PS 38-55 - Variation in Water Use Strategies Among Riparian Prosopis velutina (Velvet Mesquite)

Abstract: As flow rates in Southern Arizona rivers diminish, understanding the strategies and patterns of riparian tree water use is more important than ever. Generally, older trees may root deeper than younger trees, but this does not necessarily mean older trees use deeper source water throughout the year. A better understanding of the water use patterns of a specific native tree, velvet mesquite (Prosopis velutina), is important for land management decisions along riparian areas throughout the increasingly water-stressed Southwest. We addressed three questions: 1) Are mature P. velutina trees able to access and use groundwater at times that young thickets cannot? 2) Do the patterns of source water use vary throughout the year? and 3) How do differences in water access and use impact tree water stress? To determine if P. velutina exhibit variation in their water use patterns, we measured stable water isotopes of the tree xylem over 2 years of young P. velutina thickets and a mature forest in a riparian area along the San Pedro River, AZ. We compared these isotopic signals to those of local groundwater and precipitation to determine the tree source water used as well as measured water stress across the pre-monsoon and monsoon periods. We also examined the relationship between stand density, soil type and their impacts on water use patterns. We found that trees at the plot level vary to some degree in their source water, but all primarily draw from groundwater likely due to a high water-table. There is also variation across the seasons, in part because these trees enter a period of dormancy and cease water uptake. We also found that the different stands experienced different degrees of water stress, with older, more mature P. velutina unexpectedly showing the highest stress. Perhaps more surprising still, all stands experienced their highest levels of water stress at the peak of the monsoon period. Our results indicate that, though riparian P. velutina may rely on groundwater, seasonal dynamics and soil composition should be taken into account when considering mesquite removal as a strategy to mitigate diminishing stream flow.