Old-growth forests can provide a refuge for heat-sensitive birds as temperatures rise across the Pacific Northwest, according to new research.
Ancient forests tend to provide moderate temperatures compared with their surroundings, potentially buffering some of the sharpest impacts of climate change, said Matthew Betts, a professor at Oregon State University.
With that knowledge, Betts and a team of researchers set off to see if the birds that breed in the canopies of these old groves benefit from their surroundings.
The scientists began by pulling data from the North American Breeding Bird Survey—a 52-year-old avian monitoring program that tracks hundreds of bird species across the U.S. and Canada.
Each June, more than 2,500 volunteers take to the forests to identify birds. They traverse 25-mile-long survey routes, stopping every half-mile for three minutes to count birds that are seen or heard in a quarter-mile radius. The survey, which is managed by the U.S. Geological Survey, is one of the most comprehensive long-term data sets on bird populations in North America.
Betts and his team selected 13 species often spotted in old-growth forests across Oregon, Washington and California, and they analyzed how the populations have changed over the past 30 years.
On paper, most of the species have weathered the warming trends so far. They found just two species, the Wilson’s warbler and the hermit warbler, whose populations have declined as temperatures have climbed. That’s because warming and precipitation changes can affect insects, the main food source of many bird species, Betts said.
Climate change is having an uneven effect on the Pacific Northwest. Some areas are warming, others are cooling, and it’s happening at different rates. When the researchers overlaid data from the breeding bird survey with satellite imagery that showed the distribution of old-growth forest across the region, they found a surprising result.
“It turned out if the landscape is warmed and had old growth on it, populations tended to not be declining of these two bird species,” Betts said.
On the other hand, if the landscape contained less than 20 percent old-growth forest and temperatures were up, warblers there suffered greatly.
Betts stressed these findings are correlative; no experiment was conducted. Still, he said it provides insight into how bird populations are responding to a changing climate. He and some colleagues hope to dive into why these forests seems to buffer certain bird species from the impacts of climate change. They’re planning future experiments at the H.J. Andrews Experimental Forest located in the western Cascade Range.
They already have some theories. One reason old-growth forests might be cooler could be tied to tree size. Many of the ancient giants boast trunk sizes of 6-8 feet in diameter, and they may suck up lots of heat and keep it trapped. Another reason could be that verdant old-growth forests have more layers of canopy, which intercepts sunlight.
Betts conceded it may be that birds flock to these ecosystems for other reasons despite below-average temperatures.
Birds are pretty smart, he said, despite the old adage “bird brain.”
“There are so many more nooks and crannies, cavities in the trees and shady parts in the branches … you could have them do what we would do on a hot day, naturally buffer themselves,” he said.