Over the past century, central Texas experienced rapid encroachment of Ashe juniper (Juniperus ashei) into grasslands and savannahs. The high year-round water use of this species is assumed to reduce aquifer recharge, although direct evidence that juniper clearing leads to increases in water yield is lacking. An explanation for this apparent discrepancy is that cleared areas are rapidly reclaimed by vegetation that consumes as much water as the woodland areas they replace. In this study, we measured the vegetation changes, soil water storage, predawn water potentials and sap flow rates of juniper and live oak (Quercus fusiformis) in a recently cleared area and compared it with that of adjacent oak-juniper woodland. The study was conducted at Camp Bullis Army Base in San Antonio, a karst terrain with little soil cover (ca. 0.3 m). Here we report on the results obtained in the first year after clearing, in 2009, which had a severe summer drought.
Despite similar soil water content in both sites during the summer, woodland oak and juniper came close to the lower limits of water transport capacity in August, while their counterparts in the clearing maintained sap flow rates that were half of maximal observed in the spring. During the driest part of the summer, from August to mid-September, sap flow rates of juniper in the clearing continued to decline but stayed level for oak. This suggests that local water availability increased substantially in the first year after clearing, although less in the shallow soil than the fractured rock layers below. Trees that remain standing in the clearing used this water to maintain higher water potentials and transpiration rates. Also, oak showed signs of having more complete access to water stored in fractured rock. We conclude that clear-cutting does indeed reduce water consumption in the first year, leading to increases in locally stored water. Trees left standing in a clearing offset these water savings by increasing individual transpiration rates. Rapid regrowth of woody perennials due to increases in water availability could explain the limited long-term impact of clear-cutting on water yields.