Urban Metacommunities: The role of local and regional factors on plant community assembly and functional trait patterns in urban vacant land

Abstract

The urban landscape is a collective outcome of both natural and anthropogenic forces. These forces strongly influence the structure and functioning of urban biotic communities, often in unique and unexpected ways. Contrary to prevailing assumptions, cities are capable of hosting a high diversity of plant species, which often represent distinct suites of functional strategies. Although this pattern has been identified across cities globally, few generalities about the processes responsible for driving plant community assembly patterns and the expression of functional traits in urban environments have been established. Using a metacommunity approach, this dissertations investigates the effects of local and regional processes on plant community assembly and functional trait patterns in urban vacant land habitats. Chapter 1 summarizes the literature, highlighting key ecological concepts and theories and how they apply to novel urban ecosystems. Chapter 2 presents results from an observational study that characterized seed dispersal and functional trait patterns of an urban species pool. Results showed that the majority of urban species had small (< 0.01 g), spherical seeds that disperse via anemochory (wind) and anthropochory (humans). Thereby indicating that plant species persisting in urban vacant land habitats display distinct seed trait and dispersal characteristics, commonly associated with ruderal species. Chapter 3 summarizes an experimental study that assessed the relative importance of trait variation in response to local environmental filtering effects (i.e., soil quality). Here, trait variation was decomposed and compared across three levels: within-species (ITV), between-species (BTV), and across functional groups (FGTV) for 56 urban plant species. Results showed that ITV explained a substantial proportion of the total variance for the majority of traits. Likewise, species representing specialized strategies (i.e., C4 photosynthesis and legumes) exhibited distinct functional responses compared to groups with more general strategies. These results indicated that accounting for ITV, and how it relates to general functional responses, can provide a clearer understanding of plant eco-evolutionary dynamics in urban environments. Finally, Chapter 4 presents an experimental common garden study that investigated the relative influence of local assembly processes (i.e., soil environmental filtering and competition) on the taxonomic and functional diversity of native plant communities over time. Results showed that native species were not only able to establish and persist in experimental urban habitats, they were capable of adapting to local filtering pressures by shifting functional trait values over time. These results suggested that regional dispersal limitation is the primary factor limiting native species in urban environments. Overall, both local and regional processes were shown to uniquely influence plant communities inhabiting urban vacant land. Future research focusing on the process of dispersal and within-species functional trait patterns are likely to provide valuable insights into urban plant community and eco-evolutionary dynamics, while also providing practical guidance to urban planning and the regreening of urban vacant land.