Environmental filtering and spatial processes in urban riparian forests

Questions: What are the spatial processes structuring plant trait composition in urban riparian forest communities at different spatial scales? What arc the relative roles of local conditions (including historical aspects), landscape context and spatial processes in the community assembly of these forests? Location: Montreal, Quebec, Canada. Methods: Species plant composition was inventoried in 57 riparian forests located along a gradient of urbanization. To analyse plant communities in terms of their trait composition, community-weighted means were calculated using eight functional traits. Forests were characterized by local (physical.features, hydrological regime and historical disturbances) and landscape (surrounding land use) variables. Spatial processes stnicturing comm unities were assessed using Moran's eigenvector maps and asymmetric eigenvector maps. The relative importance of these three subsets (local, landscape and spatial variables) on tree, shrub and herb functional composition was quantified by variation partitioning using redundancy analyses. Results: Functional patterns in riparian forests resulted primarily :front environmental filtering (local and landscape variables). Local conditions, especially flood intensity, exerted an overriding selection pressure on functional composition of riparian plant communities. Urbanization seemed to act indirectly on trait patterns through the alteration of hydrological disturbances caused by ongoing and historical land transformation. Nevertheless, dispersal along rivers was also a significant structuring force, while overland dispersal was negligible. Conclusions: Our study highlights that under severe natural disturbance regimes, the effect of natural filters outweighed the negative effects of urban filters. However, the alteration of natural flooding processes by human activities is also a major mechanism influencing plant trait composition in urban riparian communities as forests subjected to reduced flooding intensity experienced a greater effect of urbanization, The effects of urbanization and of past land uses on plant communities were greater for trees than for shrubs and herbs due to the high turnover rate of the latter. Finally, our results showed the importance of dispersal along rivers for biodiversity even in fragmented urban landscapes.