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Climate change might be causing the reduction of boreal forests worldwide as it propels them toward higher latitudes

The boreal forests, encircling the far northern reaches of Earth just south of the Arctic’s treeless tundra, resemble a loose-knit headband wrapped around the planet’s ears, covering substantial areas of Alaska, Canada, Scandinavia, and Siberia, particularly where an Arctic ice cap is present. Renowned for their vital role in carbon storage and historically sheltered from extensive human impact due to their remote locations, boreal forests are among the most crucial ecosystems globally. These regions are inhabited by diverse species of mammals, fish, plants, insects, and birds. Over the past two centuries, scientists have observed a northward shift in these forests and their associated soils as temperatures rise, influenced by the subzero winters and short summers characterizing the boreal region.

Nevertheless, the northward progression of boreal forests has been uneven and more gradual than initially anticipated. Simultaneously, their retreat southward has outpaced scientists’ predictions. As researchers specializing in the examination of northern ecosystems, including forests and wetlands, we are encountering troubling indications that the planet’s largest forest wilderness is seemingly diminishing as global temperatures rise.

The planet’s largest wilderness, boreal forests, encompasses billions of trees, predominantly needleleaf, cone-bearing conifers. Additionally, there are pockets of broadleaf species such as birch, aspen, and poplar. These forests provide a habitat for millions of migratory birds and iconic mammals like brown bears, moose, and lynx.

The intricate relationship between these trees and the soil around their roots contributes to the regulation of Earth’s climate by extracting carbon dioxide from the atmosphere, preventing it from acting as a greenhouse gas. The trees utilize this carbon to develop roots, trunks, and leaves, ultimately forming carbon-rich soil upon the tree’s demise. Any substantial alterations to these forests will have a direct impact on the global climate.

These forests are experiencing warming at rates significantly surpassing the global average. The escalating temperatures directly influence the growth and viability of trees, subsequently affecting their capacity to sequester carbon.

Forests are undergoing dynamic changes due to atmospheric warming, liberating trees from the constraints of cold temperatures. In response, mature trees exhibit accelerated growth, while milder temperatures create opportunities for seedling trees in the northernmost boreal forests to establish themselves in areas previously deemed inhospitable. The warmer southern boreal forests, however, present a contrasting scenario. Here, the elevated temperatures prove challenging for cold-adapted boreal trees, impeding their growth and leading to instances of mortality.

As temperatures rise, dry conditions ensue, rendering trees more vulnerable to insect infestations and wildfires, as witnessed in Canada in 2023 and Siberia in 2019 and 2020. If this trend unfolds on a broader scale, the boundaries of the southern boreal forests will diminish and deteriorate, prompting a retreat toward the north, where temperatures remain conducive.

While the conventional expectation suggests a gradual northward expansion and southern retreat of boreal forests in tandem with warming temperatures, our comprehensive research, utilizing satellite and field data, reveals a more intricate narrative.

Utilizing satellites is indispensable for monitoring the recent transformations in boreal forests and determining whether these alterations align with an overarching northward migration. Satellites enable researchers to observe year-to-year variations in forest attributes, encompassing annual tree growth and tree cover.

Our recent studies, employing satellite data, indicated an increase in tree growth and tree cover from 2000 to 2019 across substantial portions of the boreal forest, primarily in the coldest northern sectors. However, there was limited evidence suggesting an expansion beyond current tree lines.

Furthermore, our investigations unveiled a frequent decline in tree growth and tree cover in warmer southern regions of the boreal forests during the same period. In these areas, elevated temperatures and arid conditions often impeded tree growth or led to the demise of individual trees, while wildfires and logging contributed to the loss of tree cover.

Satellite data unequivocally demonstrates the influence of climate change on both the northern and southern fringes of the boreal forest. Nevertheless, if the reduction in tree cover in the south outpaces gains in the north, the boreal forest is likely to contract rather than merely shifting northward.

Zooming in to comprehend changes in the forest landscape involves understanding the progression of individual tree seeds germinating and growing. Boreal trees, characterized by slow growth that spans decades before reaching a size visible from space, present a challenge in identifying young trees indicating shifts in the tree line. In the late 1970s, David Cooper documented the growth of young spruce trees at altitudes and locations significantly beyond the highest-elevation cone-bearing trees in Alaska’s Brooks Range. Upon revisiting in 2021, these young trees had matured to several yards tall and were producing cones. Notably, the number of young spruces now surpasses tenfold those observed during initial field research. Our on-foot exploration along the boundary between Alaska’s boreal forest and Arctic tundra has revealed thousands of young boreal trees flourishing up to 25 miles north of established tree lines. Most thrive in areas with deeper snowfall, influenced by an Arctic Ocean version of the “lake effect,” where cold air moving across open water captures warmth and moisture, precipitating as snow downwind. The retreat of sea ice, leading to more open water, induces stronger winds that carry tree seeds farther, coupled with increased snowfall insulating seedlings from severe winter conditions. Consequently, trees in Alaska’s Brooks Range are swiftly encroaching into the treeless tundra. However, these expansions are currently concentrated and have not universally manifested along the entire northern tree line.

The forthcoming transformation of boreal forests is evident in our collective research, indicating their response to escalating temperatures. However, the swift pace of climate change suggests that trees may not be able to migrate northward quickly enough to offset their decline in the southern regions.

The prospect of northern trees catching up with climate shifts to prevent forest contraction remains uncertain. The recent proliferation of trees in the Brooks Range could potentially signify such an expansion. Additionally, there is ambiguity regarding whether the northern reaches of boreal forests can sufficiently accumulate carbon through enhanced growth to counterbalance carbon losses in the south.

If boreal forests are indeed nearing contraction, they will eventually vanish from their current southern boundaries. This poses a threat to numerous native and migratory animals, particularly birds, as it diminishes their boreal habitat. Moreover, these forests hold cultural significance for several million people, including Canada’s aboriginal communities.

A more meticulous monitoring of boreal forests worldwide, utilizing both satellite data and ground-level measurements, will contribute to a more comprehensive understanding. Only through such scrutiny can researchers begin to discern the future trajectory of one of Earth’s last remaining wildernesses.

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