Short-Term Responses of Clear Cutting on the Water Supplies, Water Status and Growth of Remaining Vegetation: Which Species Have the Most to Gain?
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Over the past century and a half, central Texas grasslands and savannahs have experienced widespread woody plant encroachment of Ashe juniper (Juniperus ashei). This has had negative impacts on grassland productivity, habitat variety and, many believe, aquifer recharge. As the first phase of a longer-term study, short-term effects of juniper removal on ecosystem water availability were examined to improve the understanding of the ecohydrologic effects of this management practice. The study was conducted at Camp Bullis Military Base near San Antonio. In 2008, 90.6 % of all juniper trees were removed from a 2730 m2 area on a wooded hill slope. The area was characterized by shallow soils (0-.3 m) overlying fractured bedrock, typical for the karst terrain of this area. For the first two years after clear-cutting, soil water content, predawn water potentials, sap flow rates, and sap water stable isotope ratios of live oak (Quercus fusiformis) and Ashe juniper trees were monitored and compared with those in an adjacent, unaltered oak-juniper woodland. Soil water content in the cleared area was consistently higher than in the woodland, though differences between sites declined in 2010. During the exceptional summer drought of 2009, predawn water potentials of trees in both sites declined, but woodland trees reached lower potentials and their sap flow rates declined more rapidly, suggesting a faster depletion of stored water. In 2010, precipitation levels were at or above average for most of the year and there was little difference in water potentials and sap flow rates between sites. Sap water stable isotope ratios of trees in the woodland and the clearing were evaporatively enriched and distinct from the stable isotope ratios of cave drips, indicating shallow water sources for trees at both sites. These results suggest that clear-cutting reduced evapotranspiration (ET) and lead to an increase in ecosystem water storage in the two years after clear-cutting. However, ET savings may be limited by the relatively low capacity and shallow location of water storage in the root zone of trees.