By: Lisa Hayward and Ryan McClymont

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Pacific Northwest hikers know how quickly summer temperatures can change as they cross from the heat of a parking lot into the cool shade of a mature forest. Factors like canopy cover, slope and aspect (direction that the slope is facing) all vary the temperatures that plants and animals experience within a forested habitat.

Sarah Frey, a graduate fellow at Oregon State University, was interested in what influence these aspects of local vegetation and topography would have on warming temperatures predicted by global climate models. The work was supported by the U.S. Department of the Interior Northwest Climate Science Center, which is managed by the U.S. Geological Survey National Climate Change and Wildlife Science Center.  

Specifically, Frey wondered whether the influence of microclimate might be powerful enough to lessen the impacts of climate change on forest birds like hermit warblers and varied thrushes.

Most climate models, used to predict future habitat change, usually take into account only large-scale patterns of air temperature and not the nuances of specific microhabitats. To get a better picture of the temperatures that animals actually experience Frey went out in the woods and measured the temperatures directly.

Under the supervision of her adviser, OSU Forest Ecosystems and Society professor Matthew Betts, Frey hiked into the National Science Foundation’s HJ Andrews Experimental Forest to measure temperatures at 183 sites on the west side of Oregon’s Cascade Mountains. Using temperature loggers hung throughout the forest she recorded temperatures at these sites year-round.

Twice a year, Frey and her crews of 4-6 people would visit the loggers to download data and check the batteries. She found that air temperatures in old growth forests were 2.5 degrees cooler than similar closed canopy plantations logged 60 years ago. Two and half degrees also happens to be the amount climate models predict the global temperature will increase over the next 50 years.

“We compared old growth to other closed forest types rather than to clear-cuts so we didn’t expect the difference to be so dramatic,” Frey said. “I expected to see a difference, but I was surprised by how big it actually was.”

While both types of forests have closed canopies, old growth forests have bigger trees and a more complex understory. Frey also found that the effect of vegetation on microclimate was stronger in larger patches of old growth.

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“So the size of the old-growth forest patches matters,” said Frey. “With larger patches showing stronger buffering capacity.”

Frey’s crews also recorded all the birds they saw and heard at each site. She used this point count data to investigate how microclimates influence where forest birds spend time. To her surprise, Frey found that about half (53.3 percent) of the species common to her sites prefer warmer than average temperatures, while the other half (46.7 percent) prefer it cooler. Hermit warblers, for example, seem to prefer cool sites, while Pacific wrens seem to like it warmer. Frey found that, overall, temperature difference is as good, or even better, a predictor of where birds spend time in the forest than vegetation.

Using publically available data from the annual breeding bird survey collected by citizen scientists, Frey did a quick test to determine whether the temperature preferences she was seeing influenced bird population patterns over time. The survey data, organized by Audubon and the USGS, provided population trends from 2002-2012 for each of the species for which she had determined temperature preference. Frey found bird species associated with the colder temperatures were more likely to be declining, while populations that liked warmer temperatures were more stable over time.

Frey’s findings help natural resource managers better understand how to lessen the impacts of climate change on birds and other forest inhabitants. For example, her work demonstrates how management practices that create variable canopy structure or increase biomass can help buffer against temperature increases.

The NWCSC is a federal-academic partnership established by DOI to address the challenges presented by climate change in managing cultural and natural resources in the northwestern United States. The center is one of eight nationally that provides scientific information to help natural resource managers respond effectively to climate change.

Learn More

Read the full research article on buffering capacity of old growth: Spatial models reveal the microclimatic buffering capacity of old-growth forests

Read the full research article on temperature preferences of birds: Microclimate predicts within-season distribution dynamics of montane forest birds