Forests are dying at an unprecedented rate due to climate change. Accelerated tree mortality poses a significant threat to numerous ecological, economic, and social benefits that forests provide. In California alone, over 129 million trees have died since 2010 due to prolonged drought and heat. Mounting evidence points to increased risk of mortality in large trees during extreme drought, a logical consequence of the physiology of taller trees. Much of the previous work is either theoretical or requires globally aggregated data to draw significant conclusions. By sampling ~2 million trees in California and tracking rates of mortality over 8 years, we have developed extensive empirical evidence identifying tree height as the single greatest predictor of mortality during extreme drought.
Results/Conclusions
Our temporal analysis shows 740,273 trees died in the 40,854 ha study area over eight years (41% mortality). Large trees die at over double the rate of small trees (20 % yr-1 and 7% yr-1, respectively). Testing biophysical, climatological, soil, and topographic variables, tree height emerges as the dominant driver of tree mortality under drought. The intensity of the height-mortality relationship is strongly mediated by environmental gradients. Under the high-frequency and duration extreme droughts predicted over the next century, we anticipate a widespread mortality of the largest trees on earth.