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

This scientific commentary refers to ‘Conifer desiccation in the 2021 NW heatwave confirms the role of hydraulic damage’ by Klein et al. (doi: 10.1093/treephys/tpac007).Most of the Pacific Northwest (PNW, USA) and British Columbia experienced extraordinarily high air temperatures during an extreme heat wave event (‘Heat Dome’) in the late June of 2021 (Neal et al. 2022, Qian et al. 2022). In many locations, all-time record high air temperatures (Tair) exceeding 40–45 °C were observed during the Heat Dome. Within days of the event, widespread foliar damage was observed in many locations of this region along with some reports of tree mortality. As the frequency of coincident heat waves and drought has increased and is expected to continue to increase (Alizadeh et al. 2020), understanding the environmental drivers, biophysical and physiological mechanisms, and ecological consequences of heat damage incurred by forests is of broad relevance. Indeed, there is an increasing recognition that forest vulnerabilities to heat waves can lead to widespread growth reductions and even mortality (Breshears et al. 2021).