Assessing heat dome 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 both forests and the organisms that depend on them. This study evaluated the effects of the June 2021 Pacific Northwest (PNW) heat dome (a prolonged high-pressure system that generated record-breaking temperatures) on forest canopy health and 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 through fall from 2017 to 2025 to identify canopy stress and quantify changes in canopy greenness. The experimental forest thinning treatments, designed to accelerate old-growth structural characteristics, allowed us to evaluate canopy responses relative to restoration treatments. Our results showed marked declines in the summer NDVI during and following the 2021 heat dome, indicating substantial canopy stress. These declines differed from patterns observed in preceding years and occurred across all forest treatments. Changes in seasonal NDVI dynamics were evident in the control, thinned, clearcut, and old-growth stands; however, pairwise comparisons showed limited differences among canopy types, indicating broadly similar patterns of NDVI suppression during extreme events. The NDVI trajectories during the four post-heat dome years suggested partial recovery in canopy greenness, although the variability among the treatments remained high and overall NDVI levels remained below Pre2021 conditions in some stands through 2024. The recovery trajectories differed modestly among treatments, but we found limited evidence that canopy density substantially altered either the magnitude of canopy suppression or the subsequent recovery trajectories. Overall, these findings demonstrate that extreme heat events can induce widespread canopy stress across forest types and suggest that canopy structure alone may have a limited influence on canopy-scale responses under extreme atmospheric conditions. These results provide insight for adaptive forest management strategies aimed at maintaining ecosystem function under increasingly extreme climatic conditions.