Despite a growing literature-base devoted to documenting biodiversity
patterns in cities, little is known about the processes that influence
these patterns, and whether they are consistent over time. In
particular, numerous studies have identified the capacity of cities to
host a rich diversity of plant species. This trend, however, is driven
primarily by introduced species, which comprise a large proportion of
the urban species pool relative to natives. Using an experimental common
garden study, we assessed the relative influence of local assembly
processes (i.e., soil environmental filtering and competition from
spontaneous urban species) on the taxonomic and functional diversity of
native plant communities sampled over four seasons in 2016-2018.
Taxonomic and functional diversity exhibited different responses to
local processes, supporting the general conclusion that species- and
trait-based measures of biodiversity offer distinct insights into
community assembly dynamics. Additionally, we found that neither soil
nor competition from spontaneous urban species influenced taxonomic or
functional composition of native species. Functional composition,
however, did shift strongly over time and was driven by
community-weighted mean differences in both measured traits (maximum
height, Hmax; specific leaf area, SLA; leaf chlorophyll a fluorescence,
chl a) and the relative proportions of different functional groups
(legumes, annual and biennial-perennial species, C4 grasses, and forbs).
In contrast, taxonomic composition only diverged between early and late
seasons. Overall, our results indicate that native species are not only
capable of establishing and persisting in vacant urban habitats, they
can functionally respond to local filtering pressures over time. This
suggests that regional dispersal limitation may be a primary factor
limiting native species in urban environments. Thus, future regreening
and management plans should focus on enhancing the dispersal potential
of native plant species in urban environments, in order to achieve set
goals for increasing native species diversity and associated ecosystem
services in cities.
Agricultural extensification enhances functional diversity but not phylogenetic diversity in Mediterranean olive groves: A case study with ant and bird communities
