DIFFERENTIATION OF WATER USE FOR THREE DOMINANT SPECIES ON THE EDWARDS PLATEAU
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Woody plant species that exhibit differences in root distribution are able to partition available water sources. This happens quite commonly in regions with deep soils, but are species still able to develop differences in root distribution when rooted in fractured bedrock? I examined this question by comparing three tree species, cedar elm (Ulmus crassifolia), a winter-deciduous broadleaf, live oak (Quercus fusiformis), an evergreen oak, and Ashe juniper (Juniperus ashei), an evergreen conifer, growing in a mixed stand on a site with shallow soil over fractured bedrock on the eastern Edwards Plateau, Texas, USA. For one growing season, which included a severe summer drought, I recorded monthly variation in predawn water potentials and the stable isotope ratios of stem water. I also continuously measured sap flow velocities. Minimum predawn water potentials differed between the species, reaching -8 MPa for both juniper and elm, and -5 MPa in oak. As the summer drought developed, sap flow velocities of the three species declined synchronously and stem water isotope ratios increased, reflecting the evaporative enrichment of a shared, diminishing water source. Thus, maximal rooting depths and access to stored water appeared to be similarly constrained across the three species. However, species exhibited differences in drought tolerance and response: oak appeared to hydraulically disconnect from water sources at water potentials of -4 to -5 MPa, at which point stem hydraulic conductivity was reduced by 75%. Both elm and juniper continued to extract water, but this was more detrimental to elm, which approached 90% loss of stem hydraulic conductivity and shed leaves by the end of August, instead of October in wet years. Juniper had the smallest loss of stem hydraulic conductivity (15-30%). These differences in drought response in species similarly constrained by storage capacity for water may suggest differences in species vulnerability to drought intensity and duration that may cause the eventual shift of species composition on the Edwards Plateau in a climate regime with a more frequent occurrence of extended or intense drought.