Energy balance and water use in a subtropical karst woodland on the Edwards Plateau, Texas

Woody encroachment into karst grasslands and savannas is presumed to reduce water availability and aquifer recharge, in part, because deep roots extract large quantities of water from perennial sources within the fractured bedrock underlying shallow soils. If true, energy balance partitioning and transpiration in woody ecosystems should be decoupled to an extent from rainfall, and sensitivity of the energy balance and evapotranspiration (ET) to rainfall and water deficits should be dampened. We evaluated responses of energy and water vapor fluxes to rainfall and water deficits in a live oak ( Quercus virginiana)-Ashe juniper ( Juniperus ashei) woodland on the karst Edwards Plateau, TX, USA, over a 2-year period using eddy covariance measurements of the turbulent fluxes. Total ET during the two years was 1416 mm, 92% of total rainfall. We observed large and rapid reductions in λE and increases in H during drying cycles, and high correlation between ET and soil water content in the upper 20 cm of the root zone. In most cases, ET declined at the same time as soil water content, indicating that the woodland relied heavily on water from recent rainfall events, rather than antecedent water. We found no evidence that deep roots were extracting significant amounts of water from a perennially stable supply of water. Excavations at the woodland site revealed a rock layer at 20 cm below the soil surface, with a dense root mat above the rock and penetration of relatively few roots into the rock through cracks and fissures. Thus, the most likely sources of water for trees were soil water and a limited supply of water stored in near-surface fractured rock layers.