UBC Undergraduate Research

Urban forestry : trees are not just for aesthetics Chan, Andy 2015-04-13

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UNIVERSITY OF BRITISH COLUMBIA Urban Forestry: Trees Are Not Just For Aesthetics  FRST 497   Andy Chan  4/13/2015      In this paper, the author will go through a few benefits urban forestry can provide in a community. The sections are broken down into environmental, air, health, and community health. Each section will describe different ways trees can benefit. From widely known benefits of trees such as providing shade to really obscure facts such as decreasing crime rates, and improving health recovery times. As well, the paper will mention Hong Kong’s state of urban forestry and suggestions from the author as to how it should advance. Ultimately, the goal of the paper is to aware readers of benefits of urban forestry and stressing its important.    Table of Contents List of Tables and Figures .......................................................................................................................... i Introduction ............................................................................................................................................ 1 Urban Forestry ........................................................................................................................................ 1 Environmental Benefit ......................................................................................................................... 1 Air Quality ........................................................................................................................................... 4 Health Benefit ..................................................................................................................................... 5 Community Health............................................................................................................................... 7 Case Study: Hong Kong ............................................................................................................................ 9 Discussion ......................................................................................................................................... 10 Conclusion ............................................................................................................................................. 12 References ............................................................................................................................................ 13  i | P a g e   List of Tables and Figures Table 1: Air pollution removal and value (Nowak, Crane, Stevens 2006) .................................................. 4 Table 2: Relative benefit of tree cover to UV-B (Grant, Heisler, Gao 2002) ............................................... 7  Figure 1: Temperatures during different time periods (Parker 1983) ........................................................ 3 Figure 2: Change in MED relative to tree cover (Grant, Heisler, Gao 2002) ............................................... 6 1 | P a g e   Introduction  The term urban forestry is different depending on who you ask and where you ask the question. But in this paper I will use the definition from the article “Sustaining America’s Urban Trees and Forests”.   “Management of urban trees and associated resources to sustain urban forest cover, health, and numerous socioeconomic and ecosystem services is known as urban forestry” (Comas, Carr, Alig 2010)   Why is urban forestry important people may ask? Imagine walking though a city where there are no trees but only small vegetation and bushes. Will a city be able to do well without the presence of trees? Since the human race is so innovative with our new inventions, we are able to create inventions that can replace items that we have removed. For example, we can technically create inventions that can replace the functions of trees.   If you were to think that a city without trees is a great idea, you are gravely wrong. Yes it is true that cities do not need trees, but why would we remove such a valuable resource from our cities. There are quite a few benefits trees can provide within its urban environment. With the global shift towards become more eco-friendly and sustainability, we humans are finding more ways to reduce our impact on the environment. Urban forestry is much more than simply taking care of the trees within the city. It is also how to use trees effectively benefiting all parties. When one talks about trees, the first thought that will come to their minds is forests. This is because a forest consists of many trees. But rarely will anyone connect trees and cities together. The main goal of this paper is to aware readers of the importance of urban forestry and how it can improve cities in a green and sustainable way. Urban Forestry Environmental Benefit  Trees in general have many benefits that the average citizen does not know about. Their potential have yet to be maximized due to lack of understand of those benefits that may be provided. But of course, trees are not as common as they should be in an urban environment. In the later sections, there will be some benefits urban forestry can provide in an urban setting. Some of these benefits that trees can provide are quite self-explanatory but I do provide an explanation of how trees achieve these results.     In the summertime when the temperature is high, humans tend to stay under shade. Why do they do this you may ask? This is because the temperature is lower under the shade compared to standing under direct sunlight. As a simple experiment, try feeling the pavement under direct sunlight and one under the shade. The result will be the pavement under shade will be much cooler in comparison. Trees and vegetation can serve the same purpose by providing shade keeping certain areas cooler. A study conducted by Akbari and colleagues measured the temperature difference shade can provide on two difference surfaces. The findings showed a difference of 11-25°C difference between a shaded surface and a surface under sunlight (Akbari, Kurn, Bertz  1997). To maximize the benefits, trees and vegetation may provide it would be wise to locate them in such a way which can maximize its benefits.  With the ever increasing demands of modern society, more and more energy is needed for individual homes. This is because every piece of technology will consume some form of energy which is often electricity. But this issue has been addressed with new innovative machines which have the same output but consume less energy. One of the most energy consuming machines in the household is the heater/air conditioner. These machines are mostly on most of the time to keep the area at a constant temperature. One way to reduce the energy consumption would be to reduce the need for heaters/air conditioners. A test was done by using vegetation cover to provide shade for homes, and the results show a 50% + in reduction of cooling energy needed (Parker 1983). Deciduous trees are actually preferred to provide maximum effective in both winters and summers. During summertime, deciduous trees will have leaves while all the leaves would have fallen by fall. No leaves on the tree will allow direct sunlight to penetrate buildings increasing temperature during the cold weather. This effectively decreases energy consumption by strategically growing vegetation. Below is a graph to illustrate the effectiveness of vegetation shading and energy consumption of utilities.    Figure 1: Temperatures during different time periods (Parker 1983)  Storm water has always been a problem for urban cities. Infrastructure such as buildings and especially paved roads has caused disruption to the flow of water. Under normal conditions, rain water is usually absorbed into the ground or taken up by vegetation. But in the case of urban cities, water is not able to penetrate roads or infrastructure. That is why many large cities spend large sums of money on rainwater mitigation systems such as storm drains. Trees, similar to many living organisms on Earth require certain sources of energy to grow and survive. For trees, this includes water, oxygen, and sunlight. The amount of water a tree is able to hold in its system is quite mind boggling. Although it may depend on the specie and the size, a tree may be able to hold up to 100 gallons + until saturation (Fazio 2010). That way, storm water runoff will be able to enter the soil and ecosystem like it should. The benefit of such is increased soil stability (Fazio 2010). The roots of the trees act as an anchor for soil that is in close proximity. During downpours or rain, less soil is washed away and lost. With many trees within an urban environment, it can reduce the need for rainwater mitigation systems and save money. In 2005, a report was released which analyzed trees within the city of Minneapolis located in the United States. Multiple tests were done and among one of them was the measure of rainwater interception of trees. The results were shown to have trees within the municipality to intercept approximately 447.5 million cubic feet per year. In terms of money, the amount was estimated to be 9.1million dollars in savings (Simpson et al. 2005). The use of trees is a much more “green” way to mitigate rainwater, as trees can serve more than one person unlike infrastructures built only for mitigation.    Air Quality  Out of the many existing plants and vegetation on Earth, trees are among one of the few that are able to create oxygen. Trees are not only limited to creating oxygen but can also remove pollutants from the air. A recent study called “Air pollution removal by urban trees and shrubs in the United States” displays the amount of pollutants removed from the atmosphere. The information is for the United States only as it only accounts for cities in USA .The breakdown of the results is shown below from a graph taken from the article.   Table 1: Air pollution removal and value (Nowak, Crane, Stevens 2006) Pollutant Removal (t) Value ($ × 106) O3 305,100 2,060 PM10 214,900 969 NO2 97,800 660 SO2 70,900 117 CO 22,600 22 Total 711,300 3828 Estimates are given for ozone (O3), particulate matter less than 10 μm (PM10), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO)  The removal amount is an estimate from 55 major cities in United States based off of an average tree canopy cover per city. This information is then used to determine many other variables which ultimately results in a value of pollutants removed for each city. Then, each totally is totaled together to form the table presented. The values used are in dollars per metric ton (t) : NO2=$6752 t−1, PM10=$4508 t−1, SO2=$1653 t−1, and CO=$959 t−1 (Nowak, Crane, Stevens 2006). The result is a total of 711,300 tonnes of pollutants removed from the atmosphere and a savings of $3.8Billion dollars. Although this result is not stunning or spectacular in anyway, the authors do see urban management as a future to meet demands of cleaner air. This is done by increase tree canopy cover in urban cities, which will increase the effects of trees removing pollutants from the air. Fewer pollutants present will ultimately result in a healthier living environment for residents living in the area.     Continuing from the previous discussion on air pollutants, the aim of removing air pollutants from urban forestry management is to increase the health of humans that reside in the vicinity. There are many health benefits that come from properly planned urban forestry projects. As previously mentioned, trees are able to remove pollution in the air. This is great news for people that are struggling with asthma. In the study “Children living in areas with more street trees have lower prevalence of asthma”, an experiment was conducted to see if there was any correlation between early childhood asthma and tree density in a city. The conclusion of the study was trees are associated with a lower prevalence of early childhood asthma (Lovasi et al. 2008). The data showed that for every 343 trees per kilometer squared, there were a reduction of 24% in rates for asthma (Lovasi et al. 2008). From this data, it shows the effectiveness of trees within a city and that cities should allow more space for urban trees. The decrease in rates for asthma can increase the health of its residents. Asthma at an early childhood stage can lead to further health problems in the future. Researchers are not certain that reduced asthmas levels are directly related to the presences of trees, but trees do aid in reduced asthma levels. This promotion of reduced asthma levels with the use of trees as an airborne pollutant removal tool can be a great investment.  Health Benefit  While on the topic of health, protection of the skin is important. During the summer months especially, is the period of time when skin is most vulnerable to the rays of the sun. The sun emits many harmful rays such as UV-A and UV-B which both have the ability to cause damage to skin under long periods of exposure. For a city dweller, summer time usually means wearing much less clothes that in the winter time. This is a problem as we are exposing more skin, and increasing our exposure to harmful rays. UV-B has been known to play a key role in development of skin cancer (Epstein, Wang). This is why sunscreen is always being promoted as a protective measure against the sun. With the proper type of sunscreen such as a broad spectrum sunscreen, it has the ability to protect against both UV-A, and UV-B rays. There are also other methods to protect against the sun, by simply avoiding long hours in the sun. But not to worry too much, the peak hours of UV-B emitting onto Earth is between 10am to 4pm in the afternoon (Epstein, Wang). In an article, a module was created to test how much UV a pedestrian is receiving under trees in an urban setting. The module created answered the question as to at what percentage tree cover is the most beneficial to pedestrians or in simpler terms, most optimal. Of course, the higher percentage of tree cover present, the more protection from sun rays (Grant, Heisler, Gao 2002). But it is not always feasible to achieve the highest percentage cover as there are many   restrictions in an urban setting. Below is a graph showing the change in MED (minimum erythemal dose) in relation to tree cover percentage. MED is the amount of UV radiation that will produce minimal erythema (sunburn or redness caused by engorgement of capillaries) of an individual's skin within a few hours following exposure (Heckman et al 2013). The lower in number, is the least damage to the skin.   Figure 2: Change in MED relative to tree cover (Grant, Heisler, Gao 2002)  As we can see from the graph, the change from 0 to 0.1 tree cover is more substantial than the change from 0.9 to 1. The change beginning from 0.5 begins to lose its effectiveness as it diminishes in value every interval until it hits 1 as tree cover. Since the Earth is not flat, the results had to be separated within its own latitude. This is because the amount of sun rays of different latitude receives is different. With this data, the authors developed the most optimal level of tree cover.     Table 2: Relative benefit of tree cover to UV-B (Grant, Heisler, Gao 2002)   As we can see from the graph, the authors propose that the most beneficial tree cover for latitudes 15-30° is 0.3 tree cover, while latitudes 45-60° is 0.1. This does not mean the less tree cover the better, but it begins to lose its effectiveness after that specific tree cover percentage. This research can give cities that have not put much though into urban forestry development insight into the UV protection benefits. As the authors suggest, even the 0.1 tree factor cover is quite a change from no tree cover.   On an interesting note, natural scenic views is associated with faster recovering times of patients (Ulrich 1984). The experiment goes about by observing the recovery rates of patients that have recently undergone surgery. The study is done at a suburban Pennsylvania hospital where the two different types of room views is one facing a brick wall and another facing a nature scene. Each room is configured in the exact same way with the same features. The only difference is the view seen outside the window from the room. The results were those patients with view of trees recovered in 7.96 days while those with brick walls recovered in 8.7 days on average (Ulrich 1984). The increase in recovery time without the use of any extra medication is quite interesting. These results can greatly change the way that hospitals are designed, such as the increase of vegetation and trees outside. There should also be access outdoors so patients can also interact with nature. In the study, the test was only done with the different views from the patients windows. Imagine even greater benefits if patients could be with nature, such as spending time in an area with many trees and vegetation. A nearby park or a park directly attached to the hospital will be highly effective. First it provides a great view from many windows and also it can provide a leisure space for recovering patients.   Community Health  Urban forestry has really peaked interest of many studies in the recent decade. Studies are finding really interesting benefits urban forestry can provide under certain situations. In a study done on domestic violence and aggression, the authors study how urban forestry can play a role in mediating violence. From the results they have found after a few studies, those neighbourhoods with trees and   vegetation nearby the apartment had lower levels of violence and aggression compared to the other test counterparts which lived in a barren location (Kuo, Sullivan 2001). Also discovered was that the mental fatigue level of those with greener settings generally scored higher than those with no green settings outside their apartments (Kuo, Sullivan 2001). This was discovered when individuals were called in for attentional functioning can measure the current state of mental fatigue the person is at. From this point and also the suggestion of the authors, that nature can improve the attentional functioning of a person which ultimately affects aggression. In turn, nature can indeed decrease the levels of violence in an urban setting.   Not only are trees able to decrease domestic violence, but also decreases crime rates (Donovan, Prestemon 2012). But in what way and how you may ask? A study was done in the city of Oregon in the United States to see how trees had an effect with crime rates. Data of thousands of houses were collected and analyzed to see if there was any correlation between houses that have been robbed and houses that have not been robbed. Generic criteria’s were set, such as if the tree present is in the lot or on the street of the house. The assumptions generated from this study are mostly theoretical as the results are mostly assumed based on the data available from the relatively small sample size of three thousand homes. For example, houses that are located on a corner of the intersection had a higher probability of being robbed from the data (Donovan, Prestemon 2012). As for the data for how trees affected the crime rates, the results were mixed as they could either increase or decrease crime rates (Donovan, Prestemon 2012). Of course this is all dependant on how big the tree is and where the tree is located in relation to the tree. For example, small lot trees that can block the view from a first floor window increased crime occurrence, whereas large lot trees and large street trees usually resulted in lower crime rates (Donovan, Prestemon 2012). What was ultimately summarized of how trees deterred criminals is that trees indicate to criminals a neighbourhood is more cared for and is most likely to be observed by authority such as police (Donovan, Prestemon 2012). In general, the study revolved around the fact whether the area is more likely to be observed or less likely to be observed. Of course, areas less likely to be observed will result in higher crime rates, and vice versa.   As many of you may or may not have seen, cities with highways that run through residential areas often have some sort of structure that helps with reducing sound levels. This is to help minimize sound pollution coming from the highway as lots of noise is created from vehicles moving at high speeds. High performance vehicles with loud exhausts and engine noises can be quite the nuisance when you are relaxing within in your own home space. This is also the case for large motor vehicles such as large trucks zooming down busy highways. According to an article, these vehicles can exceed 100 decibels   which is twice the level noise becomes a health risk (Simpson, Peper, Xiao, Pettinger, & Hodel, 2001). What many cities are opting to do instead of implementing thick cement barricades is to use trees as a method of sound deadening. This is rather an ingenious idea as it can promote green scenery as well as offering the benefits of sound insulation. Not to mention the other benefits tree can provide such as the ones I have mentioned within this paper. On average, tree barricades can provide approximately 6-15 decrease in decibel levels (Simpson, Peper, Xiao, Pettinger, & Hodel, 2001). But this green barricade can only provide deadening for lower frequency noises that are produced.  With the higher frequency noises that are created, other methods of prevention must be used. Alternate methods are used, such as planting plants in sensitive areas that are exposed to high frequency noises. Plants have been placed under tests and the results show high absorption of high frequency noises (Miller 1997). So with the combination of the two different methods to reduce high and low frequencies, this combination can effectively reduce sound levels in an urban setting.  Case Study: Hong Kong   The city we will look more in depth into will be Hong Kong, a small city but with a large population that is still continuing to grow. “The urban forestry programme in the heavily built-up milieu of Hong Kong” by CY Jim will be the article I will refer to for this specific case study (Jim, 2000). Why I have chosen this city in particular is because of the many constraints that are present due to its geological location. This study only takes into account the core of Hong Kong which is only 124km with a population of 3.3 million (Jim, 2000). As you can imagine, a fast growing city with large infrastructures and a growing population will be hard to develop effective urban forestry plans. The constraints for tree growth in Hong Kong is limited planting space, as well as varying steep topography making it difficult for tree development (Jim, 2000). The steep topography also creates constraints for urban development making infrastructure development compact. Without wasting space and having close infrastructure, space for vegetation or even trees is very limited. The Hong Kong before WW2 (1939) had tried to implement trees when building infrastructure, but this effort declined considerably after the war until 1980s.  The government wanted to increase the aesthetics of the city which included many urban development’s including planting of new trees. But this led to even more problems as the solution to greater aesthetics resulted in “intensification of development and infilling of hitherto low density sites” (Jim, 2000).   In 1994, all the roadside trees were mapped and put into a computer database to help create a new planting plan. Many of the roadside trees suffered from growth problems and resulted in small   trees. This was noted to be the issue of confined spaces both above ground and also underground due to utility lines and whatnot (Jim, 2000).  Another problem was the lack of even distribution as some roads may have many trees while some will have no trees. As for trees that already exist, they are under threat as redevelopment often leads to tree relocation or limb damage (Jim, 2000). For the planting plan of 1995 to 2000, the aim was to plant 10,000 trees of varying species, as well as effectively distributing trees evenly across Hong Kong roadsides.   As for public green space, as one can imagine is lacking in terms of area and only occupying 1.5% of Hong Kong (Jim, 2000). But the problem does not stop here, these green spaces are mostly used up by recreation activity sites such as soccer fields leaving behind very little area for groups of trees to grow in. Also since there are so many limited parks, usage is very high since the population is very large. Parks had to experiment with different methods such as fencing certain areas to keep visitors confined within a certain area. But doing so restricts the freedom of its visitors which is counter intuitive of the purpose of park.   For private lands under development, very seldom will there be planning to include trees. And even if there is space, those spaces are often threatened by redevelopment as we have learned; space is very scarce in Hong Kong (Jim, 2000). The government is to be blamed suggested by the author, because there are no laws or guidelines present that encourages the planting of trees. Developers are left to decide on their own accord whether they wish to invest space in planting trees or not. This generally results in having smaller scale developments having little to no trees due to the lot size constraint. As for larger scale developments, designers usually set aside space for tree planting mostly because there is extra space available and trees would complement the visuals.  Discussion  As you can see, Hong Kong is a great example of constraints that make it hard for urban forestry planning. The city itself is not neglecting the fact that the city is lacking in trees and vegetation. As you read earlier, Hong Kong is in a very unique geological area which has very steep slopes. Buildings and such are forced into very centralized location which makes space very scarce. Reserving space for tree planting truthfully speaking does not outweigh the monetary value of that space. Trees that were planted long ago now have to compete with constant development increasing the chance of tree damage and mortality. Also, the additions of trees in already developed areas are not possible because there is physically no space available. The government of Hong Kong has recently begun radical changes to urban forestry and are planning to increase the number of trees. But with so many constraints,   implementing any urban forestry development is hard. Solutions are available but they are not feasible logically. Ideas that would work in North America would generally not work in Hong Kong. Ideas such as setting aside land for a park within the urban core would be feasible in large North American cities. With a city such as Hong Kong, there is already a lack of housing, and not developing an area to create housing would not be logical. According to the article, in Hong Kong no guideline exists for greenery provision in different types of land use zones or roads (Jim, 2000). The first step Hong Kong should take would be to set up policies or laws that can influence developments to reserve space for trees. This would greatly increase the amount of green space set aside in Hong Kong which is a step towards a greener city. But to be fair, the government should introduce alongside a tax break or incentive for those that go beyond the required requirements for space reserve. This would give corporations and developers a reason to go beyond the requirements to earn the tax incentive. Ultimately, all parties will benefit from the new changes as there will be more green space and companies are also benefiting from the tax incentives. But as of now, Hong Kong in my opinion is taking the right direction in regards to tackling the problems of urban forestry development.   With regards to the benefit of trees, Hong Kong can greatly benefit from trees and air quality. With the dense population, air quality is not something Hong Kong gloats about. Air pollution is quite high if you were to ask any locals. The difference is astronautically if you were to compare to a city like Vancouver. Aiming for more trees would definitely help improve with the air pollution problems Hong Kong is faced with at the moment. Not to mention the rainwater runoff benefits tree are able to provide. As we established earlier, much of the city is covered in cement which is an impenetrable surface.  Many drains are utilized to properly mitigate rainwater. This is especially crucial during certain seasons because of the location along the coast. In the case of drains becoming dysfunctional, trees as a way of rainwater mitigation should be emphasized. It can also recharge the watershed of the area as to not waste valuable water. . As a suggestion, it would be best if urban forestry programmes begin as soon as possible. For example, Hong Kong used to have many trees within the city. But for the sake of development, many trees were removed or relocated to make room for new infrastructure. As their urban forestry program is still quite new, it is hard now for trees to be implemented within the core of the city as there is no space physically to make room for trees As long as the city utilizes all of its resources well, they will be able to achieve an exceptional urban forestry programme comparable to its western counterparts.    Conclusion  The goal of the paper was structured so that new information can be directed towards readers into understand more about urban forestry. A handful of benefits were discussed throughout the paper which can provide some insight on the topic of urban forestry. The information hopefully inspires individuals to continue research on specific benefits that are still not so clear in terms of its direct relationship.    Urban forestry planning as shown can be really beneficial to a city in terms of improving quality of life. What city and governments need to do is invest more resources into the program as the pros outweigh the cons. As noted in many results from different articles, the benefits of urban trees are not as significant as their respective counter parts in certain cases. But they key idea is urban trees are natural and sustainable, whereas man made inventions are generally not sustainable and require many resources to create. As for places that are just beginning to indulge into urban forestry, the common constraint is the government itself. As the government does not have laws or statues to accommodate urban forestry. This is a known issue for many places such as the case study done on Hong Kong, as it is recently developing its urban forestry programme to higher levels to meet its own demands. Ultimately, the public should be aware that trees within the city are planted not just for its aesthetics values but also its ability to provide an alternative to a healthier environment.       References Akbari, H., Kurn, D. M., Bretz, S. E., & Hanford, J. W. (1997). Energy and Buildings, 25(2), 139-148.  Comas, S. J., Carr, M. A., & Alig, R. J. (2010). Sustaining America’s urban trees and forests. Retrieved on  March 23, 2015, from https://mi.mi.gov/documents/dnr/SustainingAmericas_333340_7.pdf  Donovan, G. H., & Prestemon, J. P. (2012). The effect of trees on crime in Portland,  Oregon. Environment and Behavior, 44(1), 3-30.  Epstein, J., & Wang, S. (n.d.). Skin Cancer Foundation. Retrieved March 16, 2015, from http://www.skincancer.org/prevention/uva-and-uvb/understanding-uva-and-uvb  Fazio, Dr. James R. "How Trees Can Retain Stormwater Runoff." Tree City USA Bulletin 55. Arbor Day  Foundation. Retrieved March 3, 2015, from  http://www.fs.fed.us/psw/programs/uesd/uep/products/11/800TreeCityUSABulletin_55.pd  Grant, R. H., Heisler, G. M., & Gao, W. (2002). Estimation of Pedestrian Level UV Exposure Under  Trees. Photochemistry and Photobiology, 75(4), 369-376.  Heckman, C. J., Chandler, R., Kloss, J. D., Benson, A., Rooney, D., Munshi, T., & Oslin, D. W. (2013). Minimal erythema dose (MED) testing. Journal of visualized experiments: JoVE, (75).  Jim, C. Y. (2000). The urban forestry programme in the heavily built-up milieu of Hong Kong. Cities, 17(4),  271-283.  Kuo, F. E., & Sullivan, W. C. (2001). Aggression and violence in the inner city effects of environment via  mental fatigue. Environment and behavior, 33(4), 543-571.  Lovasi, G. S., Quinn, J. W., Neckerman, K. M., Perzanowski, M. S., & Rundle, A. (2008). Children living in  areas with more street trees have lower prevalence of asthma. Journal of Epidemiology and  Community health, 62(7), 647-649.  Miller, R.W. (1997). Urban forestry: planning and managing urban greenspaces. 2nd. ed. Upper Saddle  River: Prentice-Hall; 502 p.  Nowak, D. J., Crane, D. E., & Stevens, J. C. (2006). Air pollution removal by urban trees and shrubs in the  United States. Urban forestry & urban greening,4(3), 115-123.  Parker, J. H. (1983). Landscaping to reduce the energy used in cooling buildings. Journal of    Forestry, 81(2), 82-105.  Simpson, J. R., Peper, P. J., Xiao, Q., Pettinger, D. R., & Hodel, D. R. (2001).Tree guidelines for inland  empire communities. Local Government Commission.  SIMPSON, J. R., PEPER, P. J., MACO, S. E., GARDNER, S. L., COZAD, S. K., XIAO, Q., & HERMANN, J.  (2005). City of Minneapolis, Minnesota: Municipal Tree Resource Analysis. Center for Urban  Forest Research, USDA Forest Service, Pacific Southwest Research Station.  Ulrich, R. (1984). View through a window may influence recovery. Science,224(4647), 224-225.       


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