Assessing heatdome effects on forest dynamics in the Pacific Northwest using Planet imagery

Increasingly frequent extreme heat events are imposing significant stress on forest ecosystems, with implications for forests and the organisms that live in them. This study evaluates the effects of the June 2021 Pacific Northwest (PNW) heatdome (a prolonged high-pressure system that generated record-breaking temperatures) on forest canopy health and its subsequent recovery within the conifer-dominated Ellsworth Creek Preserve in Washington state, USA, using high-resolution Planet Labs satellite imagery. We analyzed normalized difference vegetation index (NDVI) trends from spring to fall from 2017 to 2024 to identify canopy stress and quantify changes in canopy greenness. Experimental forest restoration thinning treatments implemented to test their effectiveness in restoring old-growth forest characteristics at our sites allowed us to assess impacts of the heatdome event relative to these restoration treatments. Our results show a marked decline in summer NDVI following the 2021 heatdome, signaling substantial canopy stress. This decline, different than patterns in the other seven years, was particularly strong in dense, unmanaged (control) forest stands, which exhibited more pronounced NDVI declines than thinned (treatment) stands. These more pronounced stress signals during and after the heat event were associated with greater tree canopy height, canopy complexity, and tree density. The summer NDVI trends over the three post-heatdome years suggest a gradual recovery in tree canopy greenness, with thinned stands showing faster NDVI recovery than controls. In all, these results suggest that restoration thinning may improve canopy resilience to extreme heat. However, recovery trajectories varied widely within stand types, with persistent evidence of stress in high-density areas that suggest incomplete recovery as of 2023. Broadly, these findings show the vulnerability of dense forest canopies to extreme heat events, suggesting that forests with high canopy density may be less adaptable to recurring heat events. Nonetheless, managing forests through restoration thinning may enhance forest resilience to these climate-induced stressors - providing potential guidance for adaptive management strategies to maintain forest health in a world with more extreme climates.