Since the commencement of large-scale greenhouse gas emissions (about 1830), Earth’s average temperature has grown by more than 1 degree Celsius. This may not seem like much, but it’s important for our woods, which are used to modest natural variations in temperature over thousands of years. During the last Ice Age, temperate woods of the Northern Hemisphere expanded further south, reaching the Mediterranean area of Europe and Northern Africa. While it was happening, parts of the world that now desert, like the Southwest of the United States, were covered in trees. As the ice caps melted and temperatures rose, they withdrew north to their current places. Human-caused global warming has recently accelerated this tendency.

Mainly owing to climate change, trees are on the move. Individual trees, of course, are immobile. But the seeds they generate can get moved by animals, wind, and water to new regions, where they develop to form new trees. If trees on one end of a range die out while the trees on another edge survive and expand, then the population of trees is said to migrate. US Forest Service researchers studied 30 states over decades, comparing where older trees grew with newer development. They observed that 70 percent of tree species surveyed were migrating. This type of movement is complex in mixed forests, where more than one species of tree predominates. The forest community will lose variety if many species travel in opposite directions.

Climate change is also endangering trees across the United States through forest fires. Due in part to hotter, drier summers from climate change, forest fires are increasing– in the last three decades, the area of extensive forest fires in the western United States has doubled. The relationship between forest fires and global warming is a self-reinforcing feedback loop. A warmer temperature fosters forest fires by drying out plants and soil, and shortened periods of snow cover imply that fires can occur during colder months. As a result, forest fires have a positive impact on global warming. A burnt forest not only releases previously-stored carbon but also prevents the trees in the area from taking in any other carbon.

Invasive organisms that aren’t native to the forest will have an easier time spreading due to climate change. Many invasive species previously limited in range by their inability to survive frigid winters are expanding to new areas made vulnerable by climate change. Invasive plant species take up space and resources, pushing out native trees and sucking up water and soil nutrients. Invasive insect species could harm certain native tree species. Millions of native ash trees have been decimated by an invasive insect called the Emerald ash borer since it was first discovered in Michigan in 2002. It came to Illinois in 2006 and has wreaked severe devastation ever since. A tree census done in 2020 indicated that the number of ash trees in Chicago has declined by half since 2010. Invasive species may expand their range due to global warming, putting our forests and local wildlife at risk. They can also kill a huge number of trees, which has the side effect of contributing to climate change.

On the other hand, climate change is a rapidly increasing process, although trees may survive for many decades. As a result, land managers must consider future climate circumstances while making planting decisions. Many different kinds of trees can be seeded in the hopes that some of them would flourish in the future, a practice known as “casting a wide net.” Another is to plant plants that appreciate warmer conditions. A group called the Climate Change Response Framework brings together scientists and land managers to understand better how trees will adapt to climate change and develop management plans accordingly. The Climate Change Tree Atlas, produced in collaboration with the U.S. Forest Service, forecasts changes in tree range under scenarios of mild and severe climate change. By using these instruments and monitoring the local circumstances of their region, foresters are making judgments about what to plant.

The future of our woods is in jeopardy. Human-caused climate change is transforming our forests far more rapidly than natural climatic fluctuation has over millennia, both in rural and urban settings. Recent studies of tree migrations have shown crucial tendencies, but scientists are unsure how our forests will appear in the future. Understanding how climate change will affect trees will become a primary concern for conservation so that we can determine the best solutions to safeguard our forests.

Joe Parker