Drainage networks as determinants of ecological pattern and process in a Sonoran desert basin

A central hypothesis in landscape ecology is that the structure and function of an ecosystem is influenced by its spatial position within a surrounding mosaic. In both terrestrial and aquatic systems, hydrologic flow and connectivity are important determinants of those spatial patterns associated with landscape position. Arid landscapes are drained by a channel continuum that constitutes a gradient in hydrologic permanence, and represents a potentially important driver of ecological pattern and process in the surrounding terrestrial environment. To better understand the ecological role of drainage networks in arid landscapes, changes in the structure and function of riparian ecosystems were quantified along a channel continuum in an upper Sonoran desert basin. Results show that riparian plant size, overall cover, and aboveground primary productivity all increase with downstream position in the drainage system. Foliar delta 13C signatures indicate that these longitudinal patterns in vegetation structure reflect increases in water availability along the channel continuum. Landscape gradients in plant size and productivity in turn influence the storage of carbon (C) and nitrogen (N) in soils. Material accrual in soils beneath shrubs increased as a function of plant size and productivity across a broad range of stream sizes. In the riparian zone of larger streams, however, flooding acted to further modify patterns of material storage. Position along the drainage systems also influenced landscape patterns of soil respiration (SR), both at seasonal and post-rainfall time scales. At seasonal scales, soil respiration increased by up to 13-fold with downstream position in the channel continuum. SR varied in space and time with patterns of plant productivity, and seasonal patterns likely were driven by root respiration. Immediately following simulated rainfall, SR increased by up to 30-fold, and varied across the landscape as function of organic matter storage in surface soils. Post-storm pulses of microbial activity were found to be rapidly limited by labile organic matter substrate, and, as a consequence, increased as a nonlinear function of rainfall amount. Overall, the results indicate that, at basin scales, drainage networks are important determinants of ecological pattern and process in arid landscapes.