Boreal Forests of North America are Shrinking

Katarina Samurović

Updated:

The northern snow forests, also called boreal forests of North America, are a part of the largest land biome in the world – the global boreal forest. Boreal forests are found in the high northern latitudes between about 50°N and 70°N.

The biome, also known as the taiga, covers most inland of Canada, Alaska, part of the contiguous US, most of Scandinavia, much of Russia (including Siberia), forested parts of Iceland, and northern segments of the Scottish Highlands, Kazakhstan, Mongolia, and Japan. 

Boreal forests are often referred to as “snow forests” due to the substantial and persistent snow cover they receive during the long, cold winters. The term reflects the climate characteristics of the biome, where the winter season dominates and snow can persist for many months.

Blue sky over a snowy forest.
Long, cold winter seasons and snow are characteristics of the boreal forest climate. Photo: Denali National Park, Tim Rains/NPS, public domain.

What are boreal forests?

The boreal climate is characterized by long, cold winters and short, mild summers, which significantly influence the types of vegetation and animal life that can thrive there.


Free weekly newsletter

Fill out your e-mail address to receive our newsletter!
Email:  

The keystone species of boreal forests are conifer trees adapted to cool temperatures throughout much of the year. Spruce trees constitute most of the North American taiga.

North American boreal forest


The North American boreal forest represents a significant portion of the global boreal or Taiga biome, stretching across Canada and into the U.S. state of Alaska. This region encompasses approximately 1.5 billion acres of dense coniferous forest primarily consisting of species such as spruce, fir, and pine.

A shaded relief map of the northern part of the United States and Canada showing areas in green that are boreal and hemiboreal forests.
Map of the North American boreal zone. Data from Natural Resources Canada developed for Brandt, J. P. (2009). The extent of the North American boreal zone. Environmental Reviews, 17(NA), 101-161.

The ecosystem is adapted to the cold, with long, harsh winters and short, mild summers, and it provides critical habitats for a diverse range of wildlife, including large mammal species such as moose, caribou, and wolves.

Additionally, the North American boreal forest serves as a vital carbon sink, sequestering significant amounts of carbon dioxide, and plays an essential role in climate regulation and the hydrological cycle of the continent.

Did you know? The 5.5 million square kilometers of the North American boreal forest is in Canadian territory, while 0.74 million km2 belongs to the United States (largely in Alaska).

The effect of climate change on the North American boreal forest

Because the boreal forest biome depends on long and stable cold or cool seasons, it is highly vulnerable to global warming and its effects – heatwaves, droughts, forest fires, and the shrinkage of snow cover

The predictions of the North American boreal forests’ habitat change under the influence of climate change were straightforward and based on a logical assumption – but, from a new perspective, maybe a bit naive. 

What the research says about changes to the boreal forest extent with climate change

Despite the shrinkage and disappearance across the southern line, it was predicted that the northern line of boreal forests would move far north, replacing the supposedly-shrinking Arctic shrub and tundra due to the now-longer and warmer growing season. In that scenario, the forests would lose some of their range, but they would make up for a significant portion of it by moving north, in tune with the shifting temperatures.

However, the actual field research doesn’t confirm this notion. Instead, it’s telling us that boreal forests in North America seem to be contracting – losing the southern part of the range and not gaining much northern ground. This is in line with greater trends among trees in North America that are experiencing climate change-driven declines.

That is exactly what the title of the ground-breaking study, “Northern expansion is not compensating for southern declines in North American boreal forests,” suggests. It was conducted by Ronny Rotbarth of the Environmental Sciences Department of Wageningen University (The Netherlands) and his team and published in Nature Communications.

Using remote sensing to map out changes to boreal forest extent in North America

The team used remote-sensing data – specifically, the MODIS Vegetation Continuous Field – to analyze tree cover and thus quantify the changes across the American portion of the boreal biome from 2000 to 2019. Further, they looked at the relations between these changes and environmental factors, such as temperature, precipitation, and albedo, as well as their connection to disturbances like forest fire damage and timber exploitation. 

What they found is a strong distinction between the changes in the southern and the northern border of snow forests – they called it the “north-south asymmetry in tree cover change.

The southern boundary of the range declined as predicted, with large-scale, global-warming-fanned forest fires being one of the leading causes (coupled with the original forest’s failure to regenerate). This portion of the boreal forest is increasingly stressed due to warmer and drier conditions, bringing about a reduction in growth rates, declines in vitality, insect infestations, and increases in tree mortality.

 Contrastingly, the novel conditions benefit many temperate broadleaf species that gradually replace the lost cold-adapted conifers. However, not all of the decline came due to the shifting climate; timber logging was also revealed to play a huge role.

On the other hand, there was no evidence of tree cover expansion on the northern border of the biome.

Expansion of conifers into the inhospitable northern and high-latitude areas is an extremely slow process, dependent on many factors. Other studies have already shown that northern forests do not always follow the expansion of their climatic niche along the northern boundary. 

One hypothesis is that, for trees to grow, the shrubs that protect the seedlings from the elements need to move in first. However, the permafrost collapse and the surface water accumulation that comes with it seems to lead to browning of both shrubs and new trees, possibly limiting the boreal forests’ northward expansion.

The only place where the tree cover markedly increased is the core of the biome’s range.

A diagram showing the current, predicted, and observed latitudinal changes of boreal forests in North America with green trees for boreal, cloud/ice icons for tundra and dark yellow trees for temperate trees.
A diagram showing the current, predicted, and observed latitudinal changes of boreal forests in North America. Diagram A shows the current extent. Diagram B shows the predicted shift of boreal forest north while contracting to the south. Diagram C shows what remote sensing observations are showing, that boreal forests have contracted to the south but have not expanded north into tundra habitat. Diagrams: Rotbarth et al., 2023, CC BY 4.0.

The team suggests that their study’s findings could indicate that, instead of the expected northward migration, a biome contraction is either coming or is already underway.

The Consequences of Snow Forest Range Contraction

Snow forest shrinkage could have grave consequences for the continent’s biodiversity. The North American boreal forests account for one-fourth of the world’s intact forests, with 80% of its territory considered intact (however, only 8-13 percent are formally protected). 

Let’s look at some examples. 

  • The North American boreal forests still facilitate some of the longest mammal and fish migrations in the world (think salmon). 
  • The biome is one of the rare places on Earth with a remaining steady population of large predators such as wolves, bears, lynx, and others, resulting in a natural predator-prey balance and healthy, functional ecosystems. 
  • It is a seasonal home to 1-3 billion of America’s nesting birds, most of which fully depend on the snow forests as their mandatory nesting ground. 
  • The snow forest territory contains some of the world’s largest lakes and longest undammed rivers, resulting in an abundance of aquatic organisms.

Consequently, the biome shrinkage could influence millions of people that depend on the forests and their biodiversity for sustenance, many of them indigenous and still nurturing a traditional way of life (much of the boreal forests in Canada stretch over the indigenous territories). 

Indigenous governments have aligned with scientific conservation efforts to preserve their pristine landscapes; the synergy created some of the world’s biggest land conservation projects. Still, with resources such as timber, minerals, and hydropower, the intact boreal forests remain a lucrative territory for extractive and exploitative industries, resulting in an increasing industrial activity that often lacks conservation-friendly land management.

The fact that boreal forests also store immense amounts of carbon is also a cause for concern. Significant parts of stored carbon are found in the biomass of the forest itself (about 15 billion metric tons), but even more so in the permanently frozen soil (permafrost) beneath it. 

A picture of a bog in the foreground and boreal forest in the background in the Interior of Alaska.
The foreground of this photo of the Alaska Interior is a bog with no permafrost bordering a boreal forest growing in permafrost. Photo: Kristen Manies, USGS, public domain.

The shrinkage of the snow forest range will impair the abilities of arboreal carbon storage and sequestration, but maybe even more alarmingly, cause the release of the carbon stored in the permafrost.

Unless this carbon release is compensated by the carbon gains elsewhere, it will inevitably accelerate the global warming process. It essentially creates a negative feedback loop for the snow forest – as the warming intensifies, forests die off and release more carbon, leading to the further escalation of climate change that’s destroying them.

Resources

The Studies

Rotbarth, R., Van Nes, E.H., Scheffer, M. et al.  (2023) Northern expansion is not compensating for southern declines in North American boreal forests. Nat Commun 14, 3373. https://doi.org/10.1038/s41467-023-39092-2

Wells, J.V., Dawson, N., et al. (2020) The State of Conservation in North America’s Boreal Forest: Issues and Opportunities. Front. For. Glob. Change, 30 July 2020. Sec. Temperate and Boreal Forests Volume 3 – 2020 | https://doi.org/10.3389/ffgc.2020.00090

USGCRP, 2018: Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report. Chapter 11: Arctic and Boreal Carbon. [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 878 pp., doi: 10.7930/SOCCR2.2018 https://carbon2018.globalchange.gov/chapter/11/

Articles

Nikk Ogasa. The snow forest of North America may be about to shrink. Science News. 29 June 2023 

MODIS Vegetation Continuous Fields. NASA MODIS. https://modis.gsfc.nasa.gov/data/dataprod/mod44.php

Related

Photo of author
About the author
Katarina Samurović
Katarina Samurović is an environmental analyst and a freelance science writer. She has a special interest in biodiversity, ecoclimatology, biogeography, trees, and insects.