A decade of decline: How trees in a semi-arid ecosystem respond to long-term drought

R. Alex Thompson and Henry D. Adams, School of the Environment, Washington State University, Pullman, WA, Jeff Dudek, DeAndre Garrett and Kaitlyn McNiel, Plant Biology, Ecology and Evolution, Oklahoma State University, Stillwater, OK, Shealyn Malone, Forest and Wildlife Ecology, University of Wisconsin, Madison, WI, Nathan McDowell, Atmospheric Sciences & Global Change, Pacific Northwest National Laboratory, Richland, WA, Cameron McIntire, United States Forest Service, Durham, NH, Drew Peltier, Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, Drew Peltier, Center for Ecosystem Science & Society, Northern Arizona University, Flagstaff, AZ, William T. Pockman, Department of Biology, University of New Mexico, Albuquerque, NM, Nathan Robertson, Biology, University of New Mexico, Albuquerque, NM, Amy Trowbridge, Entomology, University of Wisconsin-Madison, Madison, WI

Background/Question/Methods
Water is a necessary resource supporting all plant life. The diversity of earth’s vegetation spans a gradient of water availability, from the cloud forests of Latin America to the xeric deserts of Central Asia, and nearly everything in between. Thus, plants are highly adapted to a variety of water regimes, yet we know little of their capacity to acclimate to abrupt and permanent reductions in water availability. Using a rare field drought experiment in the southwest USA, we compared the physiological response of mature Pinus edulis trees to a 10-year (long-term) and 1-year (short-term) experimental drought. Trees in both treatments were exposed to a 45% reduction in precipitation using throughfall exclusion structures. To assess the response of trees in each plot to their respective drought condition, we analyzed changes in sapwood area to leaf area ratio (As:Al), leaf-mass per unit area (LMA), maximum rates of photosynthesis (Amax) and predawn and midday water potential Ypd/md. We present four hypotheses indicative of drought acclimation: 1.) Trees exposed to long-term drought increase As:Al and LMA, 2.) maintain Amax at levels consistent with a control treatment, and 3.) exhibit Ypd similar to trees experiencing short-term drought, while acclimating Ymd to control treatment levels.
Results/Conclusions
We found no evidence of hydraulic adjustment through changes in As:Al with drought duration, in addition to a lack of changes in LMA. Interestingly, trees exposed to a long-term drought maintained Ypd and Ymd that did not differ significantly from the control, which observed no drought, even while trees exposed to a short-term drought showed significantly more negative Ypd and Ymd. During the pre-monsoon dry season, Amax of trees exposed to long-term drought was significantly higher than Amax in the new drought and control trees. However, following the arrival of monsoon rains, Amax of trees in all plots increased equally. Our results are the first to suggest that P. edulis may possess a mechanism for acclimating to long-term reductions in precipitation. We showed that this acclimation may not occur via traditional mechanisms of changes in As:Al or LMA. Other possible mechanisms include increased rooting depth to access deep water supplies or decreased osmotic potential of leaves via accumulation of soluble sugars.