A Buffer Zone for Trees

On most trails, a hiker climbing from valley floor to mountain top will be caressed by cooler and cooler breezes the farther skyward they go. But there are exceptions to this rule: Some trails play trickster when the conditions are right. Cold air slips down the slope to pool at the bottom, leaving the crest of a mountain warmer than its base. If you’ve ever reached the top of a hike and looked down from a clear sky to a fog-shrouded valley, you’ve probably seen this phenomenon at play.

It turns out that these meteorological islands of cold air, which exist across the mountainous and hilly regions of the world, do more than just fool hikers. A new study shows they can reshape local ecosystems—and may even provide an escape route for some species threatened by climate change.  

In most mountainous regions, cold-loving trees like spruce and fir grow at higher elevations while warm-adapted ones like maple and beech grow at lower elevations. But a team of researchers from the University of Vermont, who studied temperature and forest composition in three New England forests, found these patterns of growth can be inverted when the cold islands of air blanket lower elevations.

“These areas prone to cold-air pooling, for now, are harboring plant communities that are vulnerable to climate change in areas where we wouldn’t expect to find them,” says Melissa Pastore, an ecosystem ecologist at the U.S. Forest Service who led the study when she was a post-doc at the University of Vermont.

Frost pockets can function as “stepping stones” for vulnerable plants and trees.

The scientists established a network of 48 plots in the forests under study and continuously measured sub-canopy air temperature for six to 10 months. Their data showed that cool-air pooling is a relatively common, but spatially complex phenomena, and that it tends to alter growth patterns most when it occurs during the daytime.  

Given that these meteorological islands seem to be somewhat isolated from atmospheric warming, the flora that have flourished inside them might last longer there than they would elsewhere, says Pastore.

While the frost pockets are never continuous enough to form corridors for cold-adapted species to migrate, they can function as “stepping stones,” Pastore says, from which plants can leap and hop and disperse to other areas that are amenable to lifeforms that like a good chill. It wouldn’t be the first time that such shifts were supported by “microrefugia”—micro-climates that sustain species whose natural ranges have narrowed during periods of climate change. 

“In the past, microrefugia, in general, have helped to explain species migrations during glacial advances and retreats in the Pleistocene,” Pastore says, referring to the geologic epoch of the Ice Ages. The unprecedented pace of climate change makes it unclear whether refuges like the cold pockets in valleys can play the same role, “but it’s worthy of investigation, and they may be the best chance that some populations have.”

Though cold-air pooling is a phenomenon that scientists have documented for many years, our scientific understanding of it remains limited. Other researchers have investigated individual cold-air pooling events in great detail or used satellites to understand, from a distance, where they might occur. The study Pastore led, however, appears to be the first to dig into how frequently they may occur and what impact they have on local plant communities.

Pastore and her colleagues say their findings suggest that cold-air pooling is a fundamental ecological process and should be folded into climate change modeling of future forest adaptation patterns.

The findings, they say, also suggest an additional measure conservationists can take to preserve flora threatened by climate change: While most efforts tend to focus on preserving mountaintop communities where cooler species typically migrate, land managers might also consider identifying and conserving these pockets that are prone to pooling cold air, too, as an added migratory path for threatened species.

“If they are buffered and/or decoupled from climate change, then they may warm at a slower rate,” Pastore says—and help to preserve some species that might otherwise perish on the landscape. “They’re not the entire answer, but they make the picture a little bit brighter.”

Lead image: Valentin Valkov / Shutterstock

• Syris Valentine

  Posted on May 10, 2024

  Syris Valentine is an essayist and freelance journalist with a focus on climate and social justice. Outside of his newsletter “Just Progress,” his work has appeared in The Atlantic, Scientific American, Grist, and elsewhere.