The importance of the water-energy limitation threshold in drought impact studies

Forest diebacks have increased in magnitude in many regions in response to greater water limitation. "Hotter droughts" are predicted to increase under climate change and result in significant restructuring of forest composition and ecosystems services. Many drought-related studies, however, focus on water limited systems, where water—not energy—poses the greatest constraint on photosynthesis. Contrary to expectation, hotter, drier conditions in energy limited systems may cause greater growth, as the growing season is extended. Thus, identifying the location of the λE (water-energy limitation) threshold is critical to predict forest mortality under climate change. Here we assess the impacts of the recent extreme drought in California (~2012-2015) on subalpine trees across the central and southern Sierra Nevada. We use a combination of 700 tree-rings and over 1,000 stable isotope samples to test whether extreme drought led to greater growth or greater physiological stress. We show that during extreme drought, the energy-water limitation threshold shifted upslope into higher elevation forests; trees growing near this threshold experienced some physiological stress, but trees far from this threshold experienced positive growth. These results suggest that extreme drought has a more nuanced effect on average productivity for forests that occur across strong climatic gradients.