Occurrence of Vegetation on Geomorphic Surfaces in the Active Floodplain of a California Alluvial Stream

Abstract

In the active floodplain of Cottonwood Creek, an alluvial stream in California, conditions for the establishment and growth of plants are largely controlled by periodic flooding. Flooding creates and modifies erosional and depositional surfaces which are occupied by different species. To evaluate patterns of species occurrence and dominance, geomorphic surface on Cottonwood Creek''s floodplain were defined on the basis of vertical and horizontal position relative to the stream (i.e., flood frequency), microtopography and particle size. Vegetation sampling on these surfaces provided data for classifying communities by relative cover of common riparian species. The results indicated zonation of communities dominated by different species in relation to flood-induced disturbance. Plant communities dominated by Salix hindsiana and annual grasses were found on surfaces frequently flooded and subject to severe scouring or deposition. Populus fremontii attained dominance or shared dominance with S. hindsiana on less frequently flooded surfaces and where particle size indicated less disturbance by erosion or deposition. Juglans hindsii, Quercus lobata or mixed stands of P. fremontii/J. hindsii/Q. lobata/S. hindsiana were dominant on infrequently disturbed surfaces.