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 the encroaching 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 three years showed that in shallow upslope terrain, the dynamic water storage in bedrock was roughly twice that of soil, while 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 two years, when precipitation was below average. Importantly, in upslope terrain we found substantially larger water storage under woody plants, which suggests that they not only can access and utilize rock moisture but also play a role in enhancing bedrock water 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.

The use of stable isotopic tracer methods is becoming a popular approach for in situ evapotranspiration (ET) partitioning studies at the ecosystem scale. Ecosystem evapotranspiration is usually partitioned based on different isotopic composition among the aggregated ET flux (δET) and its two components: physical evaporation process (δE) and biological transpiration process (δT). Uncertainties in calculating these three isotopic compositions, especially δET, are usually not negligible and could propagated to large uncertainty in the ET partitioning outcomes. In this study, we present a new ET partitioning approach utilizing dual isotope-based evapotranspiration partitioning based on d-excess (d-excess = δ2H - 8 × δ18O) . A field deployable laser absorption spectrometer was used for in situ measurements of isotopic composition (δ2H and δ18O) of atmospheric water vapor at different heights within the turbulently mixing ecosystem boundary layer for tallgrass prairie. This study incorporated a new refinement to the Craig-Gordon model to describe δE. During non-growing season, estimates of δET—based on Keeling plot and flux-gradient approaches—were compared against δE values derived by the Craig-Gordon model, under the assumption of no transpiration and thus the equality between δET and δE. During the growing season, coupled with isotopic sampling in plant and soils, we partitioned ET into transpiration and evaporation. This refined dual isotope-based approach of ET partitioning shows promise in reducing the uncertainties of the fractional contributions of evaporation and transpiration, thus enhance our understanding on the mechanisms underlying plant water use efficiency and the role of vegetation ecophysiological processes in eco-hydrologic processes under the changing environment.