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Contributed Talk

Session: : Physiological Ecology 3

COS 180-6 - Impacts of the June 2021 Heat Dome event on trees and forests of the Pacific Northwest, USA

Wednesday, August 9, 2023
11:15 AM – 11:30 AM PDT
Location: E142
Abstract: Background/Question/Methods

Following 2 years of drought, many areas of the Pacific Northwest (PNW) experienced record high air temperatures during an extreme heat wave event (“Heat Dome”) in late June of 2021. Notably, all-time record high air temperatures (Tair) during the heat dome coincided with the longest photoperiod and greatest solar insolation of the year, driving sunlit foliage temperatures 5-10 ºC above Tair. Upper canopy microclimate measurements at the H.J. Andrews Experimental Forest and LTER site, showed that at midday on the hottest day Tair was ~42-44 ºC and sunlit foliage was around 50 ºC for several hours. These temperatures exceed known thresholds for leaf damage and there is an increasing recognition that heat waves can lead to widespread growth reductions and forest vulnerabilities. As the frequency of coincident heat waves atop drought is expected to increase across the conterminous US, understanding the environmental drivers, ecological consequences, and biophysical and physiological mechanisms underlying heat wave damage incurred by forests is essential. In this work we analyze tree-, ecosystem-, and regional-scale responses to the 2021 Heat Dome using a combination of stem dendrometry data, flux data from the Wind River Experimental Forest (old-growth forest) and ABBY (plantation forest) NEON sites, and aerial damage survey maps.



Results/Conclusions

The extreme heat elicited multi-scale responses in trees and forests of the PNW, including widespread reports of foliage scorch from coastal locations and the western Cascades. Aerial mapping of scorch damage suggests at least 93,000 ha were moderately to severely affected, primarily on south- and west-facing aspects. The scorch led to unusual early pulses of mostly non-scorched litterfall at the Andrews and ABBY forests within a week or two of the Heat Dome. Aside from direct leaf impacts, the extreme heat strongly affected forest-atmosphere fluxes. At both NEON forest sites the net ecosystem exchange of CO2 was on average positive, i.e., the forests were C sources throughout the event at a time when they are normally carbon sinks. At both sites, evapotranspiration increased markedly ( >20%) during the event compared to before and after it, leading to canopy cooling and reduced sensible heat fluxes. Dendrometers at the Andrews Forest and sites of the Douglas-fir Seed-Source Movement Trial recorded pronounced stem shrinkage from dehydration during the event, followed by reduced or minimal growth for the remainder of the growing season. Not all monitored trees were negatively affected, suggesting some resilience in response to future extreme heat events.