Drought-tolerance, vulnerability to embolism, hydraulic capacitance and stomatal sensitivity in four co-occurring tree species in central Texas, USA
Distributions of tree species under climate change may be attributed to their hydraulic parameters, particularly as they pertain to drought-tolerance. The Edwards Plateau of central Texas experienced a severe drought during 2011-2013 which resulted in the death of hundreds of millions of trees. To determine if certain hydraulic parameters were related to greater mortality in different species we measured gas exchange, water potentials, leaf, root and branch hydraulic conductance and vulnerability to embolism and sapwood hydraulic capacitance in four co-occurring tree species (Quercus fusiformis, Prosopis glandulosa, Diospyros texana, and Juniperus asheii). Based on the summer aridity of this plant community (approximately 140 mm July-September rainfall and August mean maximum temperatures of 37.5˚C), we hypothesized that these species would be highly resistant to embolism. We also hypothesized that these species would have stomata that were highly sensitive to changes in vapor pressure deficit.
Stomatal conductance was more sensitive to changes in vapor pressure deficit and leaf water potential in Quercus and Juniperus than in Diospyros and Prosopis. Vulnerability to embolism in roots and stems varied across species with Juniper roots and stems being the most resistant (The water potential at which 50% of hydraulic conductivity was lost [P50] = -5.3 and -8.4 MPa, respectively) and Quercus being the least resistant (P50 = -1.4 and -1.8 MPa, respectively). Predawn leaf and midday branch water potentials indicated that roots and stems of Quercus and Prosopis lost more than 50% of their hydraulic conductivity during mid-summer. Quercus leaves lost no hydraulic conductivity, but Juniperus, Prosopis and Diospyros all lost nearly 100% of their hydraulic conductivity by midday. All species had low values of hydraulic capacitance (< 200 kg m-3 MPa-1) and experienced water potentials low enough for their hydraulic capacitance to be exhausted. Overall, these four co-occurring species have remarkably different hydraulic strategies. In central Texas, Juniperus appeared to have the greatest mortality of the four species studied here. Although Juniperus was the most embolism resistant, it was the least able to tolerate the extreme drought conditions of 2011-2013.