Disentangling the role of environmental filters and spatial processes in structuring plant functional communities in urban riparian forests
Urbanization is a global change driver and a major threat to biodiversity. In cities, many ecological filters exert a strong selective pressure on species traits, leading to new distribution patterns of biodiversity. Species and functional trait composition within riparian plant communities is rapidly changing in response to urbanization, raising concerns about the sustainability of associated ecological functions.
This project aims to understand the effects of urbanization on riparian functional communities. Using a trait-based approach, we address the following questions. What are the key environmental filters shaping riparian communities in cities? And, how important is dispersal by water current in functional composition patterns? Floristic inventories were carried out in 57 riparian forests of Montreal area in 2013. Community weighted means (CWM) were computed using eight functional traits. To assess the role of environmental filtering, each forest was characterized by a subset of local variables, describing their physical features (e.g., area), hydrological regime and historical disturbances, and a subset of landscape variables, describing surrounding land use. A third subset of unidirectional spatial variables, modelled by asymmetric eigenvectors maps (AEM), was used to assess species dispersal along rivers. The relative importance of these three subsets was quantified by variation partitioning using redundancy analyses.
Across the riparian forest network, together, landscape context, local conditions and spatial processes explained about 50% of the functional composition variation and most of this variation was spatially structured. According to the variation partitioning, plant trait patterns were mainly explained by the unique fraction of local variables (9%), followed by spatial variables (5%) and, to a lesser extent, landscape variables (3%). The large shared variation between environmental and spatial subsets (20%) suggest that local and landscape conditions presented a strong spatial structure, reflected in community composition patterns. Indeed, while urbanization favored annual shade-intolerant species, flood intensity favored water-dispersed wetland species.
The functional patterns in riparian forests resulted from both environmental filtering and dispersal limitation processes, although local conditions, especially flood intensity, exerted an overriding selection pressure on riparian functional communities. The effect of the landscape was possibly indirect, acting on trait patterns through altered hydrological disturbances in riparian forests caused by ongoing urbanization and historical land transformation. Dispersal along rivers was also an important process in structuring riparian forests, suggesting that the establishment of vegetation corridors along rivers could be an effective method for preserving functional diversity in urban areas and mitigate urbanization impacts on plant communities, such as fragmentation.