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Boise State University (2015)

Stable Isotopes Reveal a Disconnect Between Biotic and Abiotic Hydrological Processes in a Seasonally-Dry, Semi-Arid Watershed

McCutcheon, Ryan James

Titre : Stable Isotopes Reveal a Disconnect Between Biotic and Abiotic Hydrological Processes in a Seasonally-Dry, Semi-Arid Watershed

Auteur : McCutcheon, Ryan James

Université de soutenance : Boise State University

Grade : Master of Science in Hydrologic Sciences 2015

Résumé
Until recently, it had been thought that humid catchment woody plants transpired primarily mobile soil water that would otherwise flow to streams or recharge groundwater. However, several recent studies have suggested that trees in seasonally-dry humid catchments use primarily tightly-bound, immobile soil water that does not fully mix with new precipitation or participate in translatory flow. McDonnell (2014) called this existence of two, hydrologically-distinct, water pools “the two water worlds hypothesis.” This ecohydrological behavior has important implications for understanding a wide range of catchment processes, including the spatial and temporal variability of evapotranspiration and nutrient cycling, and our abilities to accurately model those processes. Yet, similar studies have not been conducted in drier environments. This study aims to improve our understanding of semi-arid woody plant root water sources. Isotopic analyses were conducted on the xylem water of nine common semi-arid woody plant species, in conjunction with bulk soil water, groundwater, and streamwater samples. The isotopic concentrations of plant xylem water and potential root water sources were plotted in dual-isotope space and qualitatively assessed. Additionally, the SISUS model was used to computationally characterize all feasible water source contributions to each plant individual. In total, 112 out of 121 plant samples had isotope values indicative of use of at least fractional immobile soil water use. These findings were consistent, regardless of plant species, geographic location, or time of sample collection. Nevertheless, some Yellow willow, Fire willow, Chokecherry, Rabbitbrush, Douglas-fir, and Ponderosa pine individuals plotted between immobile and mobile water sources, and were modeled to use greater than 30% mobile water sources (groundwater and streamwater) in the late growing season. Additionally, median modeled use of mobile water sources increased for each field site between July and September, indicating that many plants may have actively rooted to groundwater as the soil column dried significantly below the permanent wilting point. Therefore, we accept the two water worlds hypothesis in terms of immobile soil water use, regardless of mobile soil water availability. However, we also acknowledge that many semi-arid woody plants also use mobile groundwater and streamwater sources

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