Science, Faculty of
Botany, Department of
DSpace
UBCV
Grewal, Devlin Singh
Master of Science - MSc
University of British Columbia
Urban forests, the trees that are planted and managed within built-up areas, provide valuable services to humans and wildlife, including lowering air temperature, carbon sequestration, improved quality of life for humans, and habitat and resources for insect, birds, and other animals. However, tree species planted within these forests are often selected for aesthetics or their ability to tolerate urban stressors, and are dominated by introduced tree species. Whether the selected trees can support urban biodiversity has largely been ignored. Trees are host to important plant-dependent organisms such as leaf-feeding insects and epiphytic microbes. These organisms are important for nutrient cycling, pest control, and tree health. Limited previous research suggests that tree provenance (i.e. native or introduced) might affect the diversity of insects and microbes supported by specific trees. Native trees are hypothesized to be a more palatable resource for phytophagous insects (and hence support a higher diversity and abundance of insects) but evidence linking tree provenance to microbial diversity or community composition is mixed. This thesis tests the hypothesis that native and introduced trees support unique communities of insects and microbes. In 2023 we sampled insect communities throughout the growing season from 80 trees (16 tree species) on the University of British Columbia Vancouver campus. We also sampled the leaf bacterial community once from each tree. We found the same overall abundance of insects on native and introduced trees, but native trees supported higher insect richness. Functional diversity and insect community composition also differed between native and introduced trees. Tree provenance did not affect microbial richness or community composition. Insect and microbial community composition were also unique between the coniferous and deciduous hosts. Overall, our findings indicate that native trees support higher insect diversity than their introduced counterparts, and suggest the continued replacement of native trees with introduced species may exacerbate the continued loss of insects worldwide. We recommend prioritizing native trees to maximize wildlife habitat and resource provisioning in urban areas.
QUANTIFYING THE INSECT AND MICROBIAL DIVERSITY SUPPORTED BY NATIVE AND INTRODUCED TREES ON A LARGE URBAN UNIVERSITY CAMPUS by Devlin Singh Grewal B.Sc., The University of Toronto, 2022 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Botany) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2025 © Devlin Singh Grewal, 2025 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: QUANTIFYING THE INSECT AND MICROBIAL DIVERSITY SUPPORTED BY NATIVE AND INTRODUCED TREES ON A LARGE URBAN UNIVERSITY CAMPUS submitted by Devlin Singh Grewal in partial fulfilment of the requirements for the degree of Master of Science in Botany Examining Committee: Dr. Michelle Tseng, Assistant Professor, Botany, UBC Co-supervisor Dr. Danielle Ignace, Associate Professor, Forest Resources, University of Minnesota Co-supervisor Dr. Matthew Mitchell, Assistant Professor, Forest Resources Management, UBC Additional Examiner Additional Supervisory Committee Members: Dr. Warren Cardinal-McTeague, Forest and Conservation Sciences, UBC Supervisory Committee Member Dr. Laura Parfrey, Associate Professor, Zoology, UBC , Supervisory Committee Member iii Abstract Urban forests, the trees that are planted and managed within built-up areas, provide valuable services to humans and wildlife, including lowering air temperature, carbon sequestration, improved quality of life for humans, and habitat and resources for insect, birds, and other animals. However, tree species planted within these forests are often selected for aesthetics or their ability to tolerate urban stressors, and are dominated by introduced tree species. Whether the selected trees can support urban biodiversity has largely been ignored. Trees are host to important plant-dependent organisms such as leaf-feeding insects and epiphytic microbes. These organisms are important for nutrient cycling, pest control, and tree health. Limited previous research suggests that tree provenance (i.e. native or introduced) might affect the diversity of insects and microbes supported by specific trees. Native trees are hypothesized to be a more palatable resource for phytophagous insects (and hence support a higher diversity and abundance of insects) but evidence linking tree provenance to microbial diversity or community composition is mixed. This thesis tests the hypothesis that native and introduced trees support unique communities of insects and microbes. In 2023 we sampled insect communities throughout the growing season from 80 trees (16 tree species) on the University of British Columbia Vancouver campus. We also sampled the leaf bacterial community once from each tree. We found the same overall abundance of insects on native and introduced trees, but native trees supported higher insect richness. Functional diversity and insect community composition also differed between native and introduced trees. Tree provenance did not affect microbial richness or community composition. Insect and microbial community composition were also unique between the coniferous and deciduous hosts. Overall, our findings indicate that native trees support higher insect diversity than their introduced counterparts, and suggest the continued iv replacement of native trees with introduced species may exacerbate the continued loss of insects worldwide. We recommend prioritizing native trees to maximize wildlife habitat and resource provisioning in urban areas. v Lay Summary Trees planted in cities offer many benefits like cooling air temperatures and improving quality of life. Yet most urban trees are chosen for their looks or resilience in the city, not for how they interact with insects or bacteria that live on their leaves. Healthy insect communities help to pollinate city plants, and serve as food for other animals, while bacterial communities can prevent the spread of pathogens. We studied insect and bacterial communities of 80 trees of 16 species on the University of British Columbia campus in Vancouver, BC. We found that native trees hosted more types of insects, and had different insect communities than introduced trees. Bacterial communities collected from leaves or needles did not differ between native and introduced trees. Our results suggest that planting a variety of native tree types can help support urban biodiversity, especially for insects. vi Preface Conceptualization and funding acquisition for this thesis was conducted by Michelle Tseng and Tara Moreau, with support from Warren Cardinal-McTeague, Danielle Ignace, and Richard Hamelin. Laura Parfrey replaced Richard Hamelin in 2024 because the focus of the microbe research changed from fungi to bacteria. Tree selection was conducted by Savitri Raghuraman, Oliver McDermott, and Michelle Tseng. Insects and leaves were collected by Savitri Raghuraman, Julie Sieg, and Oliver McDermott. Insect specimen preparation, identification, and preservation were performed by Devlin Grewal, with assistance from Julie Sieg. Leaf microbial DNA isolation methodology was developed by Devlin Grewal and Parker Lund, with supervision by Laura Parfrey. Microbe sequencing and initial data cleaning were led by Parker Lund and Siobhan Schenk, with supervision from Laura Parfrey. All data curation, cleaning, and analysis, and thesis writing was led by Devlin Grewal, with editing and supervision by Danielle Ignace and Michelle Tseng. Generative AI was not used for any aspects of the work. vii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ........................................................................................................................ vii List of Tables .............................................................................................................................. viii List of Figures ............................................................................................................................... ix Acknowledgements ...................................................................................................................... xi Dedication .................................................................................................................................... xii Chapter 1: A Brief Introduction ...................................................................................................1 1.1 Urbanization shapes novel ecosystems ........................................................................... 1 1.2 Ecosystem services in the urban forest ........................................................................... 2 1.3 Species provenance and biotic interactions .................................................................... 4 1.4 Beyond pollinator communities ...................................................................................... 6 Chapter 2: Insect and Microbial Diversity on Urban Trees ......................................................8 2.1 Introduction ..................................................................................................................... 8 2.2 Methods......................................................................................................................... 13 2.3 Results ........................................................................................................................... 25 2.4 Discussion ..................................................................................................................... 36 Chapter 3: Conclusion .................................................................................................................41 Works Cited ..................................................................................................................................44 Appendices ....................................................................................................................................62 viii List of Tables Table 1. Tree species included in this study. Order, family, and species are listed. Common names are from the Database of Vascular Plants of Canada (http://data.canadensys.net/vascan). â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦24 ix List of Figures Figure 1. Locations of the 80 sampled trees (denoted in red) on the UBC Vancouver campus. The UBC campus is located to the west of Vancouver, British Columbia (not shown). Campus is bordered on the east by Pacific Spirit Regional Park (indicated in green)â¦â¦â¦â¦.15 Figure 2. Effect of tree origin, tree type and sampling period on insect abundance (log). Orange and green colours represent introduced and native tree species, respectively. Triangles and circles represent coniferous and deciduous trees, respectively. Error bars are 95% confidence intervals. Sample periods are as follows: Period 1: June 6th â 22nd, 2: June 27 â July 17th, 3: July 27th â August 1st, 4: August 2nd â August 17thâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦..27 Figure 3. Effect of tree origin, type and sampling period on insect richness (log). Orange and green colours represent introduced and native tree species, respectively. Triangles and circles represent coniferous and deciduous trees, respectively. Error bars are 95% confidence intervals. Sample periods are as follows: Period 1: June 6th â 22nd, 2: June 27 â July 17th, 3: July 27th â August 1st, 4: August 2nd â August 17thâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦..28 Figure 4. NMDS plots of insect communities colored by (a) tree origin or (b) tree type and sampling. (a) Orange and green represent introduced and native tree species respectively. (b) Blue and pink represent coniferous and deciduous trees respectivelyâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦...29 Figure 5. NMDS plots of insect communities on native (a), introduced (b), deciduous (c) and coniferous (d) trees over sampling time. Sample periods are denoted by solid (1: June 6th â 22nd), long dashes (2: June 27th â July 17th), short dashes (3: July 27th â August 1st) and dotted lines (4: August 2nd â August 17th)â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦..30 x Figure 6. Insect functional trait proportions on native and introduced coniferous and deciduous tree hostsâ¦â¦â¦â¦â¦â¦â¦...â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦32 Figure 7. NMDS plots of individual trees clustered by their microbial communities. (a) Orange and green dots represent introduced and native trees respectively. (b) Pink and blue dots represent deciduous and coniferous trees respectivelyâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦..34 Figure 8. Relationship between (a) insect abundance and microbial richness and (b) and insect richness and microbial richness in sampling period 1. Points are jittered slightly to prevent overlap. Dots represent individual treesâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦.....35 xi Acknowledgements This thesis would not have been possible without the help, guidance and insight of an incredibly supportive community of mentors, advisors and friends. Michelle, thank you for being an amazing supervisor, for being a guiding light through the academic aspects of my thesis, and for always being willing to hear about the latest shenanigans. You've shown me how important it is to use your platform to stand up for the values you believe in, and that a successful degree is just as much about a supportive community as it is academics. Thank you for your patience and encouragement, and for your dedication to a job well done. Danielle, Laura, and Warren, thank you for your insight, critiques and extensive guidance throughout this degree. I am incredibly grateful for having such a supportive and thoughtful committee who care about me as a person, and a student. To all my friends: get fucked. I have been incredibly blessed to be surrounded by a community that has encouraged me to grow to heights I could not attain alone. In no particular order, I would like to thank Markus Thormeyer, Katie Gyte, Natasha Klasios, Parker K Lund, MJ Herrin, Bernadette Johnson, Wouter van der Bijl, Sean Edgerton, Ara Jamaldin, Shabnam Tejani, Caleigh Charlebois, Jacelyn Shu, Duncan MacNoughton, Lilith South, Regine Manglicmot, Maia Palka, Alexandria Thompson, Bea Santos, Ayuni Ratnayake, Jawad Sakarchi, Javad Meghrazi, Niki Love, Nathan Malamud, Julie Sieg, Mari Wilson, Amy Forsythe, Bryn Wiley, Asad Hasan, Avery Kruger, Claudie Pageau, Finola Fogarty, Kwasi Wrensford, Nick Bard, Nicole Bison, Mia Waters, Marcella Cross (and Ollie), Milos Simovic, Jess Kowalski, Derek Tan, Sivanna Trainor, John English, Menilek Beyene, Francoise Cardou, Gloria Gao, Cissy âBigstickâ Zhang, Zoe Schaefer, EK Unger and Jess Stockdale and Lydia Fong. xii Dedication In memory of Shruthi Sailesh,1 Chapter 1: A Brief Introduction 1.1 Urbanization shapes novel ecosystems Urbanization, the process by which previously undeveloped or rural land is converted into built-up spaces, is continuing unabatedly, with more than 5 billion people projected to live in urban areas by 2030 (Seto et al. 2012). The restructuring of natural systems into urban areas leads to increased impervious surface cover, altered resource availability, and novel anthropogenic pressures including pollution, the urban heat island effect, and reduced habitat connectivity (McDonald et al. 2013). Intense urbanization leads to a decrease in richness for most species, while moderate levels of urbanization can lead to increased species richness for some taxa within these novel ecosystems (McKinney 2008, Piano et al. 2020). Understanding the ways in which species interact within these novel urban environments is key to predicting future patterns of biodiversity and ecosystem services that most humans will interact with in the future. Urban ecosystems are comprised of the remnants of natural habitat, urban green spaces, green infrastructure, and parks within cities. These green spaces play critical roles in supporting biodiversity and ecosystem services within the city (Elmqvist et al. 2013, Aronson et al. 2017). such as carbon sequestration, pollution mitigation, thermal heat island effect amelioration, mental and physical health benefits, and improvements to property values (Pandit et al. 2012, McDonald et al. 2013, Livesley et al. 2016, Ziter et al. 2019). Inhabitants of urban ecosystems, including the floral, and faunal found within urban centers, provision ecosystem services such as nutrient cycling through their actions and interactive effects (Hooper et al. 2005). With an ever-2 increasing proportion of global human populations residing in urban areas, the conservation and promotion of biodiversity for these ecosystem services becomes ever more important. Urban biodiversity is shaped through a series of ecological filters, wherein species establish via their dispersal ability, tolerance to anthropogenic disturbances, and other traits favored in urban centers (Aronson et al. 2016). However, urban centers are often home to relatively harsh or highly disturbed ecosystems, which are more homogenous than their natural analogues (McKinney 2008, Knop 2016). Although often overlooked, insect and epiphytic bacterial species are among the most abundant species within urban ecosystems. Insects serve several ecological functions such as pollination, nutrient cycling, herbivory, and serving as prey for organisms at higher trophic levels (Schowalter et al. 2018). Similarly, bacterial species residing on the surfaces of plants (the phyllosphere) help host plants access nutrients and resist pathogens (Bacon and White 2016, Sohrabi et al. 2023). Urbanization may be changing these communities through pollution, temperature, habitat disruption and introduction of novel species interactions, across all scales (McKinney 2008, Nowak and Greenfield 2018, Stevens et al. 2021). My thesis aims to characterize some of these interactions, to better improve our understanding of insect and microbial communities residing on urban trees. 1.2 Ecosystem services in the urban forest Urban trees provide a variety of ecosystem services, encompassing human services (i.e. emotional and psychological services, physiological services, and monetary services), abiotic services (i.e. hydrological services, thermal services, and air quality services), and biotic services 3 (i.e. habitat provisioning, landscape connectivity) (OâBrien et al. 2022). However, research encompassing ecosystem services tends to disproportionately focus on the services that urban trees are able to provide to humans. (Roy et al. 2012, Wolf et al. 2020, OâBrien et al. 2022, Tartaglia and Aronson 2024). Both native and introduced trees are able to support a wide variety of biodiversity services and interactions with other faunal residents of urban centers including habitat provisioning, herbivory and predation opportunity, nesting and reproductive resources (Tartaglia and Aronson 2024). Despite this, urban trees are typically selected for their viability and aesthetic considerations to their planting site, (Conway 2016), with property managers retaining trees for their provisioning of shade, privacy and aesthetic value to their planting location (Guo et al. 2019). These planting patterns result in urban forest communities wherein property managers select trees that are on average smaller, non-native ornamental and whose ecosystem services typically are human-centric (Dilley and Wolf 2013). There is growing sentiment among practitioners and property managers towards selecting trees for urban plantings with their biodiversity services potential in mind (Woodall et al. 2010, Sjöman et al. 2016). Urban trees can ameliorate some of the negative effects of the urban matrix on faunal and non-human residents (OâBrien et al. 2022). Increased arboreal habitat richness can support greater diversity of lepidopteran species (Goodwin et al. 2017), while green infrastructure can provision important floral resources for pollinator species (Liang and Huang 2023). 4 1.3 Species provenance and biotic interactions The ability of insects and other species to interact and utilize plant species as food, forage, or habitat is a combination of that speciesâ familiarity and degree of ecological overlap with the host plant (Brändle et al. 2008). While many insect species show marked history of co-evolution to plant defenses or morphology, and the ability to take advantage of a host plant is a product of the speciesâ ability to recognize the host, and circumnavigate these defenses (Gatehouse 2002, Grunzweig et al. 2015). As such, plant species introduced beyond their native ranges represent novel players to communities, to which native insect species are naïve to both their defenses and how to utilize these introduced plants. Introduced plants in novel ranges thus tend to show lower degrees of herbivory by insects and other species than their native counterparts (Siemann et al. 2006). This release from herbivory and predation can enable some species of introduced plants to proliferate and succeed outside of their native ranges(Carpenter and Cappuccino 2005), which can lead to decreases in native flora (Bernard-Verdier and Hulme 2015, Rojas and Zedler 2015) and changes to existing community structure (Heleno et al. 2009, GrzÄdzicka 2024, Basile et al. 2025). While the richness of insects on introduced plants increases with their time since introduction, the faunal communities interacting with introduced plant species tend to be dominated by generalists rather than specialists (Brändle et al. 2008, Kurylo et al. 2020). However, this effect may be less pronounced if introduced plants are preceded by congenators within the native range (Mata et al. 2021). The proliferation of introduced plant species has well documented links to declines in insect richness and biomass (Heleno et al. 2009, Tallamy and Shropshire 2009, Richard et al. 2019, 5 Kurylo et al. 2020, Tallamy et al. 2021). Alien species introductions are considered to be a contributing cause in 25% of native plant extinctions, and 33% of animal extinctions (Blackburn et al. 2019), and insect species are not exempt from this trend. With insect abundance and diversity declining across the world (Hallmann et al. 2020, Kawahara et al. 2021, Wagner et al. 2021), the introduction of novel plant species represents additional pressures contributing to population declines. Furthermore, habitat invaded by introduced plants can see shifts in their floral communities, with invaded habitats often showing decreased richness and abundance of native host plants (Bernard-Verdier and Hulme 2015, 2015, Rojas and Zedler 2015), decreasing available habitat and forage opportunity for insects reliant upon them (Bassi and Staude 2024). As such, introduced plant species may be reducing the complexity of biotic interactions, leading to communities that are more homogenous, and predominantly populated by generalists. This biotic homogenization poses threats to communitiesâ ability to resist large scale environmental events, adapt in future generations, and reduces their resilience to environmental stressors (Olden et al. 2004). With introduced plant species potentially paving the way for subsequent invasion by non-native insect species (Rodríguez et al. 2019, Bonnamour et al. 2023), their effective and informed management is vital to future biodiversity conservation. While the detrimental impacts of introduced plants are well documented among annual and perennial species, the role of trees is less well known. Native trees may act as bastions against introduced plant invasion, reducing introduced plant richness within invaded areas (Bernard-Verdier and Hulme 2015). My thesis seeks to contextualize the impact of introduced tree species within the urban ecosystem, especially as it relates to insect and microbial diversity. 6 1.4 Beyond pollinator communities Urban forest research has increasingly recognized the importance of tree diversity in supporting ecosystem services within cities, however, there is a striking overrepresentation of pollinator support within the literature. Recent reviews have shown that studies on pollinators equals or outnumbers publications on all other arthropod taxa combined (Millard et al. 2021, Liang and Huang 2023, Tartaglia and Aronson 2024). While pollinator services are ecologically and economically vital (Klein et al. 2006, Ollerton et al. 2011), this narrow focus potentially overlooks the contributions of other insect and microbial taxa to urban forest health and functioning. Insects beyond pollinators, including predators, herbivores and detritivores, perform critical ecosystem functions including nutrient cycling, biomass control and act as food sources for higher trophic levels (Seastedt and Crossley 1984). The examination of broader arthropod diversity in urban forests can allow us insights into resilience of ecosystems in the face of climate change, resistance to biotic invasion, and provisioning of desirable ecosystem services for human health and well-being. Non-pollinator insects provide a suite of services within urban landscapes, supporting the foundations of ecosystem functioning of these anthropogenically modified ecosystems. Through their feeding, nesting, and excretory activities, insects contribute to carbon and nitrogen fluxes, act as biotic control agents for floral and faunal species, and expedite decomposition and resource cycling within the urban matrix(Kotze et al. 2022). Parasitoid and predatory taxa contribute to biotic control of âpestâ species such as aphids and leaf miners, which can reduce human reliance on chemical pesticide management strategies (Fenoglio et al. 2013). In addition, 7 insect biomass supports the feeding requirements of many terrestrial vertebrates including birds, bats and small mammals, and their continued existence is vital to maintain these rich webs of species communities within urban centers. Urban trees and their insect communities are not isolated, but sit within complex species networks (Valdés-Correcher et al. 2021, Clark et al. 2024). Microbial actors, including the bacterial communities residing on the phyllosphere can influence nutrient acquisition of the host tree, impact tree herbivory defense against insect predators, and change disease dynamics for both (Bacon and White 2016, Laforest-Lapointe et al. 2017b). Likewise, insect herbivory can in turn shape the microbial community on leaves, and host behaviour and traits are thought to be predominant factors in epiphytic microbial communities (Schlechter et al. 2019, Humphrey and Whiteman 2020). Expanding urban forest research to include phyllosphere bacterial communities within the scope of our research is important for a more holistic understanding of the ecosystem services hosted upon urban forest trees. 8 Chapter 2: Insect and Microbial Diversity on Urban Trees 2.1 Introduction Urbanization, the process by which existing land is transformed for residential, commercial and industrial purposes alongside increasing human population density, affects ecological processes through the creation of novel ecosystems (Kowarik 2011, Aronson et al. 2016, Cadotte et al. 2017). In particular, urban plant communities are shaped by public and private preferences in addition to natural processes (Conway 2016, Aronson et al. 2017, Gwedla and Shackleton 2019). Urban trees, for example, are planted for a range of reasons, including those related to improving human health or property values (Grinde and Patil 2009, Pandit et al. 2012, Wolf et al. 2020), reducing urban heat island effects (Ziter et al. 2019), decreasing air and noise pollution (Livesley et al. 2016), and improving stormwater management (Kuehler et al. 2017, Berland et al. 2017). Alongside these ecosystem services, urban trees are also important for city wildlife as they provide habitat and decrease the effects of anthropogenic stressors (OâBrien et al. 2017). The variety of decisions underlying tree selection in cities has resulted in a heterogeneous mixture of native and introduced trees in urban areas (Aronson et al. 2016, McCoy et al. 2022). This unique configuration of tree species presents a valuable opportunity to improve our understanding of the importance of plant provenance (i.e. native or introduced) for urban biodiversity. In particular, trees are host to thousands of species of plant-dependent organisms such as phytophagous insects and epiphytic plant microbes. Insects perform a variety of ecological and ecosystem functions within urban systems, including pollination and pest control (Klein et al. 2006, Schowalter et al. 2018, Dangles and Casas 2019), while the tree microbial 9 community has been linked to plant health, pathogen resistance, and human health (Sohrabi et al. 2023, Lu et al. 2024). As such, understanding the factors shaping tree-associated communities of insects and microbes can help enhance the benefits of urban forests to humans, as well as illuminate ecological processes acting in unique urban ecosystems. Additionally, insect abundance and diversity are declining globally (Didham et al. 2020, Hallmann et al. 2020, Kawahara et al. 2021, Wagner et al. 2021),(but see Crossley et al. 2020) and the disappearance of native host plants is thought to be a key mechanism underlying these declines (Bassi and Staude 2024). The degree to which insects are able to utilize specific tree species as food resources is a function of the insectsâ ability to circumvent plant defenses and the duration of geographic overlap between insects and their plant hosts (Southwood 1961, Cornell and Hawkins 2003, Tallamy et al. 2021). Most phytophagous insects are only able to tolerate the plant secondary compounds of the species to which they have co-evolved (Tallamy et al. 2021). As such, native trees are predicted to support higher abundance and diversity of native insect species (Tallamy et al. 2021). This prediction has been supported by studies conducted in botanical gardens (Kirichenko and Kenis 2016) and forest preserves (Grunzweig et al. 2015, Wildermuth et al. 2023), as well as in field experiments (Berthelot et al. 2023) and database studies (Tallamy and Shropshire 2009, Ogushi et al. 2024). Studies investigating insect abundance and diversity on urban trees are surprisingly rare. In a study in southern Sweden, Jensen et al (2022) found fewer insects on non-native urban trees. Similarly in Melbourne, Australia, Mata et al (2021) reported that native trees hosted significantly higher diversity of insects. Comparisons of the insects supported by native vs. introduced plants in urban areas have mostly focused on non-arboreal plants and have found 10 both higher insect diversity and abundance on native plants or no effect of plant provenance (Zuefle et al. 2008, Matteson and Langellotto 2011, Kowarik 2011, Padovani et al. 2020, Fenoglio et al. 2021, Philpott et al. 2023, Lerch et al. 2024). Our study helps fill a gap in our understanding of the effects of native vs. introduced urban trees on insect abundance and biodiversity. Along with being an important habitat for insects, the above-ground surfaces of urban trees (the phyllosphere) represent one of the largest habitats for microbial life. Microbial communities are typically recruited from sources such as the local environment (e.g. soil, atmospheric and neighboring phyllosphere sources), or seed microbiomes (Abdelfattah et al. 2021), and follow host-specific successional trajectories (Redford and Fierer 2009, Lajoie and Kembel 2021, Sohrabi et al. 2023). Phyllosphere microbial communities can have important implications for host plant health by affecting host plant nutrient uptake, pathogen resistance, hormone regulation and toxin degradation (Bacon and White 2016, Laforest-Lapointe et al. 2017b, Zhang et al. 2024). Environmental conditions such as heavily urbanized environments, low nutrient availability, poor sunlight, and heavy herbivory can contribute to reduced microbial diversity and poor host health (De Mandal and Jeon 2023, Zhang et al. 2023, Seabloom et al. 2023). Host plant identity is thought to be a predominant driver in shaping these communities, due to close associations between specific microbes and leaf-surface characteristics such as exudates and leaf secondary characteristics (Lajoie and Kembel 2021, De Mandal and Jeon 2023). Morphological characteristics of the trees can also shape the microbial community, with conifers supporting more diverse bacterial communities compared to their broadleaf neighbors (Lu et al. 2024). Finally, evidence for effects of host provenance on microbial diversity is scarce and mixed, with 11 introduced plants demonstrating decreased (Lu-Irving et al. 2019) or increased (Zhang et al. 2024) microbial alpha diversity compared to their native counterparts. Given that tree provenance (in addition to tree type [i.e. coniferous or deciduous] and species identity) may also play an important role in explaining variation in the phyllosphere microbial community, here we examine microbial diversity, in addition to insect diversity, on native and introduced urban trees. In addition to the characteristics of the tree host, phyllosphere bacterial communities may also be shaped by insect visitation and herbivory (Humphrey et al. 2014, Humphrey and Whiteman 2020). Microcosm experiments indicate that herbivory by beak-feeding insects such as aphids can reduce the diversity of phyllosphere communities through the deposition of Buchnera (an aphid gut symbiont) rich honeydew (Wolfgang et al. 2023). In addition, mandibular-feeding insects such as lepidopteran larvae can unlock nitrogen resources previously sequestered within the leaf tissues, thereby increasing plant pathogen abundance within the phyllosphere community (Stadler and Müller 2000, Müller et al. 2003). Insect herbivory thus appears to influence the phyllosphere in mixed ways, at times leading to increased microbial abundance or diversity through insect-driven changes in resource availability (Müller et al. 2003, Humphrey et al. 2014, Allard et al. 2018, Humphrey and Whiteman 2020) and at others showing negative correlations with herbivore activity (Humphrey et al. 2014, Seabloom et al. 2023, Wolfgang et al. 2023), or no effect at all (Hansen and Enders 2022). Overall, these mixed findings suggest that additional research is required to better understand how insect diversity affects microbial phyllosphere communities within urban systems. 12 The overall goals of this study are to test the hypothesis that compared to introduced trees, native tree species support a higher abundance and diversity of insects. Given the importance of tree foliage for the urban microbial community, we also characterize the phyllosphere microbiome associated with native and introduced urban trees. Together, our study informs how native vs. introduced trees shape insect and microbial communities in urban environments. If native trees are a more palatable food source for phytophagous insects, we predict that compared to introduced species, native trees at our study site will support higher abundance and diversity of plant-eating insects. In addition, previous papers have also demonstrated the importance of tree leaf type (i.e. coniferous or deciduous), with coniferous trees supporting a higher abundance and diversity of insects (Ozanne 1999). We thus also investigate the role of tree type in shaping patterns of insect richness and abundance. Here we refer to broadleaved, deciduous trees belonging to angiosperms as âdeciduousâ, and cone-bearing, evergreen gymnosperms as âconifersâ. With respect to leaf microbes, it is less clear whether native vs. introduced trees will support more or less diverse bacterial communities. If native trees support higher insect diversity which in turn is positively correlated with microbial diversity, we expect native tree species to host a higher diversity of microbes. However, previous studies have documented the importance of tree species and tree type (coniferous vs. deciduous) on the phyllosphere community, and these factors may also play important roles in shaping microbial community composition. 13 2.2 Methods Study location This study was conducted on the University of British Columbia (UBC), Vancouver campus (49.2617, -123.2486) which occupies the traditional, ancestral, and unceded territory of the xÊ·mÉÎ¸kÊ·ÉyÌÉm (Musqueam) First Nation. UBC is more than 400 hectares in size, has a student population of 71,000, and approximately 15,000 full time residents. The campus functions like a small city, with restaurants, grocery stores, and residential housing ranging from single detached houses to multi-story condominiums. The climate of Vancouver is classified as âtemperate oceanicâ by the Köppen climate classification system and belongs to the Georgia Depression Ecoprovince (Demarchi 2011). The city boasts a 31% tree canopy cover within its administrative boundaries, with planted canopy made up predominantly of deciduous trees (73%) and the remainder as conifers (27%), with targets for 40% canopy cover by 2050 (metrovancouver 2024). The urban forest is predominantly made up of ornamental species, with the most prevalently planted trees being Prunus serrulata Lindl. (7.5%), Acer platanoides L. (7%), Prunus cerasifera Ehrh. (6 %), Acer rubrum L. (5.4%) (City of Vancouver Open Data Portal). The UBC Vancouver campusâ diverse assemblage of native and introduced trees make it an ideal location to test the role of tree provenance on insect and microbial diversity. Tree Selection The UBC Urban Forestry Program maintains a database of tree species on campus. Using this database as a starting point, we first surveyed campus by foot to confirm the accuracy of tree locations and identifications. We then selected 16 species of native and introduced trees based on provenance, growth stage and planting location. We used the British Columbia Conservation 14 Data Centre (BCCDC) database and the Biota of North America Program (bonap.org) to confirm whether tree species were native to British Columbia. Trees needed to be at least 30 meters apart from each other and 100 meters from the intact forest at the northern and western borders of the campus (Figure 1). Trees also needed to have attained adult growth form, possess living foliage accessible while standing on a 1.37m (4.5 ft) portable ladder (3m or less above ground level), and include a stable and accessible terrain for sampling. Our final selection of trees included 5 individuals each of: 7 species of native trees (3 deciduous, 4 coniferous) and 9 species of introduced trees (8 deciduous and 1 coniferous) (Table 1), for a total of 80 individuals across 16 tree species. Sample sizes within the four possible tree origin (native vs. introduced) by tree type (coniferous vs. deciduous) combinations were not balanced because we were constrained by the community of trees available on the UBC Campus. However, these tree species represent 30% of the tree species planted throughout the City of Vancouver (2025 City of Vancouver) and thus provide important insight into the insects and microbes that may be supported at the municipal scale. To randomize how trees were sampled for insects and microbes, the 80 trees were randomly assigned into one of 8 transects (10 trees per transect). Each transect was sampled for insects once per sampling period (Period 1: June 6th â 22nd, 2: June 27 â July 17th, 3: July 27th â August 1st, 4: August 2nd â August 17th), with the order of transects and trees within transects randomized within each sampling period. Leaf samples for microbial sampling were collected during sampling period 1 (June 6th â June 22nd). 15 Insect Collection Insects were collected from each tree during each sampling period, utilizing the âbeat-sheetâ sampling technique (see: Whitman et al. 2019). For each tree, one branch with living foliage 1.5 â 3 m above ground level was tagged with flagging tape, and the appropriate tree code was noted on the tape. During each sampling event, we hit the branch with a stick (a 1 m long arbutus Figure 1. Locations of the 80 sampled trees (denoted in red) on the UBC Vancouver campus. The UBC campus is located to the west of Vancouver, British Columbia (not shown). Campus is bordered on the east by Pacific Spirit Regional Park (indicated in green) 16 branch, 5 cm in diameter). We used enough force to agitate the tree branch and foliage such that insects clinging to branches or foliage would be dislodged and fall into an inverted white umbrella. This method is commonly used to collect insects from trees (Helden et al. 2012, Goodwin et al. 2017, Jensen et al. 2022). We transferred all insects that fell into the umbrella into vials containing 95% ethanol. We spent a maximum of five minutes collecting insects from each tree. On average, all insects were collected in approximately three minutes. Several trees contained hundreds of barklice (Psocodea). We used the remaining two minutes to collect barklice in the event that we decided to use them in the analyses. Collection vials were labelled with tree code, sampling round, and collection date, and stored at room temperature in the lab. Target taxa were classified as juvenile or adult, pinned, and labelled. Insects were identified to the lowest taxonomic group possible using reference photos from the Spencer Entomological Collection (Beaty Biodiversity Museum), bugguide.net, illustrated keys, and guides (McPherson 1982, Borror et al. 1989, Weirauch et al. 2014, Peterson 2018, Babenko et al. 2019, D. Manastyrski n.d.). Where possible, we also assigned each insect taxon to one of the following functional groups: Detritivore, Leaf Herbivore, Nectar/Blood feeder, Nectar/Pollen feeder, Polyphagous, Predator, Sap Herbivore, Sap Herbivore/Predator, and Seed Herbivore based on available feeding habit information (McPherson 1982, Borror et al. 1989, Weirauch et al. 2014, Peterson 2018, Babenko et al. 2019, D. Manastyrski n.d.). As one of the goals of the study was to investigate potential differences in insect diversity between native and introduced plants, these data on what/how insects eat provides an additional metric of diversity. 17 Leaf Phyllosphere sampling During the first sampling period (June 6th â 22nd), 50 to 100 grams of leaves or needles from each target tree were collected utilizing aseptic techniques and stored at -67 ºC for later processing. Leaf surface microbial communities were sampled using methods adapted from existing protocols for phyllosphere and cuticular fungal sampling in agricultural produce (Laforest-Lapointe et al. 2017b, 2017a, Hamilton et al. 2020, Lajoie and Kembel 2021). Before processing, frozen leaf or needle samples were allowed to come to room temperature. Ten grams of leaf material were removed from sample bags, taking whole leaves where possible. Leaves were added to flasks containing a 100 mL solution TE buffer (10 mM Tris-Cl, 1 mM EDTA) with 2% v/v Tween, and vortexed for 5 min. The liquid was decanted into 50 mL conical tubes and centrifuged at 3220 Ã g for 30 min to pellet the bacteria. A negative control of buffer was used to detect contamination during sample processing and extraction. DNA extraction was performed on the pellet using the ZymoBIOMICS DNA Miniprep Kit (cat.#D4300) following manufacturerâs protocols. 16S rRNA Amplicon Sequencing DNA was amplified using barcoded Illumina primers 515F (5â-GTGYCAGCMGCCGCGGTAA-3â) and 806R (5â-GGACTACNVGGGTWTCTAAT-3â), which target the V4 region of the 16S rRNA gene. A PNA clamp was added to inhibit plant chloroplast amplification (PNA Bio cat.#PP01-50). The following thermocycler settings were used: 98ºC for 10 s, followed by 35 cycles of 98ºC for 1 s, 70ºC for 5 s, 50C for 5 s, 72ºC for 24 s, with a final extension at 72ºC for 5 min. PCR amplification was confirmed by gel electrophoresis, then quantified using the Quant-IT Pico Green dsDNA Assay Kit (Invitrogen 18 cat.# P11496) and pooled in equimolar concentrations of 10 ng. Agilent Bioanalyzer quality control and sequencing via the Illumina MiSeq Platform was performed by the Sequencing + Bioinformatics Consortium at the University of British Columbia. Microbe Data Processing 16S rRNA data was processed using the âdada2â package (Callahan et al. 2016) to filter, trim and merge the forward and reverse reads to a length of 200 base pairs, with maxEE = (8,8) and truncQ = 2. Taxa were then assigned using the SILVA database (v138) (Quast et al. 2013). We first removed non-bacterial sequences and discarded tree samples with fewer than 1000 reads and ASVs with fewer than 99 occurrences. We next removed potential contaminants by discarding ASVs found in two samples or less and denoised the remaining data. At this point 73 tree samples and 724 ASVs remained. We then rarified to an even read depth of 2128 reads, which removed an additional seven tree samples, for a total of 66 trees and 724 ASVs. We experimented with different thresholds of filtering and rarefication (up to a read depth of 7500). Comparisons of microbial richness or community composition between native vs. introduced trees and between coniferous vs. deciduous trees were not affected by using different threshold for cleaning, filtering, or rarefication. Additional tree traits Although the focus of the study is to compare the insect and microbial communities found on native vs. introduced trees, our findings may also be affected by additional tree traits, such as age or health of the tree. As proxies for overall tree condition, we measured tree height and specific leaf area (SLA) for each tree. To measure SLA, from each tree sample, ten leaves (or needles) 19 were removed from the sample originally collected for microbial analyses, and allowed to come to room temperature. Leaves were scanned using an Epson V39 flatbed scanner, and leaf area was determined using ImageJ (version 1.54g) (Schneider et al. 2012). Scans were first scaled using a static scale bar on the scanning bed. Masks of the leaves were then created by converting the image to binary color mapping. In this way, scanned leaves (black pixels) are able to be differentiated from the background (white pixels) by the software. Area of the leaves was then calculated from scaled pixel area. Leaves were then desiccated under a laminar flow hood, and dry mass recorded (Garnier et al. 2001). SLA was calculated as the ratio between leaf area (in cm2) and dry mass (g). We measured tree height with a rangefinder (LaserTech TruPulse 200). Data Analysis R packages and software versions All analyses were conducted using R version 4.3.2 (R Core Team, 2023). General data cleaning and transformation and visualization was performed using functions from âtidyverseâ (Wickham et al. 2019).We used the âLme4â and âlmerTestâ packages for mixed effects models (Kuznetsova et al. 2020), and conducted posthoc tests using the âemmeansâ package (Lenth et al. 2025). Microbe dataset cleaning, filtering, rarefication and analyses of microbial richness were undertaken using the âphyloseqâ package (McMurdie and Holmes 2013). Analyses of similarity (ANOSIM) were performed using the âveganâ package (Oksanen et al. 2025). Leaf scan measurements were performed in ImageJ version 1.54g (Schneider et al. 2012). 20 Insect Abundance Insect abundance was defined as the total number of insects collected per tree per sampling period and included both juvenile and adult insects. Insect abundance was log-transformed to better meet assumptions of linear models. Barklice were not included in any analyses. Barklice primarily consume fungi and the hypothesis of more insects on native trees is primarily applicable to phytophagous insects. We conducted linear mixed-effects models to examine whether tree origin, tree type, and sampling period explained variation in insect abundance. Individual tree identity and tree species were included as random effects to account for repeated sampling of each tree across the four sampling periods and to account for the non-independence of tree individuals belonging to the same species. Statistical models that included all possible interactions returned the same results as the simplest model with the three main effects included. We thus report the results for the simplest model. Leveneâs Test of log-transformed data showed no heteroscedasticity and Q-Q plots of model residuals vs. predicted data showed no deviation from normality. Insect Richness Insect richness was defined as the number of unique taxonomic groups found on each tree on each sampling period. Insect taxonomic resolution ranged from order to species. The proportion of insects identified to family, genus, and species was 38% (n = 543), 11.7% (n = 167), and 50.3% (n = 719), respectively. We assigned 151 specimens to 61 morphospecies, and 1175 specimens to either species or genus level. We investigated the effect of tree origin, tree type, and sampling period on insect richness using a linear mixed-effects model. Tree origin, tree type, and sampling period were modelled as fixed factors and individual tree identity and tree species 21 were included as random effects. Richness data were log transformed to reduce heteroscedasticity and model residuals showed no strong deviation from normality. The full model including all interactions returned the same result as the simplest model with just main effects included. We thus present the results for the model with no interactions. Additional tree and location variables To confirm that any observed effects of tree origin were not due to variation in tree height or leaf SLA, we used linear mixed-effects models to test whether native vs. introduced trees differed in their average height or SLA. For these models, tree height (or leaf SLA) was included as the dependent variable, tree origin as the independent variable, and tree species as the random effect. Insect Functional Diversity We used Pearsonâs Chi-squared tests to test whether four groupings of trees: coniferous-native, coniferous-introduced, deciduous-native and deciduous-introduced trees contained different proportions of insect functional groups. Proportions of insect functional groups within the four tree groupings were relatively consistent through the growing season (Fig. A5) and thus we analyzed the proportions of insect functional groups summed across the four sampling periods. Insect community composition We visualized whether native vs. introduced trees supported unique insect communities using non-metric multidimensional scaling (NMDS) plots. For this analysis, we aggregated all insects collected over the four sampling periods for each individual tree. Each tree species thus contributes five data points to the NMDS plot. We used the following parameters: k=3, 22 maxit=999, trymax=1000, distance = bray. We next used analysis of similarities (ANOSIM) to test whether native trees clustered more closely to each other than to introduced trees. We repeated the above analyses to test whether coniferous trees supported unique communities of insects, in comparison to those found on deciduous trees. We used the non-parametric ANOSIM instead of the quasi-parametric PERMANOVA because the five replicate trees per tree species are not independent samples. Community composition over time We visualized insect communities from native and introduced coniferous and deciduous trees at each sampling point using NMDS plots. We used the parameters k=3, maxit = 999, trymax = 1000, and distance = bray for each NMDS plot. We then used ANOSIM to test whether insect communities clustered more closely within their sample period than to communities collected at other sample periods. We repeated the above analyses for each combination of tree origin and tree type (coniferous-native, coniferous-introduced, deciduous-native and deciduous-introduced trees). Microbial Richness We investigated the effect of tree origin and tree type on microbial observed richness. Microbial richness here is defined as the number of unique ASVs found per tree sample. We used linear mixed-effects models with microbial richness as the dependent variable, tree origin (or tree type) as the independent variable, and tree species as a random effect. Substituting observed richness with Chao1, Shannon, or Simpsonâs diversity indices did not affect the overall results and thus we report model results for observed richness only. 23 Microbe Community Composition We used NMDS plots to visualize whether native and introduced trees differed in their microbial community composition (distance=âbrayâ, dimensions=2). Similar to the analyses of insect community composition, we used ANOSIM to test whether native trees clustered more closely to each other (based on their microbial communities) than to introduced trees. We repeated this analysis to test whether coniferous and deciduous trees formed unique groupings based on their microbial community. Similar to the analyses for insect community composition, we used the non-parametric ANOSIM instead of PERMANOVA because the five replicate trees per tree species are not independent data points. Relationship between insect abundance/richness and microbial richness Finally, we investigated whether insect abundance or richness was correlated with microbial richness. Because phyllosphere microbes were only collected during sampling period one, we only included the insects collected at this time. The explanatory variables were insect abundance (log) or insect richness (log) and the dependent variable was microbial richness. We used linear models without the random effect of tree species for this analysis because there were sufficient trees missing from the data (after microbe data cleaning and low insect abundances in sampling period one) that several tree species were only represented once in this analysis. 24 Table 1. Tree species included in this study. Order, family, and species are listed. Common names are from Canadensys database of vascular plants of Canada. Order Family Species Common Name Provenance Type Saxifragales Cercidiphyllaceae Cercidiphyllum japonicum Siebald & Zucc. Katsura Introduced Deciduous Cornales Cornaceae Cornus nuttallii Audubon Pacific Dogwood Native Deciduous Cornales Cornaceae Cornus kousa Bürger ex Miq. Kousa Dogwood Introduced Deciduous Cuppressales Cupressaceae Thuja plicata Donn ex D. Don Western Red Ceder Native Coniferous Ericales Ericaceae Arbutus menziesii Pursh Pacific Madrone Native Deciduous Fagales Fagaceae Quercus rubra L. Northern Red Oak Introduced Decidious Pinales Pinaceae Pinus contorta Douglas ex Loudon Lodgepole Pine Native Coniferous Pinales Pinaceae Pinus nigra J.F. Arnold Austrian / Black Pine Introduced Coniferous Pinales Pinaceae Pseudotsuga menziesii Mirb. Douglas Fir Native Coniferous Pinales Pinaceae Tsuga heterophylla Sarg. Western Hemlock Native Coniferous Rosales Rosaceae Prunus serrulata Lindl. Japanese Cherry Introduced Deciduous Sapindales Sapindaceae Acer circinatum Pursh Vine Maple Native Deciduous Sapindales Sapindaceae Acer griseum (Franch.) Pax Paperbark maple Introduced Deciduous Sapindales Sapindaceae Acer palmatum Thunb. Japanese maple Introduced Deciduous Sapindales Sapindaceae Acer platanoides L. Norway Maple Introduced Deciduous Sapindales Sapindaceae Acer rubrum L. Red Maple Introduced Deciduous 25 2.3 Results Overall, we collected 1432 insect specimens across the four sampling periods. 747 specimens were collected from native and 685 from introduced trees. We collected 738 specimens from deciduous and 694 from coniferous trees. Specimens were assigned to 108 morphospecies spanning 34 families, of which Aphidoidea (343), Coccinellidae (198), Miridae (178), Cicadellidae (113), Lygaeidae (109) were most abundant. Specimens represented 40 genera, with Campyloneura (128), Kleidocerys (109), Psyllobora (88), Dikraneura (79), Ceratocapsus (78) having the greatest number of specimens. In addition, 106 specimens were Coccinellidae (53) and Lygaeidae (38) juveniles. Collected specimens belonged to two feeding types (Beaked (987), or Mandibular (368) and represented 6 functional groups (Sap Herbivore (515), Predator (360), Polyphagous (179), Seed Herbivore (159), Detritivore (60), and Others (90) [Leaf Herbivore, Nectar / Pollen, Sap Herbivore / Predator, Nectar / Blood]). For analysis involving insect richness, juvenile specimens were removed from the data set, for a total of 945 adult specimens. Insect abundance, richness, and community composition Insect abundance declined over the sampling season (Fig. 2, F3, 187 = 4.2, p = 0.007) with post-hoc tests revealing that significantly fewer insects were collected at sampling period 4 ( xÌ ï± sd: 4.83 ï± 6.6) than sampling period 1 (7 ï± 7.2). The total number of insects per tree did not differ between native vs. introduced tree species (Fig. 2, F1, 13 = 0.1506, p = 0.70, native trees: 6.2 ï± 7.3, introduced: 5.3 ï± 6.1). More insects were collected from coniferous than deciduous trees (Fig. 2, coniferous: 7.6 ï± 8.8, deciduous: 4.65 ï± 4.8) but this difference was not statistically significant (F1,13=4.2, p=0.06). 26 Native trees supported higher insect richness (Fig. 3, F1,14=7.6, p=0.02; Native xÌ ï± sd: 2.62 ï± 1.54, introduced:1.89 ï± 1.16) and richness did not change over the growing season (F3,185=1.4, p=0.23). Insect richness did not differ between coniferous and deciduous trees (F1,13 = 3.02, p = 0.1; 2.66 ï± 1.55 and 2.01 ï± 1.26 respectively). Native vs. introduced tree species supported different insect communities (ANOSIM R=0.058, p=0.012, Fig. 4). Insect communities also differed between coniferous and deciduous trees (ANOSIM R=0.253, p=0.001, Fig. 4). Insect communities on native and introduced trees changed over time (Fig. 5a-d). We observed clustering by sampling period for both native (Fig. 5a, ANOSIM R=0.135, p=0.03) and introduced (Fig 5b, ANOSIM R 0.19, p<0.001) trees, with a more pronounced shift in insect communities over time in introduced vs. native trees (Fig. 5a,b) When trees were grouped by type (coniferous vs. deciduous) instead of origin, there were clear shifts in insect community composition over time for deciduous (Fig. 5c; ANOSIM R=0.22, p<0.0001) but not coniferous (Fig. 5d, ANOSIM R=0.15, p=0.07) trees. Finally, there was no difference in tree height (F1,14.2=1.83, p=0.2) or leaf SLA (F1,14.3=1.97, p=0.18) between native and introduced trees (Fig. S1). 27 Figure 1. Effect of tree origin, tree type and sampling period on insect abundance (log). Orange and green colours represent introduced and native tree species, respectively. Triangles and circles represent coniferous and deciduous trees, respectively. Error bars are 95% confidence intervals. Sample periods are as follows: Period 1: June 6th â 22nd, 2: June 27 â July 17th, 3: July 27th â August 1st, 4: August 2nd â August 17th. 28 Figure 2. Effect of tree origin, type and sampling period on insect richness (log). Orange and green colours represent introduced and native tree species, respectively. Triangles and circles represent coniferous and deciduous trees, respectively. Error bars are 95% confidence intervals. Sample periods are as follows: Period 1: June 6th â 22nd, 2: June 27 â July 17th, 3: July 27th â August 1st, 4: August 2nd â August 17th. 29 a) b) Figure 3. NMDS plots of insect communities colored by (a) tree origin or (b) tree type and sampling. (a) Orange and green represent introduced and native tree species respectively. (b) Blue and pink represent coniferous and deciduous trees respectively. 30 a) b) c) d) Figure 4. NMDS plots of insect communities on native (a), introduced (b), deciduous (c) and coniferous (d) trees over sampling time. Sample periods are denoted by solid (1: June 6th â 22nd), long dashes (2: June 27th â July 17th), short dashes (3: July 27th â August 1st) and dotted lines (4: August 2nd â August 17th). 31 Insect Functional Diversity The proportions of insect functional groups varied among the four tree groupings (Fig. 6, X2=237.12, df=24, p<0.0001). Notable differences in insect functional groups included high proportions of sap herbivores in introduced coniferous and deciduous trees, elevated proportions of polyphagous insects in introduced deciduous trees, and high proportions of predators in native deciduous trees. These patterns were relatively consistent over the growing season (Fig. A6). 32 Figure 5. Insect functional trait proportions on native and introduced coniferous and deciduous tree hosts 33 Microbial richness and community composition Microbial richness was not affected by tree origin (F1,13.2=0.13, p = 0.72) or tree type (F1,12.9 = 0.58, p = 0.46). Native and introduced trees did not support unique microbial communities (ANOSIM R=0.013, p = 0.22, Fig. 6a) but coniferous and deciduous trees clustered uniquely (ANOSIM R= 0.27, p=0.001 Fig. 6b). Lastly, there was no relationship between insect abundance and microbe richness (log; F1,53=0.89, p=0.35, Fig. 7a) or between insect richness and microbe richness (log; F1,48=0.01, p=0.9, Fig. 7b). 34 Figure 6. NMDS plots of individual trees clustered by their microbial communities. (a) Orange and green dots represent introduced and native trees respectively. (b) Pink and blue dots represent deciduous and coniferous trees respectively. a) b) 35 a) b) Figure 7. Relationship between (a) insect abundance and microbial richness and (b) and insect richness and microbial richness in sampling period 1. Points are jittered slightly to prevent overlap. Dots represent individual trees. 36 2.4 Discussion The overall goal of this study was to test the hypothesis that native species of urban trees support higher abundance and diversity of insects as compared to introduced trees. Additionally, we investigated whether tree provenance helped explain variation in the phyllosphere microbial community, and whether higher insect diversity or abundance was correlated with elevated phyllosphere microbial richness. Across the growing season, we found that compared to introduced trees, native trees supported similar numbers of insects, but 57% higher richness, different proportions of functional groups, and unique insect communities. Native and introduced trees did not differ in their phyllosphere microbial richness or community composition. Furthermore, insect richness or abundance was not correlated with microbial richness. Our findings suggest that despite native and introduced trees being planted in close proximity in urban environments, tree provenance plays an important role in shaping insect communities. This result is important because ecosystems benefit from broader and more complex communities of species (Hooper et al. 2005, van der Plas 2019). Urban areas with a greater proportion of native trees may thus receive elevated rates of insect-related ecological functions such as nutrient cycling and pest control. From an insect conservation perspective, prioritizing the planting of native vs. introduced trees in urban areas may help to slow rates of insect declines around the world (Tallamy et al. 2021). Native and introduced trees supported similar microbial richness and community composition. Bacteria are highly mobile and widely distributed, suggesting that if phyllosphere micro-environments allow establishment, bacteria may not distinguish between ânativeâ and âintroducedâ ranges of host species. This result is supported by several studies reporting that host 37 tree identity plays a central role in the structuring of phyllosphere bacterial communities, likely driven through species-specific leaf traits (Laforest-Lapointe et al. 2016, Li et al. 2022). In contrast, Lu-Irving et. al (2019) reported lower microbial diversity on invading populations of star thistle compared to native populations but the mechanisms underlying this difference were unknown. Unlike studies reporting positive relationships between insect herbivory and phyllosphere microbial diversity (Müller et al. 2003, Allard et al. 2018, Humphrey and Whiteman 2020), we found no relationship between insect richness or abundance and microbial diversity. Our result and these previous studies may not be directly comparable as we surveyed more than just herbivores. However, given known seasonal variation in the leaf microbial communities (Li et al. 2022), the fact that we only collected phyllosphere microbiome samples during the first sampling period suggests that we likely only captured a small fraction of total diversity. In general, the relationship between insect and microbial diversity on the phyllosphere remains under-studied, focusing mostly on a few host-bacteria-species groups rather than communities as a whole (Sugio et al. 2015, Wielkopolan and ObrÄpalska-StÄplowska 2016). Given the importance of both taxonomic groups for ecological functioning and tree health, more data are needed to better understand the general relationships between these two taxonomic groups. Our finding of higher insect richness on native vs. introduced trees adds to the small number of studies that have also documented this pattern (Mata et al. 2021, Jensen et al. 2022). Unlike these two previous studies our study did not take place in city parks, but rather in a large, densely populated university campus which is largely a microcosm of the larger surrounding city. That higher insect richness was documented from both native trees in city parks in Melbourne, 38 Australia (population 5 million) and Malmö, Sweden (population 365,000) and from native trees on a city-like university campus in Vancouver, Canada (population 600,000) cautiously suggests that the pattern of native trees supporting elevated insect diversity may be generalizable across temperate cities across a range of population sizes. Clearly more studies across many different parts of the world are needed to verify this statement but for now the evidence suggests that native trees are more likely to support diverse communities of insects, compared to introduced trees. Native trees supported different proportions of insect feeding types and functional guilds, compared to introduced trees. For example, we found high abundances of the ladybug Psyllobora borealis on native trees. This species does not feed on the tree directly, but instead uses it as a habitat for predation of other insects. In this way, host characteristics extending beyond palatability to herbivorous insect species shapes the communities residing upon them. With insect richness peaking during the first sampling period (June 6th â 22nd ), supporting spring time richness is essential for greater diversity of insect species throughout the growing season, especially chewing herbivores (Richard Southwood et al. 2004). The lowest insect richness was found on five introduced trees, with Acer platanoides, Acer rubrum, Cercidiphyllum japonica, Cornus kousa, and Prunus serrulata, supporting on average less than five insect species per sampling event. Several of these species (i.e. A. rubrum, C. japonica, C. kousa, P. serrulata) are popular ornamental tree species, which are often selected by homeowners or practitioners for their floral displays or foliage (Conway 2016, Guo et al. 2019). In this way, human aesthetic preferences may be leading to the proliferation of host species that are poorly suited to supporting insect biodiversity. From our selected host species, native 39 conifers (i.e. Pinus contorta, Pseudotsuga menziesii, and Tsuga heterophylla) supported the greatest richness among all sampled trees, closely followed by native deciduous Arbutus (see supplemental figures for full list). For these reasons, co-planting of native deciduous and coniferous species may maximize the potential support for insect communities, increasing functional trait diversity, redundancy of ecological function provisioning, and encouraging robust and resilient ecological communities. We also documented differences in insect abundance (Fig. 2), proportions of functional groups (Fig. 6), and in insect (Fig. 4) and microbial community composition (Fig. 7) between coniferous and deciduous trees. Due to the availability of tree species at the study site, we were not able to create a balanced sampling design and thus there were more coniferous tree species included in the native vs. introduced tree groupings. Our results are not solely due to differences in native vs. introduced coniferous and deciduous trees. When we just compare insect richness between native and introduced deciduous trees, the same pattern of higher insect richness on native trees appears (p=0.03). The same comparison is not possible for coniferous trees because only one introduced coniferous tree species was included in the study. Our microbial communities revealed dominant representation by taxa including Pseudomonas, Hymenobacter, Mucilaginibacter, Erwinia and Buchnera (Fig S1), which are common phyllosphere residents (Hirano and Upper 2000, Wu et al. 2021), opportunistic pathogens (Hirano and Upper 2000) and gut symbionts (Feng et al. 2019). Pseudomonas and Hymenobacter include species that are integral to nutrient cycling, stress tolerance and opportunistic pathogen infections (Wuyts et al. 2020). Mucilaginibacter contribute to nutrient cycling through 40 polysaccharide degradation (Khomutovska et al. 2024). Additionally, the presence of Buchnera, an obligate aphid gut symbiote, represents contributions to the phyllosphere via insect interactions, further illuminating the need for study into the interplay between host, insects and bacteria within these systems. Our findings provide preliminary evidence that native trees on this large university campus supported greater insect richness compared with introduced trees. Showy ornamental tree species were especially poor at supporting insect diversity within urban centers. With cities looking to increase urban forest canopy cover, we recommend prioritizing native coniferous and deciduous trees to maximize wildlife habitat and resource provisioning. Urbanization has resulted in unique plant assemblages, with introduced trees forming the majority of the urban canopy (McCoy et al 2021). Our data suggest the continued replacement of native trees with introduced species may further exacerbate the loss of insects worldwide. Our study adds to a small body of literature demonstrating the importance of native trees for maintaining or augmenting insect diversity. 41 Chapter 3: Conclusion Our study provides key insights into the communities of insects and bacteria hosted by urban street trees. Compared with introduced trees, native trees supported higher a higher number of insect species. This finding suggests that native trees are important resources for preserving insect diversity within urban centers. Coniferous trees within our communities supported higher abundance of mostly small-bodied insects than their deciduous neighbors, suggesting that mixed co-plantings may support greater provisioning of biotic services. Introduced trees supported similar abundances of insects to native hosts, but decreased species richness, which may be contributing to greater biotic homogenization across larger scales. We additionally noted decreased abundance of sampled insects as the growing season progressed, suggesting that sampling during early season monitoring windows is vital for capturing the greatest proportion of biodiversity that may be supported within urban communities. We found large tree-to-tree variation in the microbial community composition in the phyllosphere, which mirrors findings within the field. Finally, while we found no correlation between insect and microbial diversity, we encourage research to further illuminate the factors shaping these complex interactions. Our tree selection represents 63.9 % of genera planted by the City of Vancouver, with 48% of all trees within the city belonging to Acer or Prunus genera (City of Vancouver). While insect collection was limited to beat-sheet collection methods to minimize disturbance and avoid the use of chemical pesticides, our abundances of collected specimens are similar to other studies in the field, accounting for sampling efforts (Helden et al. 2012, Berthelot et al. 2023). Many studies predominantly sample during narrow temporal windows, offering limited insight into the 42 dynamics of communities as a whole (Matteson and Langellotto 2011, Helden et al. 2012, Goodwin et al. 2017). Our season-long sampling effort instead offers insights into the communities of insects throughout the growing season. Finally, although our microbial community investigation was less extensive than our insect collections, it offers valuable insight to future research directions. Street trees are often selected by homeowners for ornamental characteristics and short growing stature, disproportionately creating demand for introduced tree species within urban centers (Dilley and Wolf 2013, Conway and Vander Vecht 2015, Conway 2016). Our study sampled 80 street trees individuals with limbs readily available below 2 meters in height, indicating that native trees are able to support demands for short ornamental growth statures, while also provisioning vital support to insect communities within the urban matrix. In addition, our study supports research indicating that conifers and deciduous trees differ in their insect community composition and functional trait provisioning. This suggests that co-planting and intermixing of host foliage types may promote the widest cross section of insect functionality. Our investigation into the leaf phyllosphere bacterial communities indicated clear tree-to-tree variation, regardless of provenance (Lajoie and Kembel 2021). To maximize bacterial diversity, we suggest practitioners select trees from a variety of genera to promote wide breadth of microbial diversity. Planting of native species would further promote the synchronous provisioning of insect habitat and forage opportunity. Finally, while our results did not show a relationship between insect and microbial communities, laboratory studies indicate that high degrees of pest insect infestation can lead to changes in the microbiome of the host (Humphrey et 43 al. 2014, Humphrey and Whiteman 2020). Our study provides exciting insight into preliminary investigations of these host-herbivore-epiphyte community dynamics. 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Brown, and D. W. Tallamy. 2008. Effects of non-native plants on the native insect community of Delaware. Biological Invasions 10:1159â1169. 62 Appendices Additional Tree Traits Figure A1 The relationships of tree origin and tree height (a) and specific leaf area (SLA). There was no difference in tree height (F1,14.2=1.83, p=0.2) or leaf SLA (F1,14.3=1.97, p=0.18) between native and introduced trees. a) b) 63 Figure A2. Insect abundance by tree species. Tree sampling events are summed such that each tree contributes 1 point per species. Introduced species are represented with orange dots, and native with green. Error bars are 95% CI. Tree Species 64 Figure A3. Insect richness by tree species. Tree sampling events are collated such that each tree contributes 1 point per species. Introduced species are represented with orange dots, and native with green. Error bars are 95% CI 65 Figure A4. 10 most prevalent microbial taxa for each tree host. Tree host identity and growth form is represented by icon color and shape, respectively 66 Figure A5. Proportion of insect functional groups on introduced and native coniferous and deciduous trees at each sampling time point. 67 Figure A6. NMDS plot of insect communities on native and introduced coniferous and deciduous trees. Orange and green dots represent introduced and native tree species, respectively. Triangles and circles represent coniferous and deciduous species, respectively. Figure A6 displays the same ordination as Figure 4, but with both tree origin and type displayed.
Thesis/Dissertation
eng
Botany
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Tseng, Michelle
Ignace, Danielle
Quantifying the insect and microbial diversity supported by native and introduced trees on a large urban university campus
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