Frozen forest reveals climate impacts on alpine ecosystems

Researchers at Montana State University (MSU) have unearthed a remarkable ancient forest preserved under ice on the Beartooth Plateau, providing a rare look into how warming climates could reshape alpine ecosystems. 

The study, published in the Proceedings of the National Academy of Sciences, analyzes the remnants of a mature whitebark pine forest that once thrived at an elevation of 10,000 feet during a period of warmer temperatures roughly 6,000 years ago.

Ancient forest exposed by melting ice

The forest, which endured for centuries before cooling temperatures forced it to retreat downslope, was sealed under ice approximately 5,500 years ago due to a shift in summer solar radiation and subsequent volcanic cooling. 

“This is pretty dramatic evidence of ecosystem change due to temperature warming. It’s an amazing story of how dynamic these systems are,” said David McWethy, one of the study’s authors and an associate professor in MSU’s Department of Earth Sciences.

Unlike glaciers, ice patches do not flow and instead preserve deposited materials such as pollen, charcoal, and macrofossils in their frozen layers. These conditions allowed the forest to remain intact until recent warming caused the ice to melt, exposing the ancient remnants.

Collaborative effort to decode climate history

The idea to explore the Beartooth Plateau for ancient climate clues stemmed from earlier discoveries by Craig Lee, now an assistant professor at MSU. In 2007, Lee uncovered a 10,300-year-old atlatl fragment in the region, suggesting that layers of ice held vast records of cultural and environmental history.

“Most of our best long-term climate records come from Greenland and Antarctica. It’s not a small thing to find ice patches that persisted for that long a time period at lower latitudes in the interior continent,” McWethy noted.

Building on this discovery, McWethy, Lee, and Greg Pederson, a paleoclimatologist with the U.S. Geological Survey, began investigating the ice patches in 2016. By 2018, their team expanded the project to study additional alpine ice patches in the region. 

This collaborative effort involved tribes, federal agencies, and multiple universities and focused on reconstructing the area’s long-term climate history and its influence on Indigenous peoples.

Reconstructing the ancient alpine ecosystem

To piece together the story of the forest, the team examined multiple elements of this alpine ecosystem. Using ice cores, they analyzed water isotopes and organic material, while Pederson harvested cross-sections of ancient wood for radiocarbon dating. 

The findings confirmed that during a period of moderate and moist climate, tree lines shifted upward, allowing whitebark pine forests to thrive for 500 years.

“The plateau seems to have been the perfect place to allow for ice patches to establish and persist for thousands of years, recording important information on past climate, human activity and environmental change,” Pederson said.

The study suggests that as today’s climate warms, tree lines could rise again, transforming areas of alpine tundra into forest. However, Pederson cautioned that factors such as precipitation, wind, and snowpack would influence the density, distribution, and composition of these potential new forests. 

“Growing season temperatures are the primary control on tree line elevation and latitude, but other factors such as moisture, wind, snowpack and human disturbance may play an important role in dictating forest structure and elevational limits,” he explained.

Implications for alpine ecosystems

The researchers warn that these changes could have profound impacts on the region’s ecosystem and resources. Rising tree lines might alter high-altitude snowpack, which serves as a critical water source for irrigation and hydroelectric power. 

Additionally, if forests encroach on tundra areas, they could increase the risk of wildfires due to changing fuel conditions.

Cathy Whitlock, a climate expert and MSU Regents Professor Emerita, emphasized the broader implications. She noted that reduced snowpack could strain water supplies, while McWethy added that the establishment of forests in tundra areas might fundamentally alter the region’s fire dynamics.

Learning from the past to predict the future

The study emphasizes the importance of understanding past ecological changes to prepare for future challenges. 

“That’s the reason why studies of past ecological change are more than interesting pieces of science. They have much larger implications for the resources we all depend on,” Pederson said.

As climate change accelerates, the Beartooth Plateau’s ancient forest serves as both a stark reminder of the Earth’s dynamic ecosystems and a window into the potential future of high-altitude environments.

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