Old-growth forests in the Pacific Northwest do a much better job of mitigating the effects of climate change than less complex forests, even mature tree plantations with full canopy cover, a new study from Oregon State University and U.S. Forest Service scientists concludes.
While that might seem a tad obvious, the amount of temperature difference the researchers observed — 2.5 degrees Celsius, or about 4.5 degrees Fahrenheit — was well beyond what they expected to find, according to lead author Sarah J.K. Frey, a postdoctoral scholar with the Forest Biodiversity Research Network in the OSU College of Forestry.
“It’s common sense to think that it’s going to be cooler in an old growth or more mature forest than in a younger forest,” she said. “For us what’s so surprising is this gradient across structures that you wouldn’t think would be so large.”
The study, titled “Spatial Models Reveal the Microclimatic Buffering Capacity of Old-Growth Forests,” was published Friday in Science Advances, the journal of the American Association for the Advancement of Science. Frey’s co-authors were Adam S. Hadley, Mark Schultze, Julia A. Jones and Matthew G. Betts of OSU and Sherri L. Johnson of the U.S. Forest Service Pacific Northwest Research Station in Corvallis.
The researchers collected temperature readings in 2012 and 2013 at 183 locations on the H.J. Andrews Experimental Forest in the Cascade Mountains east of Eugene, sampling sites that ranged from 40-year-old tree plantations to undisturbed old growth forests. They used lidar imaging to create detailed maps of forest structure and computer modeling techniques to analyze the data.
“This is showing quite a large difference,” said Frey, noting that the cooling effect of old growth forests compared with that of mature, closed-canopy tree farms was roughly equivalent to the amount of temperature increase predicted to occur over the next half-century by the Intergovernmental Panel on Climate Change.
“That’s telling me that the way we manage our forests can have large implications for temperature regimes in our forests and also the plants and animals that live there,” she added. “We have the ability to buffer the effects of climate change.”
In addition to remaining significantly cooler during the spring and summer months, the researchers noted, old growth stands were also slightly warmer (by about 0.7 degree Celsius) during the winter. In general, that kind of temperature stability is beneficial to plants and animals.
One factor in this stabilizing effect appears to be the far greater amount of living plant material in old growth forests with fully developed understories, which can create a sort of “thermal inertia,” according to Frey.
“There’s so much more mass there to be heated up and cooled down, so it buffers the temperature more,” she said.
Converting forests to single-species, even-aged tree plantations is likely to reduce their ability to mitigate rising global temperatures, the study notes, while managing forest lands to promote old growth characteristics could create “microrefugia” for temperature-sensitive species, from insects and amphibians to large mammals.
While the study stops short of making any specific policy recommendations, Frey said it could provide guidance to land managers in the Pacific Northwest who must juggle competing priorities for timberland.
“We’re faced with a changing climate but also the demand for wood,” she said. “This has the potential to be useful to managers if they’re trying to preserve old growth forests.”