The Effect of Canopy Cover and Wildfire Smoke on Near-Surface Temperatures

Recent shifts from clearcutting to partial retention harvesting on many ownerships and the importance of microclimate dynamics on biotic responses to regional climate highlight the need to understand how microclimate conditions in forest understories differ across gradients of partial overstories. This study compares below-canopy near-surface temperatures at 2cm above ground to open-air temperature at 1.5m across a gradient of canopy cover and wildfire smoke. Findings are interpreted in the context how extreme events, such as the Northwest heat dome in Jun 2021 and large wildfires, as well as general warming trends may influence seedling performance and mortality. Overall, for every 10% increase in canopy cover, below-canopy air temperature at 2cm was 1.3 deg C lower and the odds of exceeding stress thresholds for conifer regeneration declined by a factor of 0.26. The reduction in temperature due to wildfire smoke was found to be equivalent to an additional 15% canopy cover. This study showed the benefits of residual canopy in lowering temperatures, but extreme events may overpower the ability of forest canopies and topography to provide microrefugia for temperature sensitive species and processes in managed forests. However, wildfire smoke may provide extra buffering during the hottest and driest conditions.