ground is strongly related (R2=0.99) with the leaf area of the tree. Unlike carbon storage, the avoided runoff shows a positive linear trend, which means that rainfall interception constantly increased with the increment of the leaf area. Note: the entire tree can intercept precipitation; however, to calculate annual avoided surface water runoff, i-Tree Eco6 only considers the precipitation intercepted by the leaves. At the neighbourhood level (Figure 3.4) this exact trend is observed. The abrupt drop from Del Carmen (DC) to Parque San Andres (PSA) is a consequence of DC having a leaf area 2.23 ha while PSA has only 0.58 ha. However, after that the decrease in the volume of avoided runoff shows a constant trend.    00.20.40.60.811.21.40102030405060708090100DC PSA SDC BT CN IC PT OU CSF CA LC PCGROSS C SEQ (TONNE/YR)DBH CLASS (%)ZONE SEGMENT< 30 cm > 30 to 60 cm > 60 cm  Gross Carbon SequestrationFigure 3.3. Distribution of the gross carbon sequestration among the neighborhoods. The stacked columns represent the percentage of each DBH class in every zone segment (neighborhood).    Figure 3.4. Distribution of the total avoided runoff according to the total leaf area of each zone segment.   Figure 3.8. Strong linear tendency between the amount of pollutants removed and leaf surface area.   Figure 3.10. Change in the amount of pollutants removal according to the total leaf surface area of each zone segme t (neighbo od).Figure 3.8. St ong linear tendency b tween the amount of pollutants removed and leaf surface area.Figure 3.7. Distribution of the total avoided runoff according to the total leaf area of each zone segment. Figure 3.3. Distribution of the gross carbon sequestration among the neighborhoods. The stacked columns represent the percentage of each DBH class in ery zon  segment (neighborhood).     60    3.3.3 Air pollution removal Another benefit that shows a strong positive relation with leaf area is pollution removal – the higher the leaf area the more pollutants are removed. The dry deposition of particles increases with the increment of the leaf area and with the size of the tree. Neighbourhoods with a leaf area lower than 0.1 ha remove less than 0.01 metric tons/yr (Figure 3.5).  Neighbourhoods with higher percentage of small trees (<30 cm DBH) have smaller leaf areas, which reduces the amount of pollutants that can be removed (Figure 3.6)     Figure 3.4. Distribution of the total avoided runoff according to the total leaf area of each zone segment.  02040608010012014016000.511.522.5DC PSA SDC BT IC OU PT CN CSF CA PC LCAvoided runoff (m3)Leaf Area (ha)Zone segmentLeaf Area Avoided Runoff  61    .              0.000.010.020.030.040.050.060.070.0800.511.522.5DC PSA SDC BT IC OU PT CN CSF CA PC LC Total pollution removal (tonne/yr) Leaf area (ha)Zone segmentLeaf Area (ha) Total Pollution RemovalFigure 3.5. The amount of pollutants removal according to the total leaf surface area of each zone segment (neighbourhood). 00.511.522.50102030405060708090100DC PSA SDC BT IC OU PT CN CSF CA PC LCLEAF AREA (HA)DBH CLASS(%)ZONE SEGMENT< 30 cm > 30 to 60 cm > 60 cm Leaf AreaFigure 3.6. Total leaf area distribution among zone segments. The stacked columns represent the percentage of each DBH class in each zone segment.    .    Figure 3.7. Annual pollution removal (kilograms). Modified from the i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017.Figure 3.6. Total leaf area distribution among zone segments. The stacked columns represent the percentage of each DBH class in each zone segment.    .     62  Total pollution removal was greatest for ozone, followed by nitrogen dioxide, sulphur dioxide, particulate matter less than 2.5 microns, and carbon monoxide (Figure 3.7). The months with the highest O3 removal were May and July. Estimates of removal fluctuated over the year, for all the pollutants except carbon monoxide for which fluctuations remained between 0.3 and 0.6 kilograms per year.    3.4 Street tree per capita Half of the sampled neighbourhoods were < 35 ha in size and most of these had high population densities (Figure 3.8). The number of street trees per capita were lower in the densely populated neighbourhoods (Figure 3.9).   -20246810121 2 3 4 5 6 7 8 9 1 0 1 1 1 2POLLUTION REMOVAL (KILOGRAMS)MONTHCONO2O3PM2.5SO2Figure 3.7. Annual pollution removal (kilograms). Modified from the i-Tree Ecosystem Analysis Assessment of street trees in Coyoacan, 2017.   63      0.00.10.20.30.40.50.60.70 50 100 150 200 250Street trees per capitaPopulation density (persons/ha)Figure 3.9. Number of street trees per capita as a function of population density. Zone segments with low population density have higher number of street trees per capita than densely populated zone segments.        Figure 3.10. Tree density according to street layout.     Figure 3.16. Age group distribution of the 186 respondents.  Figure 3.15. Difference in tree density according to street layout.   0501001502002500255075100125150175200DC PC IC PSA SDC OU PT LC CSF CN CA BTPopulation densitySurface area (ha) Zone segmentArea (ha) Persons/haFigure 3.8. Comparison between the surface area (ha) and the population density among zone segments. Zone segments with small surface areas are occupied with higher number of people.      64  3.5 Street layout The street layout conformations had significantly different (n=12, p value= 0.048) numbers of trees present (Figure 3.10). Rectilinear streets had 32.0 (± 5.23SE) trees per street unit, while curvilinear streets had 15.6 (±3.89 SE) trees per street unit.            3.6 Survey results Of the 186 respondents, 53% were 36 years old or younger (Figure 3.11). There was also a predominance of male respondents over female respondents in all age groups (Figure 3.12) respondents.  Figure 3.10. Tree density according to street layout.     Figure 3.16. Age group distribution of the 186 respondents.  Figure 3.15. Difference in tree density according to street layout.     Figure 3.11. Age group distribution of the 186 respondents.    Figure 3.17. Gender distribution among age groups.  Figure 3.16. Age group distribution of the 186 respondents.   65       Figure 3.11. Age group distribution of the 186 respondents.    Figure 3.17. Gender distribution among age groups.  Figure 3.16. Age group distribution of the 186 respondents.    Figure 3.12. Gender distribution among age groups.    Figure 3.18. Percentage of respondents with residency in Coyoacan  Figure 3.17. Gender distribution among age groups.  Figure 3.11. Age group distribution of the 186 respondents.   Figure 3.12. Gender distribution among age groups.    Figure 3.18. Percentage of respondents with residency in Coyoacan  Figure 3.17. Gender distribution among age groups.  25% 25%18% 18%7%1%010203040506070808 to 22 23 to 36 37 to 50 51 to 64 65 to 78 79 to 92FrequencyAge Group9%7%9%6%2%0%16%18%10%12%4%1%01020304050608 to 22 23 to 36 37 to 50 51 to 64 65 to 78 79 to 92FrequencyAge GroupFemaleMale  66   42% of the respondents reside in Coyoacan, while 58% did not (Figure 3.13).            For the question on favourite tree species and favourite tree traits, respondents were at liberty to mention as many species and favourite traits as they liked. Since each respondent gave more than one answer and all were considered during the analysis, the sum of all replies includes over 186 for favourite trees as well as for favourite traits. Table 3.4 shows the ranking of favourite species from most frequently mentioned to least frequent. The most appreciated trees were Jacaranda mimosifolia and the genus Pinus sp. The category ‘other species’ includes 19 additional tree species, mentioned by less than 2.5% of the respondents.  Table 3.4. Tree species mentioned by respondents as the "favourite" trees.      Tree species Frequency  Jacaranda mimosifolia 55 (29.6%) Pinus sp. 28 (15.1%) Fraxinus uhdei 11 (5.9%) Eucalyptus sp.   7 (3.8%) Figure 3.13. Percentage of respondents with residency in Coyoacan    Figure 3.18. Percentage of respondents with residency in Coyoacan    Figure 3.13. Percentage of respondents with residency in Coyoacan    Figure 3.18. Percentage of respondents with residency in Coyoacan   42%58%020406080100120Yes NoFrequencyDo you live in Coyoacan district?  67     Table 3.5 provides an overview of trees’ most preferred traits like leafiness, flowers, and smell. Apart from the categories mentioned, seven additional traits were mentioned, but by less than 5% of the respondents.  Table 3.5.  Most liked tree traits mentioned by respondents.        When asked about most disliked trees and their respective traits, respondents also had the liberty to mention as many trees and traits as they want. The results in Table 3.5 show that 52.5% of the respondents agreed there were no trees they disliked. The remaining 47.5% of the respondents mentioned varies species, being the most disliked Eucalyptus sp., followed by Schinus molle and Ficus elastica. The category ‘other’ includes 16 tree species. Tree species Frequency  Schinus molle   7 (3.8%) Taxodium mucronatum   7 (3.8%) Others ≤2.5%  45 (24.2%) No answer 20 (10.8%) Traits Frequency Leafiness 69 (37.10%) Flowers 57 (30.65%) Smell 50 (26.88%) Size 48 (25.81%) Beauty 45 (24.19%) Shade 44 (23.66%) Shape 36 (19.35%) Color 27 (14.52%) Fruits 23 (12.37%) Mental health providers  16 (8.60%) Trunk 13 (6.99%) Others ≤2.7% 30 (16.13%) No answer    3 (1.61%)   68  Table 3.6. Tree species respondents mentioned as most disliked trees.     The tree traits that people disliked the most (Table 3.7) were litter generation (12.9%), associated mainly with Jacaranda mimosifolia and Ficus benjamina, and feeble appearance (11.3%) associated almost exclusively with Eucalyptus sp. Tree species like Ficus elastica, Jacaranda mimosifolia, Schinus molle and Ficus benjamina were associated with infrastructure damage which includes roots damaging sidewalks, drainage or house foundations, and leaves clogging the drainage. However, most respondents (33.33%) left the space in blank; for these cases, it was assumed that respondents did not have any disliked trait. The category ‘others’ included an additional 11 traits. Table 3.7.  Most disliked tree traits mentioned by the respondents.                             Respondents were also asked for a list of the benefits provide by trees. The most frequently mentioned benefit (Table 3.8) was oxygen production, followed by aesthetic improvement Tree species Respondents (n=186) None 106 (52.5%) Eucalyptus sp.   23 (11.4%) Schinus molle     9 (4.5%) Ficus elastica     8 (4%) Jacaranda mimosifolia     8 (4%) Ficus benjamina     6 (3%) Ligustrum lucidum     6 (3%) Others ≤2.5%   36 (17.6%) Traits       Frequency  None 62 (33.33%) Litter generation 24 (12.9%) Feeble appearance 21 (11.29%) Infrastructure damage 20 (10.75%) Others ≤2.5% 71 (38.17%)   69  of the city, pollutant removal, and shade. It seems that respondents are aware of the benefits provided by trees, since twelve benefits were mentioned repeatedly plus the 11 benefits less frequently mentioned, included under the category ‘others’  Table 3.8. Tree benefits mentioned by respondents.        As for disliked traits (Table 3.7), infrastructure damage (23%) and accidents (19%) caused when trees fall, especially during storms and strong winds, were considered the most common tree disservices (Table 3.9). However, more than half of the respondents considered that trees do not cause any harm, while another 19% stated that trees themselves do not cause any harm, but any disservices are caused by poor management practices. Although litter generation was the most common disliked trait (Table 3.7), it was one of the least mentioned disservices (Table 3.9).   Benefits Respondents (n=186) Oxygen supply 108 (58.06%) Aesthetic improvement 79 (42.47%) Pollution removal 65 (34.95%) Shade 63 (33.87%) Habitat for animals 34 (18.28%) Regulation of temperature 27 (14.52%) Health and recreational activities 14 (7.53%) Resources 13 (6.99%) Water retention 12 (6.45%) Carbon sequestration 10 (5.38%) Avoiding soil erosion 10 (5.38%) Others ≤2.5% 55 (29.57%)   70   Table 3.9. Tree disservices mentioned by respondents.      For the question about preferred tree location, placement both near respondents’ houses and in green areas were chosen by 70% of the respondents, based on the argument that having trees in both places will provide visual beauty (31%) and will increase the canopy cover of the city (29%) (Table 3.10). Another 26% of the respondents preferred to see trees planted in green areas because they will receive more attention and it is a more suitable places for trees to grow.  In either of the three cases, respondents agreed that wherever the trees are planted they provide visual beauty. Table 3.10. Preferred tree location in the city and reasons.  On the left side are the tree locations chosen by the respondents and on the right side the reasons for the specific tree placement listed by percentage of the respondents.  Disservices Respondents (n=186) Trees don't cause harm 95 (51.08%) Infrastructure damage 43 (23.12%) Accidents 36 (19.35%) Disservices are caused by poor management practices 35 (18.82%) Electric service damage 10 (5.38%) Litter generation 7 (3.76%) Allergies 2 (1.08%) Tree location - respondents (n=186) Reasons (respondents, percentage of respondents that chose the given location) Near my house - 4 (2.2%) Improve air quality 2 (50%)  Provide visual beauty 2 (50%)  Provide shade 1 (25%)  No reason 1 (25%) Green areas - 48 (25.8%) There they receive better attention 12 (25%)  Have more space to grow 9 (18.75%)   71      From a list of actions that are using or should be using public resources, the respondents considered street garbage recollection (20%), tree planting (18%) and improvement of the public transportation (16%) the three most important (Table 3.11). It is worth mentioning that 15 respondents chose all seven actions as they consider them equally important.  Table 3.11. Most important management actions mentioned by the respondents. Urgent management actions Respondents (n=186) Street garbage recollection 118 (20.5%) Fixing broken roads and sidewalks 72 (12.5%) Reduce vehicular traffic 50 (8.7%) Detection and elimination of water leaks 87 (15.1%) Street lighting maintenance  53 (9.2%) Planting trees in sidewalks and median stripes 102 (17.7%) Public transportation improvement 91 (15.8%) No answer 2 (0.3%)  Tree location - respondents (n=186) Reasons (respondents, percentage of respondents that chose the given location)  To increase greenery 8 (16.67%)  Use for recreational activities 8 (16.67%)  Provide visual beauty 6 (12.5%)  Provide oxygen 4 (8.3%)  Others 15 (31.25%) Near my house and in green areas - 128 (68.8%) Provide visual beauty 40 (31.25%)  To increase canopy cover 37 (29%)  Provide oxygen 18 (14.06%)  Obtain their benefits 13 (10.16%)  Provide shade 13 (10.16%)  Other 83 (64.84%)  No reason 1 (0.78%) No answer - 6 (3.2%)    72  For dwelling preference, respondents based their answer on tree location (Figure 3.14a). The great majority (70%) of the respondents selected option D (Figure 3.14b), a house with trees in front (sidewalk) and on the back (garden). Only 2% chose option A, a house with no trees around. This suggest that people would prefer to live in places with trees either as garden or street trees, or preferably both.                 A) B) Figure 3.14.  Dwelling preference according to tree location. A) Four options for tree location around houses. B) Respondents’ house preference percentage.   2%20%9%70%020406080100120140A B C DFrequencyHouse preference  73  Finally, respondents chose from another set of drawings the one that resembled the most the current distribution of street trees on their neighbourhoods (Figure 3.15).  More than half (52%) of the respondents chose option B, neighbourhoods with a fair number of street trees present, followed by option C (32%), neighbourhoods with few trees (Figure 3.16).                B) Figure 3.15. Relative number of trees in your neighbourhood. A) neighbourhoods with plenty of street trees, b) neighbourhoods with a fair number of street trees or c) neighbourhoods with very few street trees.  Figure 3.16. Relative number of street trees in people neighborhoods. Subjective opinion of the number of trees in respondents’ neighborhoods. Figure 13.2. Tendency of carbon storage according to DBH dimensions. The power trendline is the red color, 16%52%32%020406080100120A B CFrequencyRelative number of trees in your neighborhood  74  The results of the chi-squared (x2) where not significant (Table 3.12) for any of the grouped variables. This suggests that the number of benefits associated with trees are independent of age, gender and relative number of street trees in the neighbourhoods. Similarly, preference for where trees should be planted – near the houses, in green areas or near houses and green areas – was also independent of age, gender, and the number of street trees in the neighbourhoods. Table 3.12. Statistical results of the non-parametric test chi-squared. The contingency tables were created using a pair of variables (variable 1 x variable 2) as shown on the table.    Non-parametric test chi-squared (x2)   Variable 1 Variable 2 X2 Value df p value Age group  Number of benefits 14.086a 10 0.169 Age group  Preferred planting site 14.150a 10 0.166 Sex  Number of benefits .192a 2 0.909 Sex  Preferred planting site 1.651a 2 0.438 Relative number of trees in your neighbourhood   Number of benefits 5.204a 4 0.267 75  Chapter 4: Discussion 4.1 Urban forest structure 4.1.1 Species composition The ten most common species found in the study, especially Fraxinus uhdei and Ligustrum lucidium, are also the most common species in the streets and green areas of Mexico City (Benavides-Meza & Segura-Bailon, 1996; Chacalo Hilú, Grabinsky, & Aldama, 1996; Falcon Lara, 1994; Mizerit Trivi, 2006; Rojo Negrete, 2006; Waluyo Moreno, 2013).    From an urban forestry perspective, the twelve neighbourhoods have a low tree species diversity, since 84% of the total population is represented by only ten species. To achieve high diversity, it is recommended in the literature that no single species should account for more than 5% and no genus should account for more than 10 % of the total population (based on e.g., Grey and Dencke, 1986; Moll, 1989; Miller and Miller, nd; Bassuk et al., 2009 in Thomsen, Bühler, & Kristoffersen, 2016). On the other hand, the ISA guidelines are more flexible and follow Santamour’s (1990) rule of thumb, aiming for tree densities that do not exceed 30 percent from a single plant family, 20 percent of a single genus and 10 percent of a single species (ISA, 2010; Santamour, 1990). If we consider Santamour’s rule of thumb (10%, 20%, 30%) for tree density, none of the ten most common species reach the 10% (Table 3.1). The closest species to the 10% are Ficus benjamina (8.5%) and Cupressus sempervirens (7.4%). While Fraxinus uhdei (27.4%) and Ligustrum lucidium (17.5%) exceed it. The other six most common tree species account for 2 to 5% of the total population, and finally, the group named as ‘others’ compile tree species that account for ≤1.6% of the total population (Table 3.1).    76  The dominance of the ten species along the streets suggests that they may have been chosen for reforestation programs in past years, while the least dominant species were maybe chosen according to residents’ preferences (G. Mcpherson & Rowntree, 1989; Rojo Negrete, 2006; Rowan A. Rowntree, 1986). Unfortunately, with no public data available about the quantity and the species used during past reforestation programs it is not possible to determine which trees were planted with public resources and which ones where planted by residents.  According to Martinez-Gonzalez (2008) these ten species (Figure 3.1) have characteristics that allow them to thrive in stressful urban environments. For example, Fraxinus uhdei and Ligustrum lucidium are species that tolerate drought and poor soils (Martinez-Gonzalez, 2008), but are susceptible to four semi-parasitic plants Cladocea loniceroides, Struthanthus quericola, Cuscuta corymbosa and Phoradendron velutinum (Sandoval et al., n.d) (Appendix 2). The indiscriminate use of these ten dominant species lowers the diversity of the street-tree community and reduces its resilience to pest and disease outbreaks. For example, Sandoval et al. (n.d) reported that 83% of the trees in the city are infected (mildly to severely) by semi-parasitic plants. During fieldwork we encountered four neighbourhoods where street trees had signs of mistletoe (Appendix 2). The most severe case was San Diego Churubusco (SDC), where 26% of the sample trees were infected or already killed by the mistletoe (Appendix 2).   77  4.2 Ecosystem services 4.2.1 Carbon storage and sequestration Chacalo et al. (1996) reported a predominance of young street trees (0-20 cm) in Mexico City, and attributed this to government efforts to reforest the city, and to low survivability of the trees. In 2016, Coyoacan still showed a predominance of younger trees (<30 cm), which suggests the same low survivability, possibly due to poor site conditions such as inappropriate tree pit sizes, short distance among planted trees, and the pest outbreaks observed during fieldwork (Appendix 3). Having younger trees is better than having no trees at all, but the amount of benefits provided by trees is proportional to their size (Pauleit & Duhme, 2000; Waluyo Moreno, 2013), so large healthy trees provide the most benefits.  McPherson et al. (1994) reported that large trees had carbon storage rates 90 times greater than small trees. For Coyoacan, neighbourhoods with a higher percentage of medium-to-large trees had the highest amount of stored and sequester carbon (Figure 3.2 and 3.3). However, the findings suggest that having more trees not necessarily means more benefits. Indeed, more trees will increase the overall carbon storage and gross carbon sequestration, but if there is no space available to plant more street trees in small or dense populated neighbourhoods (Figure 3.8 and 3.9), then the increase in the carbon storage and gross carbon sequestration would depend entirely on the survival and healthy growth of the young trees in the neighbourhoods (Figure 3.2 and 3.3).      78  4.2.2 Avoided surface runoff The results of this study support the hypothesis that rainfall interception increases with the leaf surface area (Thomas, 2016). However, other factors may also have a strong influence on rainfall interception capacity, such as tree size (G. Mcpherson & Rowntree, 1989; Xiao & McPherson, 2002), tree species, canopy gap fraction and meteorological factors (Xiao & McPherson, 2002).   Earlier studies have reported higher rainfall interception by broadleaf evergreen species and conifers than broadleaf deciduous species (Thomas, 2016; Xiao & McPherson, 2002; Xiao, McPherson, Simpson, & Ustin, 1998). The ten most common species in Mexico City include three broadleaf evergreens (Ligustrum lucidium, Ficus benjamina and Ficus retusa), three conifers (Cupressus sempervirens, Cupressus lusitanica and Thuja orienalis) and four broadleaves deciduous species (Fraxinus uhdei, Jacaranda mimosifolia, Erythrina collaroides and Liquidambar styraciflua) (Martínez González, 2008). This composition will most likely play a role in rainfall interception. However, studying these possible differences was outside of the scope of this thesis. Future studies should explore the influences of tree species on rainfall interception in Mexico City.  Xiao (2002) reported that intensive pruning practices can reduce the amount of rainfall intercepted by opening significant gaps in the canopy and reducing leaf surface area. In Coyoacan and in general in Mexico City, the most common tree pruning practices consist of utility pruning to free utility lines and topping (Benavides-Meza & Segura-Bailon, 1996; Falcon Lara, 1994; Mizerit Trivi, 2006). ISA (2010) considers topping as an inappropriate pruning technique to reduce the size of the tree, since it can lead to branch dieback and decay. Poor pruning practices can also negatively impact the trees’ capacity for storm runoff reduction. More studies like the one by Xiao (2002) are needed to further assess this.   79  4.2.3 Air pollution removal Given that Mexico City has a very poor air quality (Garza, 1996), pollution removal is one of the most important tree benefits to consider.  In this case study the 13,397 street trees in the twelve neighbourhoods remove an estimated of 5.05 tonnes per year (5050 kg/yr) (Table 3.2). This quantity is very low compared to the number of trees. For example, in Gainesville, Florida, it was estimated that 2000 trees removed 390 tonnes per year of air pollutants (Escobedo, Seitz, & Zipperer, 2009), i.e. 77 times more than the trees in our case study.  The strong positive linear relation between leaf area and the air pollutants removal supports Maco and McPherson (2002) findings about the positive relation between the canopy area and the amount of tree benefits received, as well as Nowak’s (2002) report on large healthy trees with diameter greater than 70 cm removing 70 times more pollutants than small healthy trees.  The small amount of air pollutants removed seems to be related the predominance of small trees with small canopies within the neighbourhoods (Figure 3.5). 4.3 People’s tree perceptions and preferences Street trees are subject entirely to human intervention (Rowntree, 1984), and the selection of the species to be planted can follow a governmental purpose, such as using Eucalyptus spp. to purify the air (Martinez-Gonzalez, 2008) or it can be based on residents’ preferences (Rowntree, 1986). In any case, the perceptions and views that citizens have towards trees strongly influence their planting or removal (Bonnes, Passafaro, & Carrus, 2011; Kirkpatrick, Davison, & Daniels, 2012; Zagorski, Kirkpatrick, & Stratford, 2004).  In Coyoacan, all secondary roads, and usually residential streets, are under the Parks Department jurisdiction, which unfortunately does not have a management program for   80  street trees. Instead tree pruning and tree removal relies on residents’ requests, and there is no option for tree restitution or tree reforestation (Delegacion Coyoacan, 2016). In this scenario, knowing the perception of Coyoacan residents towards street trees provides important insights into the current state and possible future of the street trees in the district.  Respondents showed a high appreciation for the trees rather than a dislike for them (Table 3.5). Other studies report that people tend to associate more benefits than disservices to trees  (Camacho-Cervantes, Schondube, Castillo, & MacGregor-Fors, 2014; Kirkpatrick et al., 2012; Lohr, Pearson-Mims, Tarnai, & Dillman, 2004; Schroeder & Coles, 2006).  As in Camacho-Cervantes et al. (2014), my results show that the most disliked tree traits were litter generation, weak appearance, and root damage to infrastructure (Table 3.6). According to Head & Muir (2005); Kirkpatrick et al. (2012) and Zagorski et al. (2004), disliked traits can motivate residents to remove trees because they are perceived as disservices, especially infrastructure damage (Table 3.8). However, 19% of the respondents attributed tree disservices to poor management practices and not to the trees themselves. Also, not all disliked traits were considered disservices; for example, litter generation was the most disliked trait (Table 3.6), but this was not often considered a disservice (Table 3.8). This could be explained, perhaps, by leaf drop being considered more of a nuisance rather than something causing actual harm to the people or the city. According to Bonnes et al. (2011) and Kirkpatrick et al. (2012), the way people value the benefits and disservices varies among different cities and even among locations within a city due to differences in cultural attitudes and exposure to trees. For example, in Mexico City infrastructure damage is the most common disservice and allergies the least common (Table 3.8), while in large metropolitan areas in the US, allergies and blocking signs in   81  business districts rank higher than the sidewalk damage cause by roots (Lohr, Pearson-Mims, Tarnai, & Dillman, 2004).   The valuation of tree benefits can be influenced by the climate regions in which the cities are located (Schroeder & Coles, 2006). Schroeder and Coles (2006) found that shade was more important in warmer and sunnier locations in the US than in cooler and less sunny locations in the UK. A similar situation might be occurring between Morelia and Mexico City. For Morelia, a warmer city than Mexico City, shade and temperature regulation were highly valued, while in Mexico City shade was less valued but still among the four benefits most valued. The poor air quality in Mexico City makes pollution removal slightly more important (Table 3.7). However, the data for this study cannot determine if these differences really operate in this case, and further research is needed to investigate possible roles of culture or climate on the valuation of tree benefits and disservices.  Although cultural attitudes and exposure to trees may influence the valuation of benefits among locations (Bonnes et al., 2011; Kirkpatrick et al., 2012), the aesthetic improvement that trees provide appears to be constant and highly valued among different cities around the world: Mexico (Camacho-Cervantes et al., 2014), Australia (Kirkpatrick et al., 2012), China (Jim & Chen, 2006), USA (Lohr et al., 2004) and UK (Schoerder & Coles, 2006). What is more, the visual beauty provided by trees impacts respondents’ preferences for planting of trees near their houses and in green areas (Table 3.9) or to live in houses with trees nearby, preferably those with trees on both the streets and on the backyards (Figure 3.14a) (Camacho Cervantes, 2014; Schroeder et al., 2006). Similarly, according to my findings, people would prefer to see trees planted both near their houses and green areas   82  (Table 3.9) because this would provide the place with visual beauty while also increasing the canopy cover. It is then not surprising that respondents in our study considered that a portion of public resources should be designated to planting tree on sidewalks and median strips, along with garbage recollection and elimination of water leaks (Table 3.10). Finally, the results suggest that two-thirds of the respondents live in places with a high or fair number of trees, while the remaining third lives in places with very few trees (Figure 3.16). Although these results are mostly subjective and not all respondents live in Coyoacan, they do indicate the uneven distribution of street trees among the neighbourhoods in Mexico City. What is more important, it seems that despite the uneven exposure to trees, most respondents have a high appreciation for street trees and are familiar with the benefits they provide. Lohr et al. (2004) reached a similar conclusion –regardless of demographic differences people consider trees very important for the quality of life. 4.4 Unexpected findings The main purpose of this project was to quantify the benefits that street trees in Mexico City provide, in order to raise awareness of the importance of protecting what I see as the most vulnerable component of the city’s urban forest. However, in the process, the notable differences in benefit provision due to an uneven distribution of street trees between the different neighbourhoods raises the topic of Coyoacan being subject to environmental inequity or injustice (Landry & Chakraborty, 2009). This concept refers to the unequal distribution of ecosystem benefits or services resulting from the patchwork design that   83  characterize urban areas (Boone, Buckley, Grove, & Sister, 2009; Landry & Chakraborty, 2009; Lindsey, Maraj, & Kuan, 2010).  Coyoacan’s urban development shifted from an organized one during the 1940s to an anarchic and unplanned development during the period from 1960 to 1980 as consequence of the high immigration the district suffered during that time (Programa Delegacional de Desarrollo Urbano de Coyoacan, 2010). Therefore, the sizes of the twelve neighbourhoods (Figure 3.8) reflect the spatial unevenness resulting from the different development periods. As a result, differences in the number of street trees per capita (Figure 3.9) reflect the unequal distribution of tree benefits among residents of different neighbourhoods (Landry & Chakraborty, 2009). It should be mentioned that the average street tree per capita in the 12 neighbourhoods is 0.27, while in 22 USA cities the average street tree per capita is 0.37 (McPherson & Rowntree, 1989).  In the case of Coyoacan, the population distribution seems to have more impact in the number of trees (Figure 3.9). However, because I used the marginalization index to stratify my neighbourhoods and this index includes education level, population distribution, housing and household income, it is not possible to determine which of the variables directly impacts the number of trees. For example, Landry and Chakraborty (2009) found a significant lower proportion of street tree cover in neighbourhoods with lower household income in the city of Tampa, Florida, USA. More studies are required that analyze the relationship between each of the variables that constitute the marginalization index and the number of trees on the streets.  There are indications that historical development patterns (Lindsey, Maraj, & Kuan, 2010; Nagendra & Gopal, 2010; Rowan A. Rowntree, 1986), lot densities (Rowntree,   84  1986), population movement (Lindsey et al., 2010) and street layout design (Jaenson et al., 1992; Nagendra & Gopal, 2010; Zhao, Tang, & Chen, 2016) all play a role in Coyoacan’s environmental injustice. For instance, neighbourhoods with curvilinear streets (usually narrower and with no sidewalks) have half the number of street trees found on wider rectilinear streets or grid-like pattern streets (Figure 3.10). These results are in line with those of Nagendra and Gopal (2010) for the streets of Bangalore, India, and with Jaenson et al.’s (1992) assumptions that rectilinear streets contain most of the trees in a city. Tthis study was not designed to test for a difference between wide and narrow roads; but rather, to standardize a sample unit (Jaenson et al., 1992).  However, the significant difference between the two types of street found in Mexico City warrants further investigation, especially for future development plans.    Studies about environmental injustice, equity, and accessibility of green areas (e.g. Boone et al., 2009; Landry & Chakraborty, 2009; Lindsey et al., 2010, and Nagendra & Gopal, 2010) are needed in Mexico City, especially for supporting the design of better urban development and reforestation programs, and to make sure that available resources are targeted to the most needed areas. 4.5 Study limitations Initially this study intended to estimate the benefits of all street trees in Coyoacan district; however, the sample size was modified from 2000 trees (as suggested in the original method) to 497. Because i-Tree Eco takes the measurement of each tree and sums these to calculate the total value, the extrapolation to the entire population of Coyoacan district may be inaccurate.    85  One of limitations of this study is that because it was carried out on a local, district level, findings cannot be generalized to the city as a whole, nor compared with other cities. However, they do provide some indication of the issues facing urban forestry in Mexico City.  Using the marginalization index may not have been the best approach to make the initial stratification. Instead, the initial stratification could have considered the different periods of the urban development of the district, and then stratified into zone segments with similar surface area and/or population density in order to analyze how these variables affect street-tree cover across the district.  Also, because the marginalization index is comprised of four different variables it was not possible to identify which one of these was most closely related to the number of street trees among neighbourhoods. Another limitation was that the passer-by method used for this thesis could not guarantee that only Coyoacan residents participated, so the survey results are not exclusively of Coyoacan residents.  The surveys could have been carried out in the same neighbourhoods where the tree data collection occurred, but this could have impacted the results for how people perceive the trees due to tree exposure. 86  Chapter 5: Conclusions and recommendations The predominance of small trees in the 12 Coyoacan neighbourhoods studied results in the trees providing low but not negligible benefits. The amount of benefits provided by trees have the potential to increase considerably if the trees reach larger sizes. Therefore, it is important to assure the survival and healthy growth of the small trees.  The uneven distribution of the ecosystem services due in part to the low tree cover and to the design of the neighbourhoods suggests that future reforestation programs and urban development plans must be targeted to provide environmental equity among the neighbourhoods.  The high appreciation that local people had for the trees and their knowledge of the wide range of benefits of urban trees shows that there is a culture for tree-caring. People would like to live in a city with more trees, and if possible have them in front of their houses and in nearby green areas.  However, they attribute the responsibility of tree planting and management to the government, and they considered this to be one of three most important actions to which public resources should be allocated. However, the care of the urban forest should not rely only on government efforts, but also be borne by the society as a whole. The culture of tree-caring needs to be nourished and developed.  Maintaining a healthy urban forest requires the joint work of government, decision makers, city planners, engineers, certified arborists, urban foresters, academics, and society as a whole.  There is a long way to go to achieve the goal of making Mexico City a green and sustainable city, but in the field of urban forestry we can start making some changes. I conclude this work with some personal recommendations.    87  As I mentioned on the first part of this thesis, it is important to establish ‘Urban Forest’ as a unifying concept that contributes to a better communication between the organizations in charge of different green areas, so that green-space management follows a single purpose, i.e. maintaining a healthy urban forest.  The city’s environmental fund should include green areas within the boundaries of the urban land together with areas of environmental value and natural protected areas on the conservation land of Mexico City. Funding needs to be provided for more studies in the areas of urban forestry and urban ecology of Mexico City, while sufficient staff and other resources need to be provided to management to carry out a complete inventory of the street trees and urban green areas. The environmental framework of Mexico City must include tree ordinances, tree preservation orders and permits as suggested by the ISA (2010) to legally protect the trees.  A culture of tree caring should be developed through more active programs of citizen participation such as tree planting, workshops, and programs of citizen stewardship of the urban forest.   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The urban forest in Beijing and its role in air pollution reduction. Urban Forestry and Urban Greening, 3(2), 65–78. https://doi.org/10.1016/j.ufug.2004.09.001 Zagorski, T., Kirkpatrick, J. B., & Stratford, E. (2004). Gardens and the bush: gardeners’ attitudes, garden types and Invasives. Australian Geographical Studies, 42(2), 207–220. https://doi.org/10.1111/j.1467-8470.2004.00260.x Zhao, S., Tang, Y., & Chen, A. (2016). Carbon storage and sequestration of urban street trees in Beijing, China. Frontiers in Ecology and Evolution, 4(May), 1–8. https://doi.org/10.3389/fevo.2016.00053    99  Appendix 1: Street tree population of Coyoacan district Table A1-1. Estimated number of trees in each of the 95 neighbourhoods in Coyoacan. The table is arranged, first in neighbourhoods with low marginalization index, follow by medium and high. The twelve neighbourhoods (bold) use in this study kept their own estimated average number of trees per street unit.    Zone type (strata) Street layout Zone segment  Estd. Avg. # trees per street unit Actual # of street units in zone segment Estd # of trees (Ni) LM RS Atlantida 46 11 506 LM RS Avante 46 78 3588 LM RS Barrio de la Concepción 46 17 782 LM RS Campestre Churubusco 46 111 5106 LM RS Campestre Coyoacan 46 18 828 LM RS Ciudad Jardin 46 36 1656 LM RS Churubusco Country Club 46 31 1426 LM RS Emiliano Zapata 46 10 460 LM RS Educación 46 56 2576 LM RS Ex-Ejido de San Francisco Culhuacan 46 61 2806 LM RS Cafetales 46 24 1104 LM RS San Mateo 46 12 552 LM RS Santa Cecilia 46 28 1288 LM RS Culhuacan Presidentes Ejidales 2nda secc. 46 21 966 LM RS El Caracol 46 18 828 LM RS El Parque Coyoacan 46 25 1150 LM RS El Reloj 46 30 1380 LM RS El Rosario 46 9 414 LM RS El Rosedal 46 18 828 LM RS Espartaco 46 41 1886 LM RS Romero de Terreros 46 62 2852 LM RS Hacienda de Coyoacan 46 19 874 LM RS Hermosillo 46 5 230 LM RS Jardines de Coyoacan 46 20 920 LM RS Joyas del Pedregal 46 15 690 LM RS Las Campanas 46 9 414 LM RS Los Cipreses 46 26 1196   100  Zone type (strata) Street layout Zone segment   Estd. Avg. # trees per street unit Actual # of street units in zone segment Estd # of trees (Ni) LM RS Los Olivos  46 10 460 LM RS Paseos de Taxqueña 46 70 3220 LM RS Prado Churubusco 46 54 2484 LM RS Prados de Coyoacan 46 26 1196 LM RS Presidentes Ejidales 1era seccion 46 21 966 LM RS Unidad Olímpica 46 21 966 LM RS Villa Coyoacan  46 21 966 LM RS Villa Quietud 46 32 1472 LM RS Xotepingo 46 13 598 LM RS Barrio de San Lucas 46 23 1058 LM RS Copilco Universidad 46 19 874 LM RS Pedregal de San Francisco 46 25 1150 LM RS Los Sauces 46 14 644 LM RS Del Carmen 53.4 106 5660.4 LM RS San Diego Churubusco  64 15 960 LM RS Parque San Andrés 64.5 29 1870.5 LM RS Petrolera Taxqueña 41.75 10 417.5 LM CS Insurgentes Cuicuilco 45.75 19 869.25 LM CS Barrio Oxtopulco Universidad 34 9 306 LM CS Los Cedros 52 14 728 LM CS Pedregal de Carrasco 21 24 504 LM CS Alianza Popular Revolucionaria 46 20 920 LM CS Barrio Niño Jesus 46 13 598 LM CS Pueblo San Pablo Tepetlapa 46 25 1150 LM CS Barrio de Santa Catarina 46 61 2806 LM CS Culhuacan CTM Secc. 5 46 22 1012 LM CS Culhuacan CTM Secc. 6 46 16 736 LM CS Culhuacan CTM Secc. 3 46 3 138 LM CS Culhuacan CTM Secc. 7 46 19 874 LM CS Culhuacan CTM Secc. 8 46 17 782 LM CS Culhuacan CTM Secc. 9 46 8 368 LM CS Culhuacan CTM Secc. 10 46 12 552 LM CS El Centinela 46 19 874 LM CS El Mirador 46 12 552 LM CS Los Girasoles 46 24 1104 101  Zone type (strata) Street layout Zone segment  Estd. Avg. # trees per street unit Actual # of street units in zone segment Estd # of trees (Ni) LM CS Jardines del Pedregal de San Angel 46 26 1196 LM CS Pueblo de los Reyes 46 37 1702 LM CS U.H. Culhuacan CTM Piloto 46 9 414 LM CS La Otra Banda 46 6 276 LM CS Los Girasoles III (Ex-ejido Santa Ursula Coapa) 46 8 368 LM CS Copilco el Bajo 46 14 644 LM CS Culhuacan CTM Secc.10-A 46 11 506 LM CS Culhuacan CTM Secc. 9-B 46 18 828 LM CS Culhuacan CTM Secc. CROC 46 11 506 LM CS Exejido San Pablo Tepetlapa 46 11 506 LM CS ExHacienda Coapa 46 16 736 MM RS Culhucan CTM Secc. 1 46 1 46 MM RS Culhuacan CTM Secc. 2 46 1 46 MM CS Canal Nacional  35.5 21 745.5 MM CS Cuadrante de San Francisco 11 19 209 MM CS Pueblo La Candelaria  46 13 598 MM CS Nueva Diaz Ordaz 46 12 552 MM CS Pueblo de Santa Ursula Coapa 46 52 2392 HM RS Adolfo Ruiz Cortinez 46 70 3220 HM RS Ajusco 46 87 4002 HM RS Carmen Serdan 46 29 1334 HM RS Pedregal de Santo Domingo 46 230 10580 HM RS Pedregal de Santa Ursula Coapa 46 171 7866 HM RS Emiliano Zapata Fraccion Popular 46 53 2438 HM RS Viejo Ejido de Santa Ursula Coapa 46 47 2162 102  Zone type (strata) Street layout Zone segment  Estd. Avg. # trees per street unit Actual # of street units in zone segment Estd # of trees (Ni) HM CS Bosque de Tetlameya 63 10 630 HM CS Copilco el Alto 31 16 496 HM CS San Fco Culhuacan Barrio Magdalena 46 26 1196 HM CS San Fco Culhuacan Barrio San Francisco 46 7 322 HM CS Cantil del Pedregal  46 10 460 HM CS Huayamilpas 46 16 736 HM CS Sn Fco Culhuacan Barrio Santa Ana 46 14 644 HM CS San Fco Culhuacan Barrio San Juan 46 8 368 Total       2737 125866 103  Appendix 2. Trees that require maintenance tasks Table A.2-1. General maintenance task found during fieldwork. These results come from the 497 street trees sampled for this thesis. The only presence registered was trees infected with a kind of mistletoe (the species was not identified). Maintenance Task Zone segment None Some kind pruning Pest/disease treatment Remove Total (%) BT 37.7 45.9 3.3 13.1 100.0 CA 63.6 36.4 0.0 0.0 100.0 CN 15.2 57.6 0.0 27.3 100.0 CSF 45.5 36.4 0.0 18.2 100.0 DC 50.0 38.4 3.6 8.0 100.0 IC 26.1 65.2 0.0 8.7 100.0 LC 57.9 31.6 0.0 10.5 100.0 OU 50.0 33.3 3.3 13.3 100.0 PC 37.5 62.5 0.0 0.0 100.0 PSA 66.7 27.8 0.0 5.6 100.0 PT 31.0 69.0 0.0 0.0 100.0 SDC 39.6 28.3 26.4 5.7 100.0     0.020.040.060.080.0100.0BT CA CN CSF DC IC LC OU PC PSA PT SDCPERCENTAGE (%)ZONE SEGMENTNone Some kind pruning Pest/disease treatment RemoveFigure A.2-1. Distribution of the maintenance task among the zone segments. The treatment for pest or disease refers to signs of mistletoe in the trees.  104         Figure A.2-2. Trees infected with mistletoe. On the left side is a Ligustrum lucidium (Glossy privet) with mild infection of mistletoe in the crown. The tree present codominant stems, sign of a poor structure and suckers which take away energy from the tree to grow. On the right side is a Fraxinus uhdei (Mexican ash) severely infected by mistletoe, the tree should be removed.    105            Figure A.2-3. Poor pruning practices. Pruning for utility line clearance is one of the most common practices. However, it is poorly performed as the weight of the crown is left unbalance, plus is not aesthetically pleasing. On the left side is a Fraxinus uhdei (Mexican ash) and on the right side is a Jacaranda mimosifolia (Jacaranda tree).   106      Figure A.2-4. Dead-standing trees. On the left side is a dead Erythrina coralloides (Coral tree). On the right side is a Cupressus lusitanica (Cedar of Goa), this tree shows signs of aggressive pruning which left it with very weak structure. it needs to be remove since it represents a risk if it falls. 107  Appendix 3. Site conditions Table A.3-1. Sidewalk conflict. Percentage of the 497 street trees that present or not conflicts with sidewalks. This includes clearly broken sidewalks and initial signs of rupture. Sidewalk conflict Zone segment No damage Broken Total (%) BT 68.9 31.1 100 CA 90.9 9.1 100 CN 93.9 6.1 100 CSF 50.0 50.0 100 DC 76.8 23.2 100 IC 73.9 26.1 100 LC 94.7 5.3 100 OU 73.3 26.7 100 PC 100.0 0.0 100 PSA 77.8 22.2 100 PT 69.0 31.0 100 SDC 86.8 13.2 100   0.020.040.060.080.0100.0BT CA CN CSF DC IC LC OU PC PSA PT SDCPERCENTAGE (%)ZONE SEGMENTNo damage BrokenFigure A.3-1. Distribution of the sidewalk conflict among the neighborhoods.   108    Figure A.3-2. Examples of trees with sidewalk conflicts. Together with sidewalk conflicts, during the fieldwork it was noticed that trees also suffer from vandalism.  Figure A.3-3. Inappropriate tree pit size. In both cases there were no signs of broken sidewalks or planters, however, the space between is inadequate and suggests future growth problems as well as future sidewalk conflicts. 109  Table A.3-2. Trees conflicting with utility lines. Most of the cases of present and no potential conflict were small trees, while cases of present and conflicting were medium and large size trees, that present poor pruning practices. Utility conflict Zone segment No lines Present and conflicting Present and no potential conflict Total (%) BT 14.8 29.5 55.7 100 CA 72.7 9.1 18.2 100 CN 84.8 0.0 15.2 100 CSF 22.7 18.2 59.1 100 DC 0.7 26.8 72.5 100 IC 100.0 0.0 0.0 100 LC 78.9 5.3 15.8 100 OU 63.3 6.7 30.0 100 PC 75.0 0.0 25.0 100 PSA 0.0 19.4 80.6 100 PT 13.8 24.1 62.1 100 SDC 0.0 45.3 54.7 100   0.020.040.060.080.0100.0BT CA CN CSF DC IC LC OU PC PSA PT SDCPERCENTAGE (%)ZONE SEGMENTNo lines Present and conflicting Present and no potential conflictFigure A.3-4. Distribution of the utility conflict among neighborhoods.   Figure A.3-6. Distribution of the soil compaction among the neighborhoods.Figure A.3-4. Distribution of the utility conflict among neighborhoods.   Figure A.3-6. Distribution of the soil compaction among the neighborhoods.    110        Figure A.3-5. Examples of trees with utility line conflict and with no potential conflict. 

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