PS 38-54 - Understanding rock moisture dynamics and quantifying its uptake by trees: a field study in the Edwards Plateau, TX

Abstract: In many parts of the world—particularly in regions of shallow soils—rock moisture can be an important source of water for trees. This source is particularly important for trees to maintain basic physiological functions during extended droughts, which are becoming more frequent and intense due to climate change. However, our understanding of rock moisture use by trees is still limited largely due to the scarcity of direct measurements. In this study, we evaluated rock moisture use by trees growing on shallow soils underlain by weathered limestone in the Edwards Plateau region of Texas. We used a combination of methods including direct measurements of soil and rock moisture (with capacitance sensors and neutron probes), stable isotope tracers (¹⁸O and ²H), and pre-dawn leaf water potentials. Preliminary results suggest that water percolates deeper and faster under woody plants. Trees used mainly water stored in soil (0–30 cm deep) and weathered bedrock down to 2 m deep. Through mass balance analysis, we found that during dry summer periods, more than 70% of the total evapotranspiration was derived from rock moisture. We also have substantial evidence that co-occurring species show contrasting water use strategies and effective rooting depths; for example, Ashe juniper (Juniperus ashei) sustains significantly lower water potentials and relies less frequently on deeper moisture sources than Redberry juniper (Juniperus pinchotii) or Live oak (Quercus virginiana). We expect that the findings from this study will improve our understanding of the water dynamics in the Edwards Plateau and other regions with shallow soils underlain by weathered bedrock. Such information is key for predicting the impact of droughts on vegetation dynamics and for assessing the vulnerability of these ecosystems to climate change.