While much is known about the water use and hydrological adaptations of trees in deep soils, we are only beginning to examine adaptations to non-soil substrates, such as the fractured bedrock structure of the epikarst, the top layer of the vadose zone in karst systems. This substrate is spatially much more constraining than soils with possible consequences for plant water dynamics, and the dynamics of recharge and depletion. Here we ask if these constraints and differences prevent tree species from exhibiting differences in spatial and temporal patterns of water use. Our Null Hypothesis was that three co-dominant tree species of the eastern Edwards Plateau use the same water sources and experience the onset of water stress at the same time, indicating that roots share the same rooting space, despite differences in life history strategies.  The species were the evergreen live oak (Quercus fusiformis), the drought-tolerant evergreen Ashe juniper (Juniperus ashei) and the winter-deciduous cedar elm (Ulmus crassifolia). Beginning in April 2009, we monitored the sap flow rates of 6 trees per species in a wooded area in San Marcos, Texas. Predawn water potentials and stem samples for stable isotope analysis were taken in monthly intervals.  

Results/Conclusions

The exceptionally dry summer of 2009 allowed us to observe and compare the effects of extreme drought.  While live oak maintained the highest predawn water potentials, they were exceptionally low compared to other years (< - 5 MPa).  The predawn water potentials of cedar elm and Ashe juniper were similar and as low as - 8 MPa. Sap flow rates declined sharply in all species during the month of June and then remained steady at 0.1 to 1.5 m per day, with live oak having the lowest sap flow rates and Ashe juniper the highest. Stem samples taken for stable isotope analysis of xylem water throughout the year also indicated no differences in water sources. The study suggests that the water sources available to the three species in this karst area were indeed very similar, but that species experienced different stress levels due to differences in their physiological tolerance to low water potentials, which is lowest in live oak and highest in Ashe juniper.