Conifer desiccation in the 2021 NW heatwave confirms the role of hydraulic damage

The unprecedented heatwave that hit the Pacific northwest of North America in late June-early July 2021 impacted ecosystems and communities, yet evidence and analysis of this impact are still missing. Here we bring a unique dataset quantifying the impact on conifer trees, which are keystone species of many northwest ecosystems. Moreover, we take advantage of this exceptional event as a broad, extreme, “field experiment” to test a fundamental theory in plant physiology, and prepare our forests to a harsher future. Overall, the data collected confirm the role of hydraulic vulnerability in drought-induced injury to trees. Among the recorded species, we obtained P50 data for 27 species, represented by 64 cultivars. Plotting needle browning extent by P50 revealed important thresholds of drought sensitivity: (1) species with P50 <-6 MPa were unaffected by drought. (2) species with -6 MPa< P50 <-5 MPa had mild extent of needle browning, up to 25% of the canopy. (3) species with P50 > -5 MPa had browning of up to 95%. The sharp divergence among resistant and vulnerable conifer species according to their xylem vulnerability, all of which simultaneously exposed trees to the same extreme event at the same site, is evidence to the key role of P50, in agreement with previous assessments of drought effects on angiosperms. Among local, NW conifer species, some cultivars proved hardier than others. The aftermath of the 2021 NW heatwave should take advantage of this broad, extreme, “field experiment” to prepare our forests to a harsher future.