Causes of widespread foliar damage from the June 2021 Pacific Northwest Heat Dome: more heat than drought

Abstract

During an extreme heat wave event (‘Heat Dome’) in Pacific Northwest (PNW, USA) and British Columbia during late June of 2021, all-time record high air temperatures (T_air) exceeding 40-45 °C were observed. Within days of the event, widespread foliar damage was observed in many locations of this region, along with some reports of tree mortality. Indeed, there is an increasing recognition that heat waves can lead to widespread growth reductions and forest vulnerabilities. As the frequency of coincident heat waves and drought has increased and is expected to continue to increase, understanding the environmental drivers, biophysical and physiological mechanisms, and ecological consequences of heat damage incurred by forests is of broad relevance. A recent paper, Klein et al. (2022) used this extraordinary event to link mechanisms of drought-induced plant mortality (hydraulic failure) to the observed foliar damage. Klein et al. related photographic observations of foliar damage from a variety of tree species and cultivars growing in a botanic garden near Seattle, WA to hydraulic parameters from other studies of the same species. They found relationships between the degree of damage and literature-derived values of P50, the xylem pressure at which a 50% loss of conductivity occurs. Specifically, Klein et al. found no damage in species with a P50 value below -5 MPa, as well as age-related patterns, with younger trees experiencing more damage. Based on this evidence, Klein et al. concluded that drought-induced hydraulic damage was the mechanism responsible for the observed foliar damage. However, in what follows, we argue that framing this event as an opportunity to test drought sensitivity, with the foliar damage driven by dehydration and hydraulic failure, fails to consider the role of extreme heat as a primary mechanism of foliar damage.

Citation

Still, C J; Sibley, A; DePinte, D; Busby, P E; Harrington, C A; Schulze, M; Shaw, D R; Woodruff, D; Rupp, D E; Daly, C; Hammond, W M; Page, G F M. 2023. Causes of widespread foliar damage from the June 2021 Pacific Northwest Heat Dome: more heat than drought. Tree Physiology. 43(2): 203-209. https://doi.org/10.1093/treephys/tpac143.