Response of cave air CO2 and drip water to brush clearing in central Texas: Implications for recharge and soil CO2 dynamics

[1] Brush removal is commonly conducted to increase water availability in arid areas, such as central Texas, where water resources are stressed. The effectiveness of brush clearing to enhance recharge, however, remains uncertain as numerous studies have yielded contradictory results. This study assesses the effects of brush clearing on recharge to a cave at Natural Bridge Caverns, central Texas by evaluating changes in drip rate, drip water compositions, cave air CO2 concentrations, and calcite growth in a cave underlying an area cleared of brush. Drip sites were monitored for 3 years preclearing and 2 years postclearing at five drip sites beneath and seven drip sites not beneath the surface cleared of brush. Physical and chemical drip water parameters exhibit preclearing and postclearing variability. Postclearing drip rate characteristics reflect an initial interval of anomalously heavy rainfall, then a longer dry period. Drip water Sr/Sr values do not exhibit preclearing to postclearing variation at sites beneath the cleared area and indicate no change in postclearing water residence time. Significant decreases in postclearing cave air CO2 are observed at seven of nine drip sites in the cave beneath the clearing. Decreases in cave air CO2 influence calcite growth, which impacts postclearing drip water Mg/Ca and Sr/Ca. Decreases in cave air CO2 immediately following brush clearing suggest that a significant portion of soil CO2 respiration is from tree root respiration rather than from soil microbial activity. Seasonal calcite growth patterns, linked to cave air CO2, also exhibit variability postclearing and suggest that cave mineral deposits may record historical changes in vegetative cover.