Professor Oregon State University Corvallis, Oregon, United States
Abstract: Under future climate change, we expect that extreme heat events – temperatures exceeding important physiological thresholds – will become more common with forest ecosystems responses ranging from reduced growth to tree mortality. Recent extreme heat events offer an opportunity to understand how vulnerability varies across complex landscapes. In late June 2021 the Pacific Northwest of North America experienced an exceptional “heat dome” heatwave, which brought multiple days of record high temperatures to the region. Daily maximum air temperatures exceeded 40oC in many parts of Oregon, Washington, and British Columbia, with atmospheric vapor pressure deficits in excess of 7 kPa and observed leaf temperatures nearing 50oC. Within days after the heat dome more than seventeen tree species began exhibiting red/orange foliage across the region similar to impacts of fire scorch. Curiously, observed patterns of foliar scorch did not indicate that the worst scorch occurred where heatwave temperatures were highest. This initial observation led us to investigate the geographic and biophysical drivers which led to the patterns of scorch severity that were observed across the region, so that we might better understand vulnerability to future heatwaves.
To better understand biophysical drivers of scorch vulnerability we used Sentinel-2 satellite data to map scorch probability based on values of the red-green index (RGI) in imagery from before and after the heat dome and used a conservative probability threshold for classifying scorch. Classification accuracy of the scorch maps were 90.4%, with rates of commission and omission of 3.5% and 6%, respectively. Scorch maps indicated that greater than 220,000 ha of forest experienced foliar scorch. We tested several hypotheses about drivers of scorch, including the effects of hillslope aspect, solar insolation, phenological stage and thermal acclimation at the time of the heat dome, site climate normals, and Swiss needle cast fungal infection severity. Preliminary results show that west facing slopes in coastal mountain ranges and mid-elevations of the Cascades range experienced the highest prevalence of scorch. These areas correspond to a cooler climatology, less thermal acclimation at the time of the heat dome, and earlier leaf phenological stages compared to lower elevation areas. Within the high impact zones, younger managed stands were more heavily impacted than older stands. These results indicate that younger managed forests in cooler regions are likely more vulnerable to extreme heat events, particularly in the early summer, suggesting a greater need for adaptation planning in these areas.