Microclimate during an extreme heat event varies across forest management types

Background/Question/Methods

Increasing temperatures and extreme heat episodes have become more common in recent years across many of the regions of the Earth including western North America. While forests are known to be relatively buffered from increasing temperatures compared to non-forests, the impact of extreme temperature events on forest canopies is relatively unknown. The heatwave and associated heat dome that occurred during June 2021 in the Pacific Northwest region of North America was one such event, with temperatures reaching 5°C warmer than the regional June monthly mean. To assess the immediate impact of these heat episodes on forests, we monitored microclimate in a temperate moist forest (20 sites) and an adjacent non-forested area (clear-cut) in the Willapa Hills region of southwestern Washington State, USA. The mixed-conifer forests that dominate this region are part of a landscape level experiment aimed at identifying how to accelerate the development of old-growth forest characteristics in former industrially managed timberlands. As such, the experimental watershed is subdivided into control and treated (forest thinning) basins. Temperature sensors were deployed in surface and air locations from May 2020 to September 2021 and were stratified by topographic position index and management intervention (control or thinned).

Results/Conclusions

Our results suggest that, while forests are able to buffer the effects of extreme heat events, not all forest densities have similar buffering capacities. The impact of the 2021 summer heatwave varied by forest density. In line with our expectations, the non-forested area was much warmer, on average, than forested areas during the heat dome event. In addition, thinned forest sites were 5°C warmer and more variable compared to control sites (where there was no forest thinning). Our results also revealed two more insights. Firstly, air temperatures across all sites increased much more than soil temperatures during the heat dome event providing evidence for significant differences in temperature along a vertical transect. Secondly, temperatures within a site varied more during the heat dome event than before or after the event. Taken together, our findings demonstrate the heterogeneity in forest buffering capacity, which affects the diversity of organisms inhabiting these forests and the microclimate they experience in the understory. This study demonstrates not only the important buffering capacity of forests, but also the substantial effect that forest density has on surface and air temperatures which can give us insight into future climate change impacts.