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

Two prominent conceptual frameworks, environmental gradients and patch hierarchies, are used in combination to describe vegetation patterns along a riparian corridor in a semi-arid South African system. We adopt both approaches, since riparian corridors are characterized by both strong environmental gradients above, away from and along the river, as well as a mosaic of patches in the geomorphology at multiple hierarchical scales. Constrained and unconstrained ordinations were used to determine the variability in vegetation pattern accounted for by the gradient and the geomorphic patch hierarchy data sets. The gradient data set consisted of vertical, lateral and longitudinal dimensions of the macro-channel, while the patch hierarchy data set consisted of substratum type, morphological unit and channel type. Elevation up the macro-channel bank, of the gradient data set, explained the main variation in vegetation pattern, and alluded to overriding processes of flooding frequency and water availability as determinants of vegetation pattern. Along the fluvially dynamic macro-channel floor (lower elevation range), patchiness at the scale of the morphological unit best explained vegetation pattern. This relationship with morphological units suggests that the formation 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 patch mosaic at different spatial and temporal scales, the combined use of both perspectives to develop a fuller understanding of vegetation pattern is imperative and is encouraged.