Effects of fragmentation on the plant functional composition and diversity of remnant woodlands in a young and rapidly expanding city

Questions: How do plant functional trait abundance and diversity in urban remnants of a rapidly urbanizing city change with fragmentation? Is there a delayed functional response to fragmentation?

Location: Thirty remnant Banksia woodlands, Perth, Australia.

Methods: We used GLMM to examine the effects of remnant age and area, and their interaction, on the relative abundance and functional diversity (FD) of five plant functional traits: growth form, pollination, seed dispersal, nutrient acquisition and regeneration strategies. We then used fourth-corner analysis to examine the influence of a wider set of fragmentation-related factors on trait abundances.

Results: The functional composition and diversity of Banksia woodlands changed with remnant age, particularly in the smaller remnants. Plants more prone to decline with remnant age were the growth form shrubs, root-clustered trees, herbaceous obligate seeders and understorey species that are insect-pollinated, have seeds dispersal internally by animals and have arbuscular or ericoid mycorrhizas. In contrast, plants more prone to persist were growth forms trees, sedges and rushes, ectomycorrhizal trees, herbaceous resprouters, wind-pollinated and root-clustered understorey species. FD increased with remnant age in the growth forms and overstorey, but declined among the herbaceous and shrub pollination and nutrient acquisition traits.

Conclusions: Functional traits that consistently signalled the plant community response to fragmentation were growth form, pollination and dispersal. This functional response was largely delayed, suggesting a functional extinction debt, which will lead to a further decline of plants with vulnerable trait states in the future, especially in the small- and medium-sized remnants. Our study illustrates the vulnerability of small remnants to changes in community assembly and ecosystem function due to fragmentation. Furthermore, it exemplifies how a functional trait approach is valuable to understand the impacts of urbanization on remnant plant communities, before local extinctions may occur. Finally, the study shows how cities' fragmentation history and biogeographic settings provide an important context influencing plant functional responses to urbanization-related processes.