PARENT SESSION
Tuesday, August 8, 8:00-11:30 am
COS 27 - Aquatic ecology II: river ecosystems, Part II
L-14, Lobby Level, Cook Convention Center
Presiders: T Barnwell

Wetlands as an alternate state in desert streams.

Heffernan, James^*,1, Fisher, Stuart ¹, ¹ School of Life Sciences, Tempe, AZ

ABSTRACT- At the time of European exploration, many streams and rivers of the arid Southwest supported productive herbaceous wetlands (ciénegas). Sedimentological evidence suggests that ciénegas persisted throughout the Holocene, and re-established following erosion events. Region-wide channel incision around the end of the 19th century dramatically reduced the abundance of these ecosystems, but recent observations at Sycamore Creek, AZ suggest that in the absence of grazing, wetland development can occur spontaneously in desert streams. Since the exclusion of cattle in 2000, dramatic increases in vegetation within the active channel and surface stream have resulted in accumulation of fine sediments and widespread hyporheic anoxia. These new wetland patches are distributed patchily along the Sycamore Creek corridor. A simple model of plant and sediment dynamics suggests that wetlands constitute an alternative regime that is maintained by the stabilization of channel sediments by herbaceous vegetation. According to this model, the persistence of the wetland state may be influenced by both local geomorphic structure of the channel and by the hydrologic regime. Observations of plant responses to flood events support the key assumption of this model: that per-capita plant persistence during floods increases as a function of plant abundance. In addition, we observed that floods cause a shift in the distribution of vegetation abundance from a unimodal distribution towards a more bi-modal distribution. This observation is consistent with the existence of multiple basins of attraction within desert stream ecosystems.

Key words: Alternative States/Regime Shift, Ecosystem Ecology, Desert Streams