The Edwards Plateau in central Texas is dominated by live oak (Quercus virginiana)-Ashe juniper (Juniperus ashei) savannas interspersed among C3/C4 grasses. Soils on the Plateau are generally shallow with limited water storage capacity, and overly fractured limestone bedrock. It is widely assumed that deep roots of woody plants provide access to stable sources of water in bedrock, which would mitigate the impact of restricted soil water storage on ecosystem water use. We investigated this assumption using 3 years of eddy covariance measurements of water vapor and energy exchange in an oak-juniper woodland with a shallow soil (20 cm depth), and a juniper-dominated savanna (50% woody cover) on a deep soil (1.5 m depth). Rainfall during the first two years was well below normal, while the third year was one of the wettest on record.
Results/Conclusions
Evapotranspiration (ET) is controlled by available energy (net radiation minus storage heat flux) and its partitioning between latent (LE) and sensible heat (H) fluxes. The amount of available energy partitioned into LE is linked to available water, which is constrained by rainfall and water retention in the root zone. Available energy at the woodland was 8% per year higher than at the savanna, but ET was slightly lower, by an average of 18 mm yr-1. Although available energy was higher at the woodland, much less water was stored in the woodland soil, resulting in a smaller fraction of available energy going into LE (43% of available energy at the woodland vs. 48% at the savanna). ET at both sites declined rapidly during drying cycles, but the response of the savanna lagged that of the woodland on occasion. Woodland ET exceeded soil water storage, indicating that the trees did extract some water from the bedrock. However, these water sources were ephemeral and depletable, just like the soil layer above. Our results suggest that ET in karst ecosystems of the Edwards Plateau is storage limited. Woody cover increases available energy, but the degree to which higher available is manifested in higher ET depends on the amount of water retained in the root zone. Thus, ET is highly dependent on local geology. Over time, rain-limited Plateau ecosystems will most likely consume all of the available water in their root zones, regardless of plant species composition.