Semiarid karst landscapes are often the source areas for regionally important groundwater supplies. Like savannas across the globe, these landscapes are experiencing an increase in woody plant cover—often referred to as woody plant encroachment. Although this phenomenon is commonly viewed as leading to increased transpiration and reduced groundwater recharge, this may not be true of all ecosystems. For example, in the Edwards Plateau region of central Texas—where the underlying geology is karst—dramatic increases in baseflows have occurred concurrently with the expansion of woody plants. It has been suggested that in this context woody plants, especially juniper (Juniperus spp.), are partially responsible for boosting recharge by improving soil infiltrability, but this hypothesis has not been systematically evaluated. Our study examined the effects of an important encroaching shrub (Redberry juniper) on soil infiltrability in the Edwards Plateau. We carried out a large number of infiltration tests to determine soil infiltrability and used a dye tracer followed by soil profile excavation to estimate the potential for deep percolation. Tests were performed at increasing distances under juniper shrubs of five size classes, ranging from young seedlings to mature shrubs. We found that in soils underlying shrubs, infiltrability was quintupled and percolation depth almost tripled compared with soils in intercanopy zones. Surprisingly, shrub size was not a significant factor. Even the soils beneath the smallest shrubs had much higher infiltrability than intercanopy soils, showing that these woody plants modify soil properties at very early stages. We also found that both infiltrability and percolation depth gradually increased with proximity to the trunk and showed a strong correlation with litter thickness. Our results provide support for the hypothesis that in semiarid karst landscapes, woody plant encroachment—especially the invasion of juniper—can play an important role in enhancing groundwater recharge by improving the soil infiltrability.

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.

Little is known about the effects of woody plant encroachment—a recent but pervasive phenomenon—on the hydraulic properties of bedrock substrates. Recent work using stream solute concentrations paired with weathering models suggests that woody plant encroachment accelerates limestone weathering. In this field study, we evaluate this hypothesis by examining bedrock in the Edwards Plateau, an extensive karst landscape in Central Texas. We compared a site that has been heavily encroached by woody plants (mainlyQuercus fusiformisandJuniperus ashei), with an adjacent site that has been maintained free of encroachment for the past eight decades. Both sites share the same bedrock, as confirmed by trenching, and originally had very few trees, which enabled us to evaluate how encroachment impacted the evolution of hydraulic properties over a period of no more than 80 years. Using in situ permeability tests in boreholes drilled into the weathered bedrock, we found that the mean saturated hydraulic conductivity of the bedrock was higher—by an order of magnitude—beneath woody plants than in the areas where woody plants have been continuously suppressed. Additionally, woody plant encroachment was associated with greater regolith thickness, greater plant rooting depths, significantly lower rock hardness, and a 24–44% increase in limestone matrix porosity. These findings are strong indicators that woody plant encroachment enhances bedrock weathering, thereby amplifying its permeability—a cycle of mutual reinforcement with the potential for substantial changes within a few decades. Given the importance of shallow bedrock for ecohydrological and biogeochemical processes, the broader impacts of woody plant encroachment on weathering rates and permeability warrant further investigation.