For centuries, nature enthusiasts around the world have hosted events to plant and care for trees. At the first U.S. Arbor Day, held in 1872, Nebraska residents planted an estimated 1 million trees. In more recent years, some groups have called for reforestation (planting trees in areas where they had formerly grown but were cut down, burned, or otherwise destroyed) and afforestation (planting trees in areas where they have not historically grown, such as in grasslands) with a new goal in mind: to help mitigate climate change.

When a tree takes in carbon from the atmosphere, it stores it throughout its lifetime through a process called carbon sequestration. The surrounding soil can sequester carbon for even longer periods, for hundreds, sometimes thousands, of years.

Still, scientists have questioned whether tree planting is a good way to mitigate humanity’s effect on climate. In particular, a landmark study in 2000 found that for most of the world’s forests, the cons can outweigh the pros. Trees that are planted in high-latitude (near the poles) regions often reflect less sunlight than the natural formations they replace, such as snowdrifts or grasses. When this happens, more radiation is absorbed at Earth’s surface, warming the ground and the layer of air just above it. So the study found that although it might be beneficial to plant trees in low-latitude (near the equator) regions, planting trees elsewhere could actually produce more warming.

A new study conducted by Mykleby et al. generally supports this conclusion and provides additional recommendations for afforestation efforts.

In light of the deforestation occurring in many tropical forests nearest the equator, the researchers chose to focus on midlatitude and high-latitude regions where afforestation might be more effective. They used a land surface model to simulate the different types of conifer evergreen trees that grow in temperate and boreal regions of North America.

Then they checked the results of their model against data collected at field sites in the humid Pacific Northwest, the alpine Rocky Mountains, the eastern coast of North America, and boreal regions of Canada.

Overall, the researchers found that afforestation would be most effective in these regions of the United States (except for mountainous regions in the West) and coastal provinces of Canada (including Nova Scotia, New Brunswick, and British Columbia). Furthermore, they found two ways to maximize the benefits of afforestation: planting trees as densely as possible and harvesting trees once they surpass their peak rate of carbon sequestration. In most forests they studied, this time frame would be about every 35–45 years, but in the American Rockies it could be as many as 90 years.

The team hopes that by painting a clearer picture of forest-climate interactions, this study will help inform policy makers. They note that altering land use may help countries in their efforts to achieve climate goals set forth by the Paris Agreement, a worldwide effort to combat climate change. (Geophysical Research Letters, https://doi.org/10.1002/2016GL071459, 2017)

—Sarah Witman, Freelance Writer