Document: RON-3-32-44

Water source use of a riparian tallgrass, big sacaton (Sporobolus wrightii), along a gradient of depth to groundwater and rainfall regime in southeastern Arizona USA.

TILLER, R.L.* ¹, K.A.SNYDER ², D.G.WILLIAMS ² and J.C.STROMBERG ¹

Arizona State University, Tempe, AZ 85287 USA ¹
University of Arizona, Tucson, AZ 85721 USA. ²

Abstract:
Riparian grasslands dominated by big sacaton, a warm season perennial bunchgrass, were abundant in the southwest before the turn of the century, occupying millions of acres of floodplain habitat. These grasslands played important ecological functions by controlling erosion, intercepting and retaining sediment, and promoting groundwater recharge. Today, these grasslands cover less than 5% of their historical extent. There is growing interest in the conservation and restoration of big sacaton, yet little is known about the ecology of this important grassland species. Effective conservation and management of big sacaton grasslands will depend on a strong base of ecological knowledge. Recent studies by the authors have begun to elucidate how big sacaton interacts with its environment (i.e. plant-soil-hydrology relationships). An important and heretofore unstudied aspect of its ecology is the source(s) from which it derives moisture for maintenance and survivorship. This research attempts to answer the following questions: 1) Does big sacaton utilize shallow soil moisture from precipitation, moisture provided by groundwater, or both, and; 2) Is utilization of these water sources influenced by site conditions? Stable isotope technology is being used to determine the water source(s) utilized by big sacaton at five grassland sites with different groundwater conditions and rainfall regimes. Natural abundance ratios of isotopes of hydrogen (¹H and ²H) and oxygen (¹⁶O and ¹⁸O) are being used to determine its source waters. At each grassland, tissue samples were collected from six big sacaton growing in an area of representative cover and density. Soil samples were collected from three cores at depths of 5,10,15,30,60 and 100 cm. Precipitation samples were collected from standard raingages containing a layer of mineral oil and groundwater samples were collected from local wells. Preliminary results of ¹⁸O analysis indicate big sacaton derives water solely from precipitation at a site where depth to groundwater exceeds 10 meters. At a location where depth to groundwater averages approximately 4 meters, big sacaton appear to use rainfall derived soil moisture and groundwater. These early results suggest big sacaton may use rainwater and/or groundwater depending on site conditions.

Keywords: big sacaton, Sporobolus wrightii, riparian grassland, water source use, deuterium, oxygen-18

This abstract is being presented at: 9:30 AM in session:
Oral Session #45: Water Relations in Shrubs and Annuals.