Riparian zones of the semiarid Great Basin supply numerous ecosystem services and resources for humans, but many are in poor ecological condition. Riverine ecosystems are hierarchical in nature, and hydrogeomorphic conditions influence riparian vegetation and its response to disturbance at multiple spatial scales. Our objective was to determine how watershed- and local-scale geomorphic controls work independently and interactively to shape riparian vegetation distribution and sensitivity. This research was conducted in small, upland watersheds of central Nevada, USA. At the watershed scale, we identified the relative proportions of riparian forest, riparian shrub, wet/mesic meadow, dry meadow, and shrub dry meadow. We calculated geomorphic variables, such as watershed area, shape, ruggedness, and lithology, that influence hydrogeomorphic processes and disturbance regime. At the local scale we classified stream sections into “process zones” based on characteristic geomorphic behaviors and structure (geometry, channel planform, and assemblage of geomorphic units). Within process zones we characterized valley and channel attributes and riparian plant species occurrence and abundance. Sensitivity indicators included number of terraces, incision depth, and stream power. We used multivariate and regression analyses to relate geomorphic characteristics to the occurrence of vegetation assemblages at the watershed and process zone scales.
Results/Conclusions
In the eighteen watersheds surveyed for the watershed-scale analysis, meadows generally occupied a small proportion of the total riparian area, and the proportion occupied by riparian forest was highly variable. Watershed geomorphology was strongly related to riparian composition. Large watersheds with gentle valley slopes were positively associated with meadows while small, steep, and rugged watersheds were positively associated with riparian forests and shrubs. Process zones identified in a subset of four watersheds included incised alluvial valley, incised colluvial valley, aggrading alluvial valley, and bedrock valley. Vegetation communities and associated species included willow (Salix spp.), birch (Betula occidentalis), aspen (Populus tremuloides), cottonwood (Populus angustifolia), wet meadow (Carex spp.), mesic meadow (mixed graminoids and forbs), and sagebrush meadow (mesic/dry meadows with Artemisia tridentata). Species composition was related to distinct characteristics of each process zone, especially height above the channel, soil texture, and stream gradient. Sensitivity was greatest at downstream positions. Watershed scale characteristics most influential for vegetation distribution and sensitivity were related to flow magnitude and flood disturbance regime, while those at the local scale measured moisture availability and susceptibility to disturbance events. Predictability of riparian plant species composition improved when watershed characteristics were considered in addition to process zone variables.
