Are recent increases in forest mortality a symptom of drought?
Tree mortality is a poorly understood process in terms of its ecological drivers and physiological processes. Recent studies have documented rapid increases in background forest mortality rates and greater incidence of large-scale (catastrophic) forest die-back. These trends are often logically correlated with drought, whether arising from lack of precipitation, increases in temperature and evaporative demand, or both. What is the evidence that drought is an underlying cause of increasing tree mortality?
Physiologically-based investigations have suggested possible mechanisms of drought-induced tree mortality, though the relative importance of different pathways is likely to vary across taxa and specific drought conditions. New evidence suggests drought across several consecutive years may be particularly important, possibly acting through 'cavitation fatigue'. However, our understanding is too rudimentary to predict future forest response to drought. For example, very different outcomes are expected if the dominant physiological mode of drought–induced tree mortality is cavitation or (drought-induced) carbon starvation. This ambiguity is reflected in a recent modeling study where forest responses to future climates was highly variable depending if trees were more sensitive to absolute or relative changes in drought stress. An additional source of uncertainty is our relatively poor knowledge of how pests, pathogens and other disturbance agents amplify drought stress. The severe drought of 2014 across much of the southwestern United States provides a remarkable natural experiment to test our understanding of forest drought responses, including assessments of management actions designed to promote forest resilience to disturbance (such as prescribed fire and forest thinning). In sum, multiple lines of evidence suggest that drought is influencing patterns of tree mortality but we are unsure how forests will respond to continuing climatic changes.