Oregon State University forest scientists studying tree regeneration have found that wildfire smoke brings an unexpected benefit: It has a cooling capacity that can make life easier for vulnerable seedlings.
An OSU College of Forestry collaboration led by faculty research assistant Amanda Brackett made the discovery while working to determine the effect of forest cover on maximum summer temperatures near ground level.
The aim of the study was to describe how heat waves and other future climate conditions could influence the canopy’s influence on temperature. The scientists used previously established heat stress responses of seedlings from laboratory studies to look at temperature-induced stress in conifer seedlings and germinating seeds under different canopy conditions.
“Tree recovery is an indicator of forest resilience and an important ecological process, but young seedlings are particularly vulnerable to heat stress,” Brackett said. “Our results show that while excessive canopy cover can help buffer seedling temperature stress, extreme heat and climate change still pose significant challenges – and that wildfire smoke may even be beneficial.”
Brackett and colleagues Chris Still and Klaus Puettman created a metric called stress rate hours to represent the duration and intensity of high temperatures that exceed levels that hinder tree regeneration.
“This work shows that widespread wildfire smoke can lead to unexpected beneficial effects by reducing surface sunlight and thus lowering the temperatures that vulnerable tree seedlings experience, even during extreme summer weather such as the 2021 heat dome period,” said Still, a professor. which studies climate change feedbacks and impacts on forests.
In the summer of 2021, the scientists surveyed 11 different stands of recently thinned, 50-year-old Douglas firs in the Upper Blue River Watershed in western Oregon’s Willamette National Forest, measuring air temperatures 2 inches above the ground at southern slopes. The study area was heavily impacted by wildfire smoke in August 2021 due to the Middle Fork Complex and Washington Ponds fires.
Overall, and after taking into account the influences of topography, they found that for every 10% increase in canopy cover, maximum temperatures at 2 centimeters were 1.3 degrees Celsius lower, and that temperatures were more likely to exceed exceeding stress thresholds for conifer regeneration fell by a quarter. and the total number of stress level hours decreased by 40%.
“These reductions are large enough to merit attention in tree regeneration management,” Brackett said. “But data collected during the June 2021 Pacific Northwest heat dome indicates that with different climate change and heatwave scenarios, temperatures will be unfavorable for regeneration regardless of canopy coverage.”
During the heat dome, the highest temperature near the soil surface that the scientists recorded was 57.4 degrees Celsius, or just over 135 degrees Fahrenheit.
The impact of smoke on maximum and average temperatures at the surface was similar to the temperature drop caused by a 15% increase in canopy coverage, Brackett said.
The finding is consistent with the idea that while smoke can cause a range of problems, fire is a natural forest phenomenon with ecological benefits.
“Smoke absorbs and scatters incoming solar radiation, reducing the amount of solar radiation reaching the forest floor, but we didn’t expect to see this impact,” she said. “That extra cooling effect can be important during extreme heat, when canopy cover and topography do not provide sufficient protection against damaging and potentially fatal understory temperatures.”
Tree regeneration is a crucial part of creating the diverse structure associated with healthy, perennial forests, she said, adding that the study highlights the need to consider the conditions young seedlings experience when designing and implementing of forest treatments.
“Managers need to understand how the microclimate varies under a range of excessive conditions,” Brackett said. “Given climate forecasts, scientists and managers should look for opportunities to increase the resilience of individual trees and forests to a hotter and more extreme climate. Wildfire smoke will continue to impact ecosystems and while we have documented an unexpected benefit, more work is needed to untangle other potential impacts.”
