Patterns of plant diversity and plant–environmental relationships across three riparian corridors

Abstract

Riparian corridors are often the ecosystem level component most sensitive to changes within the surrounding environment. High levels of variability and heterogeneity commonly characterize plant communities and plant species within riparian corridors. The overall aim of this study was to assess and characterize riparian forest vegetation (trees and shrubs) and its relationship to environmental attributes along three different riparian corridors. The three corridors represented different levels of disturbance. We wanted to determine if and how disturbance level in the riparian corridor, defined by existing land cover, might be influencing patterns of both native and non-native woody vegetation. Our specific objectives were to: (1) characterize and classify the tree and shrub layers across the three riparian corridors; (2) determine if riparian disturbance was a good predictor of plant diversity patterns; (3) assess the relationships between geomorphology and soils and tree and shrub vegetation; (4) ascertain any linkages between the tree and shrub layers; and (5) characterize the extent of non-native tree and shrub species across corridors. Our assumption that species diversity was negatively correlated with land cover disturbances was not fully supported. In addition, shrub species and tree species appear to be responding to different underlying gradients and in different ways. Detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) ordinations showed four distinct tree species assemblages that appeared to be responding to both underlying soil (pH, base saturation, NO 3, Ca) and morphological (height above river, slope) gradients. However, no unique shrub assemblages could be distinguished in the DCA and CCA ordinations. In addition, other than pH, shrub plots did not exhibit strong correlations with any of the environmental variables measured. Thus, the tree and shrub layers appear to be uncoupled. The shrub layer does appear more susceptible to invasive non-native species than the tree layer although both non-native tree and shrub species appear to have been somewhat evenly distributed across the three riparian corridors. Overall, this study identified key underlying gradients, abiotic conditions and major soil factors that are influencing tree species in the three riparian corridors studied. The importance of shrub species as well as low frequency, non-dominant tree species contributions to diversity in these riparian forests should be emphasized in protection and conservation efforts. Disturbance, as determined by large-scale land cover patterns, provided only a partial explanation of woody species composition, structure and diversity in the riparian corridors.