by Aji John and Kavya Pradhan

The Pacific Northwest (PNW) is renowned for its temperate climate, lush forests, and diverse ecosystems. However, climate change has ushered in an era of extreme weather events, and if you remember 2021, the year the heat dome swept across the PNW in June 2021, we had record breaking temperatures that had devastating consequences for both the ecosystem and local communities. This event highlighted a critical gap in our understanding, the vital role that forest canopies play in buffering microclimates (inside the forests) and mitigating the impact of extreme heat events.

As part of looking into climate refugia, researchers at TNC and UW had set up loggers in Ellsworth preserve starting in 2019. In late June 2021, when the Pacific Northwest was gripped by a heat dome, a meteorological phenomenon characterized by a high pressure system that traps hot air in a specific area, the team had instrumentation in place to capture the impact of heatdome in the forests. Temperatures soared to unprecedented levels, with parts of Oregon and Washington experiencing highs above 100°F (37.8°C). The heat dome not only broke temperature records but also had serious implications for human health, agriculture, and local wildlife, leading to heat related illnesses and fatalities.

With the data we were able to collect, our study explored the relationship between forest canopy cover and microclimate buffering during the 2021 heat dome. We used ground based data alongside regional climate information to assess the temperature variations between forested areas and clearcut regions during the heat event.

We found three main things - forests cool, canopy density matters and vertical variation in climate is there.

Forest understories maintained temperatures that were, on average, 3°C cooler than clearcut areas and 4°C cooler than regional averages during the heat dome. This indicates that forests provide a significant cooling effect even during extreme weather events. The buffering capacity of forests is enhanced with denser canopy cover. Thicker canopies provide more shade and help retain cooler temperatures below the forest's surface, creating a microclimate that benefits flora and fauna. We also found that the greatest cooling effect was observed at the forest floor level, emphasizing the need to consider vertical microclimate dynamics when evaluating forest ecosystems.

Implications on Climate Resilience

Our study has profound implications for climate resilience strategies in the Pacific Northwest. As extreme heat events become more frequent due to climate change, the preservation and enhancement of forest ecosystems should be prioritized.

Effective forest management practices that promote dense canopy cover can enhance microclimate buffering. This could include selective thinning of trees to ensure healthy growth while maintaining a robust canopy.

As urban areas expand, integrating green spaces and preserving existing forests can help mitigate heat impacts. Urban planners should consider incorporating tree cover in their designs to create cooler urban environments.

Educating local communities about the importance of forests in climate regulation can foster support for conservation efforts. Community engagement in tree planting and forest stewardship programs can help reinforce the significance of these ecosystems.

What's Next

We emphasize the importance of forest canopy cover in buffering microclimates during extreme heat events, like the 2021 heat dome in the Pacific Northwest, firming the understanding that denser canopies maintain cooler understory temperatures, providing critical protection against rising heat levels. At TNC, the team is already implementing practices that promote dense canopies in forested areas. Raising awareness and involvement in local conservation efforts is part of TNC’s DNA. Related efforts are aiding urban planning that integrates green spaces to mitigate heat impacts.

Looking Ahead

The heat dome of 2021 served as a wake-up call for the Pacific Northwest, research on forest canopy cover and its role in microclimate buffering is a crucial piece of this puzzle. The team is now evaluating use of satellite imagery to track stress induced changes in these temperate forests.

As we move forward, it's imperative to incorporate these findings into broader climate adaptation strategies, reinforcing the intrinsic value of our forests in combating climate change. Our team is driven by prioritizing forest preservation and management, and augmenting the region's resilience to extreme heat events.