As Temperatures Rise, Poplars Replace Alaskan Tundra

In Alaska’s Denali National Park, summer temperatures have risen 2 degrees Celsius over the past century, with the majority of change occurring since the 1970s. The glaciers that cover 1 million square miles of the park are melting rapidly, exposing bare earth where there once was ice.

An Ecosphere study, published July 19, finds that the rising temperatures impacting the glaciers are also affecting the plant communities that grow in newly exposed areas, fundamentally altering the Alaskan landscape and ecosystems.

View of Denali from Stony overlook (Credit: NPS Photo / Jacob W. Frank)
View of Denali from Stony overlook (Credit: NPS Photo / Jacob W. Frank)

The research team, led by Carl Roland and Sarah Stehn, investigated how the Alaskan landscape near Denali’s Muldrow Glacier changed over time by recreating a study conducted 54 years ago by Leslie Viereck. In 1966, prior to the 2 degree temperature rise, Viereck set out to determine the plant succession in the area. Succession is the process of an ecosystem evolving over time. In mountain regions, it can occur when a glacier retreats or a river forms an outwash plain, and a new community of vegetation can grow.

Viereck studied the outwash plain of the McKinley River, which flows west out of Muldrow Glacier, and examined areas ranging from 1 to roughly 5,000 years old. Based on his observations, he determined that the bare rocky plain would transition into a meadow, followed by small shrubs and eventually becoming a tundra ecosystem, with thick moss and a low canopy of shrubs.

Half a century later, Roland, Stehn and their colleagues were able to replicate Viereck’s study to see if the temperature change has impacted the successional path laid out by their predecessor. Using a series of photographs, GPS, field notes and re-measured areas of land, the team found surprisingly different results. The newly exposed areas were not transitioning into meadows, but instead covered in balsam poplar trees. The new Alaskan landscape showed signs of succeeding into a forest rather than tundra—representing a completely different biome change.

Photographs of the Muldrow Glacier study area from the original and current study (source: Ecosphere)
Photographs of the Muldrow Glacier study area from the original and current study (source: Ecosphere)

According to the study, the temperature rise fundamentally altered the climatic conditions of the ecosystem, and as time passes, the differences become increasingly larger. Like an archer shooting an arrow hundreds of meters away, even a small shift in the starting point can change the trajectory completely, and yield a very different outcome in the ecosystem structure and function.

The poplars began to grow in the early succession landscape because they thrive in the warming climate, and require warmer soils to grow. Once the trees were established, they had the competitive advantage over other plant species—they produce seeds early and abundantly, and are able to thrive in bare soil when other species are not yet present. Once the trees begin to grow, they alter the landscape by blocking the sun from smaller plants, and allowing a different range of species to thrive. Both plants and animals that prefer woodland instead of tundra move to these newly formed forests. The trees also block the wind, allowing snow to build up where it previously would have been blown away. The thick layer of snow prevents permafrost from forming, keeping the soils warm. This one species, through a series of chain events, is able to colonize the area and alter both the species and climate of the region.

Balsam Poplar canopy (source: Adam Jones, PhD)
Balsam Poplar canopy (source: Adam Jones, PhD)

While the newer areas of exposed land showed a dramatic shift in the projected succession, the older, more established areas of the landscape followed the path as predicted by Viereck. The areas located farther from the river and the end of the glacier plain had begun to grow before the temperature increase. Once the ecosystem had begun to develop, it is much more difficult to change its course. While poplars were still found in these areas, they had a much smaller impact on the ecosystem as a whole.

As temperatures continue to rise and glaciers continue to retreat in Alaska, there will be large areas of land exposed which will be colonized by vegetation. Ecosystems will form in place of the ice, and when they do, they will be woodlands rather than the iconic Alaskan tundra.

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