An ancient fossilized forest in Montana is reemerging 600 feet higher than the present-day tree line after spending the last 5,500 years encased in alpine tundra. But as temperatures continue to warm, researchers say the ecological evidence hints at what may eventually return to the region.
Montana’s Beartooth Plateau in the Greater Yellowstone Ecosystem of the Rocky Mountains has remained covered in frozen tundra for thousands of years. But during the Mid-Holocene epoch, warm season temperatures more resembled those of the mid-to-late-20th-century. This allowed forests in the area to reach mountainside elevations as high as around 10,000 feet above sea level. These regions were largely populated by whitebark pine trees (Pinus albicaulis), a species of conifer that typically ranges between 16 and 22 feet tall.
About 5,500-years-ago, however, increased volcanic activity in the Northern Hemisphere blanketed the Earth’s atmosphere in particulates, blocking sunlight and causing the climate to cool rapidly. As the forests receded towards warmer weather, alpine tundra took its place across the Beartooth Plateau. Tundra as a biome is defined by its lack of trees due to consistently cold temperatures and short growing seasons. There are three main classifications of tundra: arctic, Antarctic, and alpine, the latter distinct for its mountainous altitudes. Unlike glacier formations, icy tundra doesn’t shift location over time, allowing for the layered preservation of fossils, pollen, and charcoal.
Those layers are now becoming accessible to researchers for the first time as temperatures trend warmer thanks to human-induced climate change. Their findings, published in the journal Proceedings of the National Academy of Sciences, hint at what Beartooth Plateau—and areas similar to it—may look like once again.
“Most of our best long-term climate records come from Greenland and Antarctica,” paper co-author and Montana State University Department of Earth Sciences associate professor David McWethy said in a statement earlier this month. “It’s not a small thing to find ice patches that persisted for that long of a time period at lower latitudes in the interior continent.”
McWethy and a team of researchers traveled to Beartooth Plateau multiple times since 2016 to gather ice core samples and analyzed cross-sections of newly exposed fossilized wood for radiocarbon dating. Their findings indicate the plateau’s treeline migrated higher over a period of around 500 years as conditions were moist and temperate.
“This is pretty dramatic evidence of ecosystem change due to temperature warming. It’s an amazing story of how dynamic these systems are,” McWethy added.
Researchers believe the recently re-exposed pine fossils may serve as a precursor to the return of higher elevation forests. They also believe that, while their study is localized, the implications likely extend to elsewhere around the world.
“Growing season temperatures are the primary control on tree line elevation and latitude,” explained Greg Pederson, a US Geological Survey paleoclimatologist and study lead author. At the same time, Pederson listed other influences depending on location, such as snowpack, wind, moisture, and “human disturbance.”
The team notes that these complex variables—especially in the context of climate change—mean that it’s impossible to know for certain what ecosystems like the Beartooth Plateau will look like in a few generations’ time. Warmer weather is necessary for forest growth, but so is rainfall and biodiversity.
The return of such woodlands isn’t necessarily a desired outcome, either. Researchers caution that the disappearance of high-elevation snowpacks in alpine tundra affects downstream water access crucial to electricity generation grids and agricultural irrigation. More trees in potentially drier areas also open the potential for increased wildfire outbreaks.
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