CHANGING SOIL CHARACTERISTICS AND BIOGEOMORPHIC SUCCESSION IN TUOLUMNE MEADOWS: IMPLICATIONS FOR RESTORATION
Ankenbauer, Kyle John
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THESIS ABSTRACT Tuolumne Meadow is an impacted groundwater dependent ecosystem that is threatened by increased drying in meadow soil and increased channel widening of the Tuolumne River channel. We investigated two mechanisms which can reduce the availability of water to meadow vegetation and further degrade the meadow or maintain it in a degraded condition: (1) decreased soil water retention as a result of soil organic matter loss in dryer soils, and (2) the reduced trapping of sediment in places with limited willow establishment. The motivation for this study was to provide a process based understanding of the interactions between meadow vegetation condition and soil maintenance and formation so that processes that inhibit and encourage meadow recovery can be identified. While the findings presented in this manuscript are based on observations in Tuolumne Meadows, they are meaningful to meadows throughout the Sierra Nevada because they all experience a similar climate and were formed by similar geologic processes. We measured soil organic content and soil water retention at distributed sites across Tuolumne Meadows and found that soil water retention is strongly influenced by soil organic content. With the use of a field-validated 1-D variably saturated groundwater flow numerical model, we demonstrated the impact of soil organic matter loss by simulating seasonal transpiration under a range of soil organic contents. We showed that the difference in cumulative seasonal transpiration between soils with 0% organic content and 20% organic content is 8.8 cm. Additionally, the simulations show this difference occurs over the latter half of the growing season, which is a critical time for root growth and seed development in plants. To characterize the role of willows in sedimentation processes along the Tuolumne River channel, we measured sedimentation deposition on top of inert clay pads placed around willow plants. To understand the role of four different biogeomorphic factors, we chose to compare (1) willows inside and outside of deer proof fencing and (2) willows on gravel bars and on banks. Additionally, we compared sedimentation deposition (3) in areas upstream, downstream, and adjacent to willows and (4) around willows close to the river and those farther away. Low peak flows over the duration of this portion of the study (2012 to 2013) limited our ability to observe sedimentation, but we were able to show the robustness of the methodology with the limited data we collected. The results show that the occurrence of deposition is significantly different for organic duff debris than for mineral sediments on gravel bars as opposed to river banks and around willows close to the river as opposed to those farther away.