Old-growth forests serve as a refuge in a warming climate

According to a new study led by the Oregon State University (OSU), old-growth forests and managed forests with old-growth features can provide relief from the effects of climate change for some bird species. Building upon earlier research showing that old forests with big trees and a diversity of tree sizes and species offer refuge to some types of birds threatened by a warming climate, the experts investigated how forest “microclimates” – local atmospheric conditions in areas ranging from a few square meters to many square kilometers which differ from those of the surrounding regions – affect bird diversity and abundance.

Since microclimates tend to be most pronounced in areas of rugged and varied topography, such as mountains, coastal areas, or islands, the researchers focused on Oregon’s Cascade Range, home to the HJ Andrews Experimental Forest. 

By analyzing eight years of breeding bird abundance data from an HJ Andrews watershed, together with subcanopy temperature readings and vegetation data, the scientists discovered that bird species tend to fare better in cooler microclimates, a phenomenon they describe as the “buffering effect.” Moreover, some bird species were found to do better in regions where the forest had more compositional diversity – referred as the “insurance effect,” since diversity helps ensure the presence of insects the birds feed upon during their breeding season.

“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 study lead author Hankyu Kim, a postdoctoral researcher at the University of Wisconsin-Madison. “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.”

In a warming climate, some birds interact with the forest ecosystem to their benefit, but others have difficulty breeding in certain areas due to decreased food availability. While seven of the 20 species that the scientists examined showed overall declines in abundance over the eight-year study period (2011-2018), nine showed increases and four did not display any detectable trends. 

Five bird species -the Swainson’s thrush, the chestnut-backed chickadee, the hermit warbler, the varied thrush and Wilson’s warbler – benefited from the buffering effect. The Wilson’s warbler and the red crossbill were found to benefit from the insurance effect.

“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 study senior author Matthew Betts, an expert in forest ecosystems at OSU. “Since leaf-out timing varies by plant species, forests with more plant diversity often have a longer period of insect availability.”

“Trends of abundance of five species declined at greater rates in warmer locations than in cooler areas. 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.” Dr. Kim concluded.

The study is published in the journal Global Change Biology.

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By Andrei Ionescu, Earth.com Staff Writer