Revealing the Secrets of Frozen Forests: Glimpses into Tomorrow’s Alpine Ecosystems

Scientists from Montana State University assert that the frozen remnants of an ancient forest found 600 feet above the contemporary tree line on Beartooth Plateau may indicate potential changes for the alpine ecosystem if the climate continues to warm.

A paper detailing this discovery is published in the journal Proceedings of the National Academy of Sciences. It explains what researchers have uncovered by examining the remnants of a mature whitebark pine forest that existed at 10,000 feet elevation approximately 6,000 years ago, during which warm-season temperatures in the Greater Yellowstone Ecosystem were comparable to those of the mid-to-late 20th century.

The forest flourished for centuries until climate cooling began roughly 5,500 years ago due to reductions in summer solar radiation, as the researchers discovered. The cooler temperatures caused the tree line to decline in elevation, transforming the high mountainous landscape from a forest into the alpine tundra observed today.

David McWethy, one of the paper’s co-authors and associate professor in the Department of Earth Sciences at MSU’s College of Letters and Science, indicated that subsequent volcanic activity in the Northern Hemisphere exacerbated the already cooling temperatures in the region. The pine forest quickly became encapsulated in ice, where it remained preserved until recent years when the ice patch began to melt. McWethy stated that this discovery provided the first evidence from an alpine ecosystem indicating that mature forests had established at higher elevations when temperatures were warmer.

“This presents quite compelling evidence of ecosystem alteration due to warming temperatures,” he remarked. “It’s an incredible narrative of how dynamic these ecosystems are.”

The paper explains that ice patches, unlike glaciers, do not exhibit flow. Until recently, the authors noted, the ice patches gradually and nearly continuously accumulated ice, “enabling the preservation of deposited materials such as pollen, charcoal, and macrofossils within their frozen layers.”

McWethy disclosed that the initiative to investigate Beartooth ice patches for clues about climate and environmental conditions over the past 10,000 years stemmed from the work of Craig Lee, currently an assistant professor in MSU’s Department of Sociology and Anthropology. In 2007, Lee recovered part of a 10,300-year-old atlatl from an ice patch on the plateau, signaling to scientists the existence of numerous millennia’s worth of cultural materials and environmental data preserved within nearby ice layers.

“Most of our most reliable long-term climate records originate from Greenland and Antarctica. Finding ice patches that have endured for such an extensive period at lower latitudes in the interior continent is significant,” McWethy noted. The ice patches on the plateau span hundreds of square meters, which remains relatively small compared to persistent ice masses found in other regions.

In 2016, Lee, McWethy, and Greg Pederson, a paleoclimatologist for the U.S. Geological Survey’s Northern Rocky Mountain Science Center and the lead author of the recently published paper, began gathering information regarding environmental changes and past human activities from the ice patches on the plateau. By 2018, they, alongside others, expanded their studies to additional alpine ice patches in the area, reconstructing the long-term climate history of the ecosystem and its impact on Indigenous North Americans. McWethy emphasized that this work involves broad collaboration among tribes, federal agencies, archaeologists, and researchers from various universities. The study of the tree line is one aspect of this collective effort.

The researchers clarify that it was crucial to investigate multiple aspects of the ancient ecosystem to reconstruct the complete narrative of the frozen forest. Team members, including graduate and undergraduate students from MSU, assessed layers of water isotopes and organic materials found in ice cores extracted from the patch, while Pederson collected cross-sections of wood from the ancient trees for radiocarbon dating.

Pederson highlighted that the findings confirmed that tree lines on the plateau rose in response to regional warming, with the pine forest flourishing for 500 years under moderate and moist climatic conditions.

“The plateau appears to have been the ideal location for ice patches to form and persist for millennia, recording vital information regarding past climate, human activities, and environmental changes,” Pederson stated.

The study’s outcomes indicate that current climatic conditions may encourage trees to migrate upslope into regions of the plateau now classified as tundra. Pederson emphasized, however, that although the study’s results are site-specific, there are significant connections to global climate controls regarding tree line elevations.

“Temperatures during the growing season are the principal determinant of tree line elevation and latitude,” Pederson remarked. “Nonetheless, at individual tree line locations, other factors such as moisture, wind, snowpack, and human disturbances may significantly influence forest structure and elevation limits.”

For these reasons, accurately forecasting the future appearance of tree line forests on the Beartooth Plateau remains a challenge.of thickness, distribution or species assortment of trees, which will fluctuate based on the degree of warming, as stated by McWethy and Pederson. The tree line is anticipated to ascend as the climate heats up, they indicate, but levels of precipitation will influence the configuration and spread of emerging forests.

Transformations will have considerable consequences for the forthcoming ecosystem, assert the study’s authors, including climate specialist and MSU Regents Professor Emerita Cathy Whitlock, who has engaged in research within the Greater Yellowstone Ecosystem for four decades. She and Pederson indicated that reduced high-altitude snowpack would impact water resources for irrigation and electricity production downstream. McWethy noted that should forests start to take root in tundra regions, the conditions for fuel could alter significantly, possibly heightening the danger of wildfires.

“That is why examinations of historical ecological shifts are more than just fascinating aspects of science,” Pederson remarked. “They carry much broader implications for the resources we all rely on.”