ECOHYDROLOGIC FUNCTION OF SMALL WASHES ON A MOJAVE DESERT BAJADA AND THE IMPACTS OF THEIR DISTURBANCE

Small washes and channels create a complex hydrological network across desert bajadas but represent only a small proportion of the bajada’s spatial area. Nonetheless, these channels may be the most important geomorphic feature influencing local vegetation properties and processes. We examined the functional influence of small channels on the vegetation of a Mojave Desert bajada by conducting a series of studies that contrast an undisturbed area versus that influenced by a ~100 year old linear disturbance (railroad and parallel road). In areas below the disturbance, where flow has been either increased due to channel coalescence into culverts, or cut off due to diversion, plant community structure and cover has changed relative to the undisturbed area. Plant physiological responses to simulated runoff experiments, conducted in active (undisturbed) and inactive (cut-off) channels, revealed subtle but consistent differences that, when compounded through time, are likely to contribute to changes in vegetation. In both undisturbed and cut-off areas, creosote bush (Larrea tridentata) plants within 3 m of a channel, and white bursage (Ambrosia dumosa) plants within 2 m of the channel had access water from the channel, however, the pulse responses were more variable for plants adjacent to inactive channels than for those near active channels. Stomatal conductance and sap-flow measurements on Larrea corroborated these findings, suggesting that root patterns and functions associated with channels are altered when water flow is reduced or eliminated over extended periods of time. These findings indicate that disturbance of small desert washes can lead to vegetation shifts through time with consequences that are not yet fully realized. Small desert washes may represent a minor spatial component of the vast bajada landscape, but runoff and higher infiltration rates, coupled with the breadth of their spatial influence on adjacent plants, suggests that these modest geomorphic features may have a disproportionate impact on plant function and community properties in arid ecosystems.