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Seismic refraction to explore the influence of belowground water storage on sap flow velocities in pines
Hicks, Emmalynn
Hicks, Emmalynn
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2023
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Abstract
Water storage in the critical zone acts as a buffer between trees and the effects of climate change by aiding in a tree community’s ability to survive water scarcity. Consequently, predicting how subsurface water-storage patterns may change under a changing climate and how trees will respond to these changes is vital for accurately assessing tree mortality. Here, we conducted a study on two opposing slopes of Gordon Gulch in the Boulder Creek Critical Zone in Boulder, CO. Sap flow—which is used as a measure of tree productivity—and seismic refraction data were collected to explore how and if subsurface water storage is related to the productivity of ponderosa and lodgepole pine trees. Four sap flow plots, two on north-facing and two on south-facing slopes, each containing roughly 8 trees, were instrumented with sap flow sensors. There is a greater depth of saprolite and unconsolidated material on the north-facing slope of Gordon Gulch, where we see an abundance of tightly spaced lodgepole pine trees. The depth of the saprolite and unconsolidated layers on the south-facing slope are shallow in comparison to the north-facing slope with the south-facing slope consisting of sparsely spaced ponderosa pine trees. Three out of the four sap flow plots displayed seasonal trends with sap flow velocity values that appear to generally correspond to subsurface water storage capacity between sites on each slope, although it was clear other variables also controlled sap flow. Sap flow velocities differed between the two plots on the south-facing slope, which had the same tree species on them, indicating that differences in the thickness of water-holding materials and other environmental and biological factors, such as slope/aspect and rooting depth, likely influence sap flow differently between sites. Sap flow velocities decreased significantly following August for the south-facing upslope plot while they remained high for the downslope plot, leading us to speculate that belowground water storage potential might be positively influencing sap flow rates into late summer. In short, we found that differences in water storage capacity may influence sap flow velocities but are convolved with other factors such as solar radiation, soil/rock moisture, and rooting depth.
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