Linkages among watersheds, stream reaches, and riparian vegetation in dryland ephemeral stream networks

Abstract

Ephemeral streams are common features of landscapes around the world, and are the predominant fluvial environment in arid zones. Current understanding of dryland riparian ecology is derived primarily from perennial stream environments, and little is known about the factors controlling vegetation along ephemeral streams. This study describes relationships between the physical characteristics of watersheds and stream reaches, and their effects on riparian plant communities, for 14 ephemeral stream reaches in the semi-arid southern Colorado Plateau, USA. Interactions of watershed properties produce gradients of hydro-geomorphic regimes throughout stream networks, which give rise to spatial patterns in the structure and composition of perennial woody plant and grass dominated communities. As watershed and network magnitudes increased, and basin slope decreased, stream reaches exhibited reduced erosive capacities and more attenuated alluvial groundwater variability. Decreased disturbance potential and increased moisture availability in the downstream direction was related to greater abundance of obligate riparian taxa, and increasing structural importance of shrub and tree species. Divisions in watershed and stream reach physical properties corresponded with riparian plant community types, implying that progress in dryland fluvial ecology could be made from research within a watershed or network context. These concepts are integrated to form a stream classification system that describes functional linkages between watersheds, stream reaches, and riparian plant ecology. Type I streams drain watersheds less than 10 km2, exhibit the greatest disturbance potential and most xeric alluvial groundwater regime, and are occupied by upland plant associations. Type II streams have watersheds between 10 and 100 km2, and have more moderate shear stresses and more persistent alluvial groundwater. Plant communities consisting of a mix of upland and riparian species occur in Type II reaches. Type III reaches drain watersheds of more than 100 km2 and hydrologic regimes are controlled mainly by upstream hydro-climatic conditions. These streams have the lowest bankfull shear stresses and perennial water tables that span expansive floodplains. Wetland tree and shrub communities dominate Type III reaches, and upland plants are a minor ecological component.