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Seasonality of subsurface electrical resistivity as a proxy for tree water use inferred from empirical orthogonal frequency (EOF) analysis
Tucker, Annie E. Dumont, Marc Lenssen, Nathan Singley, Joel Callahan, Russell Singha, Kamini
Tucker, Annie E.
Dumont, Marc
Lenssen, Nathan
Singley, Joel
Callahan, Russell
Singha, Kamini
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2025-04
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Abstract
Trees are facing increasing water stress. To understand their resilience capabilities, we must characterize their root-zone structure and ability to access soil moisture. Electrical resistivity (ER) timelapse observations reveal insights into daily root-driven, soil-moisture movement. Here, we apply a novel empirical orthogonal frequency (EOF) analysis to ER data to assess the seasonality of tree rooting-zones. EOF analysis extends traditional Principal Component Analysis to spatial data to identify dominant spatiotemporal patterns capturing the most variance in the dataset. We applied this tool on data from three sites on different hillslopes in the Kings River Experimental Watershed, Southern Sierra Critical Zone Observatory. At each site, we collected ER from two ~18-m lines for 48 hours in Oct. 2022, June and Oct. 2023 and May 2024 to assess large-scale soil conductivity. The survey periods represent both wet and dry periods. We inverted each ER survey into timelapse models. To contextualize the ER, we installed sapflow sensors on white-fir (Abies concolor) trees for transpiration, vapor-pressure deficit sensors for air water demand, and soil-moisture probes for soil water. The EOFs fitted to our data show clear conductivity (i.e., soil moisture) cycles that start rising ~6am and decreasing between ~5-8pm. The spatial modes show an active rooting zone expanding during drier surveys (Oct. 2022 and 2023) versus wetter surveys (June 2023 and May 2024), indicating deeper water uptake in dry conditions. We shows the first application of EOFs on ER and identify seasonality in root-drive diel soil-moisture cycles, enhancing understanding of trees’ response to water stress.
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