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New research finds that temperatures in 500-year-old Douglas fir forests measure up to 2.5 degrees cooler than in younger woodlands.
The world’s old-growth forests are worth saving for myriad reasons. They absorb more than their fair share of planet-warming carbon dioxide and provide nesting and sheltering habitat for a third of all wildlife. Fish and other aquatic animals also benefit from ancient forests as leaves, branches, and trunks fall into streams, providing nutrients and altering the flow of water.
Every increase of a degree in temperature means an escalation in species extinctions, according to scientists. The uptick could shift suitable habitats outside certain animals’ ranges, and that’s where old-growth forests could help out, said Sarah Frey, an ecologist at Oregon State University’s College of Forestry.
In a paper published Friday in the journal Science Advances, Frey and colleagues compared temperatures in 500-year-old Douglas fir forests in Oregon’s Andrews Forest with temperatures in 50-year-old woodlands nearby.
“We wanted to make sure that the comparisons we were getting were between mature forests, both with closed canopies supplying the same amount of shade—not just comparing old-growth regions to a recently clear-cut area or grassland,” Frey said.
By pairing temperature sensors with lidar technology, the team was able to get accurate measurements of tree-canopy density and overall biomass across 183 forest locations—one-third of which were second-growth plantation sites—while taking into account elevation and gradient changes.
They expected elevation to affect temperatures, but when they looked just at levels of forest vegetation, they found the old-growth forests had an “insulating effect” that kept temperatures up to 2.5 degrees Celsius cooler than in the second-growth forests.
“To the untrained eye, they look like similar forests with similar structure, but that’s a large difference in temperature between them,” Frey said.
But old-growth forests are in decline around the world. California has lost 95 percent of its ancient redwoods, Sweden has seen a 90 percent decline in big tree density, and fragmented rainforests in Brazil are losing half their big trees within 30 years of isolation.
“We tend to think of climate change on a global scale, but for animals, they experience it on a much smaller scale, in their immediate environment,” Frey said. “If there are animals that can take advantage of the cooler microclimates we’re finding in these old-growth forests, it could reduce the impact on those species.”
But how these old-growth forests stay cooler isn’t fully understood. The researchers suspect it has to do with the high levels of biomass (plants, lichens, mosses, and more) that exist in the canopies of old-growth trees and take centuries to develop.
“Think of it like a really deep lake compared to a small pond,” Frey said. “When it gets hot, temperatures fluctuate quicker in the pond because there is less water to warm up. We think the process of cooling and heating in old-growth forests is slower because there is so much more mass to influence.”