Print PageIn the southwestern United States, extended drought has historically resulted in widespread tree mortality in piñon-juniper woodlands. In 2007, a large-scale rainfall manipulation experiment was implemented in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland to assess the response of these two species to extended drought. The experimental design utilized four replicated treatments including; passive rainfall exclusion (50% reduction), supplemental H2O addition, plastic cover control, and ambient control. Here we address differences in canopy gas exchange during the 2010 growing season in one replicate block. We measured sap flow on five target trees of piñon pine and juniper using the Granier method, and used this data to estimate midday canopy stomatal conductance (GS) and whole-plant leaf specific hydraulic conductance (KL). KL was calculated using midday sap flow data and the difference between predawn and midday leaf water potential. We compared this data with soil water content (SWC) measurements and periodic measures of plant water potential. Because SWC varied greatly with the temporal onset of summer monsoon rains, we compared two time periods: Julian-day 181 (pre-monsoon) and Julian-day 214 (monsoon).
Results/Conclusions
We observed sharp reductions in GS for trees exposed to drought. Pre-monsoon, midday GS in piñon averaged 1.73 (0.61) and 1.86 (0.41) mmol m-2 s-1 in drought (D) and ambient control (C) plots respectively compared to 19.1 (0.83) in irrigated (I) piñon. Pre-monsoon GS in juniper averaged 5.5 (1.28), 12.3 (1.94), and 22.9 (2.56) mmol m-2 s-1 in D, C, and I trees. Clearly, both seasonal and experimentally induced soil water stress resulted in midday stomatal closure and negligible net photosynthesis in non-irrigated piñon trees. Following monsoon precipitation, piñon GS increased to 20.5 (1.70), 30.4 (3.92), and 39.5 (3.34) mmol m-2 s-1 in D, C, and I trees respectively. While GS recovered during monsoon in drought treatment piñon, rates were 33% lower compared to control trees. In contrast, monsoon juniper GS recovered to comparable levels in drought and control plots. Similar to GS, a reduction in KL was also observed for droughted piñon compared to controls. In summary, drought treatment piñon did not recover hydraulic function comparable to control trees once seasonal water stress was alleviated. This observation is consistent with recently published research where recovery of stomatal function was directly related to post drought recovery of xylem hydraulic conductivity.
