Unexpected indirect effects of field simulated drought offset direct climate-change impacts

Climate change is expected to increase the frequency, intensity, and duration of droughts in most regions around the world. The ecohydrological response will not occur immediately because it involves changes ranging from rapid physiological change to slower evolutionary and community composition change. Acclimation is the speed by which these phenomena occur while reaching a new state in equilibrium with novel climate conditions. Data show that average aboveground net primary production is relatively more sensitive to changes in average precipitation than when observing the same changes of precipitation in one site. The spatial model relating average production and precipitation represents full acclimation and the temporal model minimum acclimation. Estimating acclimation is critical for predicting the impacts of future climate change and requires an in-depth understanding of its ecohydrological mechanisms. Our long-term experimental drought in the Chihuahuan Desert in the SW provides evidence of the direct and indirect mechanisms driving acclimation. Experimental drought, when lasting less than 4 years, caused immediate reduction in primary production driven by changes in plant ecophysiology and relative species abundance. On the contrary, sustained drought caused changes in species composition that offset the direct effects and reduced the speed of acclimation. Finally, the mechanisms and the rate of acclimation show thresholds highlighting acclimation rate not just as a linear time process but a more complex phenomenon involving multiple scales and discontinuities. Thresholds result from cumulative ecological phenomena, such as reductions in grass tiller density, acting in combination with broad-scale climatic patterns, such as El Niño and the Pacific Decadal Oscillation.