Old-Growth Forests Provide Climate Change Relief for Some Bird Species, Study Finds

A Swainson's thrush songbird in a forest in Whitehorse, Canada
A Swainson's thrush songbird in a forest in Whitehorse, Canada. Stefan Gottermann / 500px / Getty Images
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Many bird species are sensitive to temperature fluctuations, especially the extreme ones that come with climate change. Global heating is especially difficult for birds, who need shade, cool water and breezes to cool down.

A new study by the Oregon State University College of Forestry suggests some bird species can get relief from climate change from old-growth forests and forests that have old-growth characteristics, a press release from Oregon State University (OSU) said.

“We have two hypotheses for this benefit, both of which received some support in our recent paper: (1) old-growth forests are cooler during the breeding season, which has a benefit for birds associated with colder climates. This effect could be due to either (a) a direct physiological advantage (the birds don’t overheat), or (b) (more likely) there is a positive effect on food availability. (2) Old forests tend to have greater vegetation diversity, which means greater insect diversity (more food to choose from) and longer periods of insect availability,” Matthew Betts, who is a professor in the Department of Forest Ecosystems and Society and one of the study’s co-authors, told EcoWatch in an email.

The study, “Forest microclimate and composition mediate long-term trends of breeding bird populations,” was led by Hankyu Kim, a former doctoral student at OSU, and published in the journal Global Change Biology.

The study builds on previous research by Betts, according to the press release. Betts’ research demonstrated that mature forests with large trees of a variety of species and sizes can provide protection to some bird species that are vulnerable to the effects of planetary warming.

“Of course, old growth forests are beneficial for birds in ways that don’t relate to climate. They tend to have more dead wood (good for cavity nesters like woodpeckers and nuthatches), and greater diversity in vertical structure (think: many canopy layers),” Betts told EcoWatch.

The researchers said their findings have crucial significance for determinations regarding the conservation of older forests.

The study examined the “microclimates” — fine-scale climatic conditions of a particular region that are different from the adjacent “free air” environment — above the forest canopy. Microclimates have a tendency to be most noticeable in regions with diverse terrain and forest types, like mountains, islands and coastal areas. The researchers conducted their study in the H.J. Andrews Experimental Forest in Oregon’s Cascade Range.

“To my knowledge, this is the first empirical evidence of any microclimate effect on songbird populations, and of the insurance effect on free-ranging birds,” said Kim, who is now a postdoctoral researcher at the University of Wisconsin-Madison, in the press release. “Each species may have a slightly different range of thermal optima — the range of thermal conditions they feel comfortable with — and it could be the same for the interaction between forest ecosystems and birds.”

The research team, which included associates from the United States Forest Service and OSU, looked at eight years of data on the abundance of breeding birds in a watershed at the H.J. Andrews forest. The authors also examined temperature readings of the forest subcanopy based on vegetation data from the ground and Light Detection and Ranging (LiDAR) laser measurements.

The way the current trend of global warming affects the forest ecosystem is beneficial to some birds, while others find it difficult to breed due to less available food and physiological challenges.

The team came to the conclusion that certain species of birds were liable to fare better in cooler microclimates. They called this tendency the “buffering effect.”

Of the 20 bird species the scientists studied, population trends for five of them — the Swainson’s thrush, hermit warbler, red crossbill, chestnut-backed chickadee and Wilson’s warbler — tended to be either steady or less adverse in microclimates with cooling effects.

“Trends of abundance of five species declined at greater rates in warmer locations than in cooler areas,” said Kim in the press release. “That suggests microclimates within forested landscapes do provide refugia for those species. Declining species that are sensitive to warm conditions, like the Wilson’s warbler, hermit warbler and chestnut-backed chickadee, seemed to benefit the most from refugia effects.”

Certain bird species did better in forested areas that were more diverse, which the researchers called the “insurance effect.” They used this term because due to the forest’s diversity there were likely to be insects for the birds to feed on during their breeding season, when they needed energy and nutrients the most.

“If plants leaf out earlier in warm microclimates, causing arthropods to emerge earlier, there is a danger of migratory birds mistiming their breeding with peak food availability,” said Betts in the press release. “Since leaf-out timing varies by plant species, forests with more plant diversity often have a longer period of insect availability.”

The adverse effects of climate warming for two of the species — the red crossbill and the Wilson’s warbler — were lower in forests that were more diverse.

“There is little doubt that climate change will continue to affect bird species into the future. The losers will be higher elevation and higher latitude birds that are not well adapted to warm climates. We’ve already seen quite negative effects after only ~1 degree C temp rises,” Betts told EcoWatch. “With projected 1.5 – 2 degree rise in the future, we could see strong climate-related declines for a number of species. This, in turn, will likely influence the ecosystem services these birds provide (e.g., insect control, recreational benefits, pollination).”

The other 14 birds the researchers looked at for the study were the hermit thrush, McGillivray’s warbler, dark-eyed junco, Pacific-slope flycatcher, golden-crowned kinglet, brown creeper, black-throated gray warbler, hairy woodpecker, Hammond’s flycatcher, red-breasted nuthatch, red-breasted sapsucker, yellow-rumped warbler, Pacific wren and western tanager.

What can humans do to help protect bird species in the face of climate change?

“First, try to limit climate change by altering our practices (burning fossil fuels)! Second, keep old-growth forests around as they seem to have this ‘buffering’ capacity for bird populations,” Betts said.

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