Trees Enhance Rock Moisture Storage: A Major Pool in Karst Drylands and Crucial During Droughts

Abstract Woody plant encroachment is a global phenomenon, observed in many of the world's drylands. In those with shallow soils overlying karst geology, rock moisture can be an important source of water for woody plants. This source can be particularly important for trees to maintain basic physiological functions during extended droughts, which are becoming more frequent and intense owing to climate change. However, our understanding of rock moisture dynamics in karst drylands undergoing woody plant encroachment is still limited because of the scarcity of direct measurements. In this study, we evaluated soil and rock moisture dynamics at a semiarid site in the Edwards Plateau region of Texas. Our measurements over the course of 3 years showed that in shallow upslope terrain, the dynamic water storage in bedrock was roughly twice that found in soil, whereas in downslope terrain, the dynamic storage was largely restricted to the soil layer. Most of the bedrock storage gains occurred during the first year, after two major storm events of approximately 95 mm, and that storage was gradually depleted during the following 2 years, when precipitation was below average. Importantly, in upslope terrain we found greater rock water storage under woody plants, suggesting that expanding woody vegetation not only has more access to rock moisture, but may also play a role in enhancing bedrock storage capacity. These interconnected abilities can help woody plants survive extended droughts—a factor crucial for understanding their persistence and proliferation in the shallow soils of the Edwards Plateau and similar karst regions.