Climate Change is Shrinking Forests in North America

Caitlin Dempsey

Updated:

From the giant sequoias in Northern California to the boreal forests that stretch across much of Alaska and Canada, North America is home to some of the worlds largest forested areas.

The United Nations’ Global Forest Resources Assessment 2020 calculated that the world has 4.06 billion hectares of forest, spread over 31% of the planet’s land surface. After Russia and Brazil, Canada and the United States are the countries with the highest amounts of forested area. Canada and the United States have over 650 million hectares of forests.

A diagram showing areas of the world covered in forests and a graph using tree symbols to show which countries have the highest amount of forests.
Proportion and distribution of global forest area by climatic domain, 2020. Images: Global Forest Resources Assessment 2020, Food and Agricultural Organization (FAO), United Nations.

How climate change is affecting forests in North America

Climate change has been significantly impacting forests in North America, primarily driven by increased levels of greenhouse gases in the Earth’s atmosphere, leading to changes in temperature, precipitation patterns, and extreme weather events.

Climate change is having a significant impact on forests in North America, and its effects are becoming increasingly noticeable. To understand this phenomenon, it’s important to break it down into several key factors:


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Temperature Increase

One of the most direct consequences of climate change is rising temperatures. As temperatures go up, it can lead to more frequent and severe heatwaves. These high temperatures can stress trees, making them more vulnerable to pests and diseases.

Additionally, some tree species may struggle to survive in warmer conditions, which can affect forest composition. As the climate shifts towards warmer and drier conditions, researchers are finding that forests in areas such as the sequoias in California’s Sierra Nevada are at risk of becoming zombie forests.

Overall, researchers are finding that less and less habitat, such as the range of the boreal forests in North America, is hospitable to the continued growth of these trees.

A gray shaded relief map of California showing in shades of green where climate mismatched forests are in the Sierra Nevada.
Vegetation Climate Mismatch among conifers in the Sierra Nevada, California. Map: Caitlin Dempsey with data from Hill et. al, 2023.

Drought and Water Stress

Climate change is altering precipitation patterns, leading to more prolonged periods of drought in some regions. Drought can weaken trees by reducing their access to water, making them susceptible to disease and insect infestations. Prolonged drought can also lead to forest fires, which can have devastating effects on tree populations.

Increased Pest and Disease Activity

Warmer temperatures allow certain pests and pathogens to thrive. Bark beetles, for example, have been responsible for large-scale tree mortality in North American forests. These pests can quickly kill large numbers of trees, especially when they are weakened by drought or other stressors.

A view across a forest with dead trees from a prolonged drought in California.
View of dead trees from the Colony Mill Trail in Sequoia National Park, taken in 2015 during California’s extreme drought. Photo: Nate Stephenson, USGS Western Ecological Research Center, public domain.

Altered Fire Regimes

As mentioned earlier, climate change can contribute to more frequent and severe wildfires. These fires not only destroy forests directly but also alter the composition of forests. Some tree species may struggle to regenerate after fires, leading to shifts in the types of trees that dominate an area.

Shifts in Growing Seasons

Changes in temperature and precipitation patterns can affect the timing of growing seasons for trees. This can disrupt the synchrony between tree species and their pollinators or seed dispersers, potentially impacting tree reproduction.

Invasive Species

Climate change can create conditions that are more favorable for invasive plant species. These invasives can outcompete native trees and disrupt ecosystems.

Rising Carbon Dioxide Levels

While not directly shrinking forests, higher levels of carbon dioxide (CO2) in the atmosphere, which is a primarily result of increased human influence on emissions, can alter the growth and physiology of trees. Some studies suggest that elevated CO2 can lead to increased tree growth initially, but the long-term effects are complex, can be negative, and may vary by species.

References

Nesha, M. K., Herold, M., De Sy, V., Duchelle, A. E., Martius, C., Branthomme, A., … & Pekkarinen, A. (2021). An assessment of data sources, data quality and changes in national forest monitoring capacities in the Global Forest Resources Assessment 2005–2020. Environmental Research Letters16(5), 054029. DOI 10.1088/1748-9326/abd81b

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.