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Mechanisms of cottonwood decline in a riparian ecosystem along the Colorado River.
Pataki, Diane1, 1 University of Utah, Salt Lake City, UT
ABSTRACT- Riparian ecosystems are a critical but rapidly degrading habitat in the arid West. Populus fremontii (Fremont cottonwood) was once a dominant species in desert riparian forests that has been increasingly replaced by the exotic invasive Tamarix ramosissima (saltcedar). In addition, reduced flooding frequency and increased salinity have been implicated in the widespread decline of P. fremontii. To elucidate some of the multiple and interacting mechanisms of this decline, I examined several aspects of ecosystem function in a control stand of P. fremontii along the Colorado River in Moab, Utah, USA, as well a disturbed stand characterized by high groundwater salinity and invasion of T. ramosissima. Water isotope data showed that P. fremontii and T. ramosissma utilized groundwater at both sites. Sap flux data showed that P. fremontii at the saline site experienced large reductions in afternoon stomatal conductance relative to the control. Thus, average daily stand transpiration was 4.8 mm d-1 at the saline site in comparison to 9.8 mm d-1 at the control. Despite the fact that P. fremontii and T. ramosissma shared the same water source, gas exchange and sap flux data scaled to canopy transpiration indicated that T. ramosissima constituted a negligible portion of canopy water use due its small contribution to total leaf area. However, tissue N data indicated that T. ramosissima was out-competing P. fremontii for nitrogen, likely due to a larger concentration of roots in the upper soil horizons. Coupled with physiological stress caused by groundwater salinity, perturbations to nutrient uptake may be contributing to the decline of P. fremontii during invasion of T. ramosissima.
Key words: riparian ecosystems, cottonwood decline, Populus fremontii, Tamarix ramosissima