Precipitation pulse responses in a piñon-juniper woodland in central New Mexico

Drought-related tree mortality has been documented across the southwestern United States, but its mechanistic basis is as yet undetermined. Plants can avoid hydraulic failure by changing the balance between root absorbing area and leaf transpiring area, altering their xylem cavitation resistance, and restricting transpiration via stomatal closure. We are evaluating species specific transpiration responses to precipitation prior to the start of a large-scale and long-term rainfall manipulation experiment. Using the heat dissipation method, we measured sap flux through Pinus edulis (piñon pine) and Juniperus monosperma (oneseed juniper) trees in conjunction with natural precipitation over a fall, winter, and spring at the Sevilleta LTER in central New Mexico. We predicted that 1) P. edulis would have a shallower rooting profile, and would therefore respond more rapidly to precipitation pulses than J. monosperma and 2) that P. edulis should achieve higher transpiration rates at high soil water potentials, but J. monosperma should maintain transpiration at more negative soil water potentials. Our preliminary data support the rooting profile hypothesis, but the relative transpiration rates of the two species were surprising. J. monosperma had a larger sap flux response than P. edulis to large and medium pulses during the monsoon, and decreased during the post-monsoon dry-down period. P. edulis, on the other hand, did not respond to either of the earlier pulses, but during the dry-down it increased sap flux to rates comparable to J. monosperma. Possibly, the extremely dry year preceding our data collection compromised the hydraulic architecture of P. edulis without affecting the more drought-tolerant J. monosperma. Understanding both species' variable responses to pulses is key to describing the mechanistic basis of how trees cope with drought in a semiarid environment and predicting the effect of changes in the precipitation pulse regime.