Riparian vegetation-environment relationships: complimentarity of gradients versus patch hierarchy approaches

Two prominent conceptual frameworks, environmental gradients and patch hier archies, are used in combination to describe vegetation patterns along a ri parian corridor in a semi-arid South African system. We adopt both approach es, since riparian corridors are characterized by both strong environmental gradients above, away from and along the river, as well as a mosaic of pat ches in the geomorphology at multiple hierarchical scales. Constrained and unconstrained ordinations were used to determine the variability in vegetat ion pattern accounted for by the gradient and the geomorphic patch hierarch y data sets. The gradient data set consisted of vertical, lateral and longi tudinal dimensions of the macro-channel, while the patch hierarchy data set consisted of substratum type, morphological unit and channel type. Elevati on up the macro-channel bank, of the gradient data set, explained the main variation in vegetation pattern, and alluded to overriding processes of flo oding frequency and water availability as determinants of vegetation patter n. Along the fluvially dynamic macro-channel floor (lower elevation range), patchiness at the scale of the morphological unit best explained vegetatio n pattern. This relationship with morphological units suggests that the for mation of well developed alluvial bars, and the degree of bedrock influence are important processes. The nested hierarchical framework used provided a good basis for identifying scale specific pattern in a relational manner. In systems characterized by strong environmental gradients as well as a pat ch mosaic at different spatial and temporal scales, the combined use of bot h perspectives to develop a fuller understanding of vegetation pattern is i mperative and is encouraged.