To save forests, cut some trees down, scientists say

Forests are feeling the heat. In places like the American West, rising temperatures and drought mean less water for trees, sometimes shriveling swaths of woodland. Now, scientists have found that thinning early in forest growth creates tougher trees that can endure climate change. What’s more, these thinned forests can suck carbon out of the air just as fast as dense forests.

“When it comes to carbon sequestration and climate change adaptation, we can have our cake and eat it too,” says Andrew Larson, forest ecologist at the University of Montana in Missoula and author of the new study. “It’s a win-win.”

As trees grow, they convert carbon dioxide to food and store it in their leaves, trunks, and roots. U.S. forests capture between 10% to 20% of U.S. emissions each year. But if trees get too crowded, they compete for light and water—and stressed trees are more susceptible to drought and insect attacks. Removing some trees can ease the competition, letting the remaining trees grow big and healthy. But scientists worry that removing trees can reduce forest carbon storage. These worries, however, are based mostly on models and short-term studies.

To see if the climate trade-off truly exists, scientists tapped into a long-term experiment in northwestern Montana. In 1961, U.S. Forest Service officials started the experiment in a young forest of western larch—a conifer common in the Inland Northwest. The forest was broken up into plots. In some plots, the 8-year-old trees were thinned from tens of thousands per hectare down to 494 per hectare (2.5 acres). These trees grew thick trunks and broad canopies. Other plots were left alone, and the teeming trees grew tall and skinny as they competed for sunlight. The original study was rooted in an interest in growing timber rapidly. But the scientists at the University of Montana sprouted a new question: How did tree density affect carbon storage?

To find out, they measured tree height, diameter, and width of branches to estimate the amount of carbon stored. They also calculated the carbon contained in other plants, dead wood, and forest floor debris.

Total carbon was nearly the same in both forests. The un-thinned forest had more trees, but the thinned forest compensated with bigger trees, the team reports this month in Forest Ecology and Management. Larson was surprised by how quickly the thinned trees had caught up.

“There are very few experiments that have ever been set up to test [carbon storage] directly,” says Mark Harmon, forest ecologist at Oregon State University in Corvallis, who was not involved in the research. Long-term studies like this are useful to validate climate models, he says. “I think we need more examples like this.”

The key is early thinning, before trees start to fight over water and light. The remaining trees grow rapidly. Thinning treatments on mature trees haven’t had such success, because the leftover trees were already weakened by competition. “The later you wait, the more impact you’re going to have on carbon storage,” Harmon says.

Climate change may bring more severe droughts to the West. Big trees are more drought-resilient, and their thick bark can resist fire better than can young trees. Also, they are healthier and can fight off disease and insects. When the large larches do die, they become homes for woodpeckers and lynx.

The results can be applied to forests that have been clear cut, boosting the recovery of trees, says forest ecologist Michael Schaedel, lead author of the study. As a forester for the Nature Conservancy in Missoula, he’s thinned similar young larch forests. But, until now, he hasn’t fully understood how the practice will affect them in the long run. “Having an opportunity to travel through time to the present day—54 years later—and see what the effect of those treatments is is invaluable.”

There are still concerns over how thinning impacts other species. In Montana, snowshoe hares—preyed on by the threatened Canada lynx—inhabit young western larch forests. Thinning in these forests could reduce hare habitat and in turn food for lynx. Still, Larson thinks thinning could become a useful tool for addressing climate change. “I think there’s no more pressing issue in natural resource management.”