Effects of disturbance type and microhabitat on species and functional diversity relationship in stream-bank plant communities

Abstract Spatial heterogeneity in site conditions and differing disturbance types are common features of natural landscapes. We asked: does the relationship between species diversityand functional trait diversity (SD-FD relationship) of plant communitiesvary among habitats experiencing different types of disturbance and among microhabitats (i.e., landscape position) within a landscape? We quantified the relationship between species richness and functional richness (FRic) and between species evennessand functional evenness (FEve) of riparian plant communitiesalong small headwater streams at five spatially adjacent microhabitats in two types of disturbed (wildfire and clearcutlogging with buffer) and reference habitats (unlogged mature forest) in northwestern Ontario, Canada. We found significant dependence of species richness vs. FRic relationship on microhabitat, but not on disturbance type. Species evennessvs. FEve relationship varied slightly among reference and wildfire sites, but not among microhabitats. A significant variation in functional trait dispersion (FDis) among microhabitats offered a mechanistic explanation to the observed variation in the SD-FD relationship. A very weak effect of disturbance type on the SD-FD relationship indicates that clearcutwith buffer retention may emulate riparian plant composition created by wildfire. However, the microhabitat dependent variation in the SD-FD relationship indicates that the same range of species diversitymay refer to different ranges of functional trait diversity depending on microhabitat. We suggest that microhabitat plays a stronger role than disturbance type in trait dispersion and ultimately modifies the SD-FD relationship in our studied communities. This result highlights the role of spatial environmental heterogeneity of a landscape and different dimensions of species diversityand functional trait diversity, e.g. richness and evenness, in understanding the functioning of a natural landscape.