How Distance from Groundwater Affects the Water Status of Trees at San Marcos Springs
The impact of perennial woody vegetation and the role of landscape topography on karst aquifer freshwater supplies is still poorly understood. To gain perspective on this question, I conducted a study at a site where the depth of a shallow groundwater table was indicated by the water level of a spring-fed lake and the distance of trees from groundwater was determined by their position on an escarpment above the lake. I tested the hypotheses that trees at higher elevations developed greater water-stress and used less groundwater than trees closer to or at lake level. I also expected that different tree species responded differently to elevation, presumably related to differences in rooting depth. I monitored several tree species common to the Edwards Plateau of Central Texas (U.S.A.) during the unusually dry summer of 2022. This included measuring predawn and midday water potentials, as well as the stable isotope ratios of hydrogen and oxygen in plant xylem-, soil-, and lake water in regular intervals. The hypotheses that trees at higher elevations developed a greater degree of water-stress and that there would be species-specific differences were supported; however, there was no evidence that any tree used substantial amounts of groundwater, not even those that grew immediately adjacent to lake level. Isotope ratios seemed to indicate a common source of water for all trees, most likely derived from winter precipitation and variably enriched by evaporation. Similar studies conducted on the Edwards Plateau and elsewhere similarly indicated that trees near flowing water sources avoid their uptake even during drought, instead taking water from some deeper regions of the vadose zone. My study adds to the mounting evidence that tree impacts on groundwater resources are complex, warranting further investigation of bedrock-associated water sources for trees of the Edwards Plateau and how bedrock storage might interact with groundwater recharge.
karst, water potentials, stable isotope ratios, Edwards Plateau, trees
Simons, E. (2023). How distance from groundwater affects the water status of trees at San Marcos Springs. Honors College, Texas State University.