Forests cooler or warmer than open areas depending on latitude, study finds

David Hollinger, a plant physiologist with the U.S. Forest Service's Northern Research Station, co-authored the article with principal investigator Xuhui Lee, a professor of meteorology at the Yale School of Forestry & Environmental Studies, and a research team that included 21 scientists from universities and research organizations in the United States, Canada, and Germany. The study was supported, in part, by grants from the U.S. Department of Energy and the Yale Climate and Energy Institute.

“Forests are complicated ecosystems with subtle climate system effects,” Hollinger said. “This study underscores the need to retune climate models to reflect that complexity so we can get a better picture of the role forests play in the landscape.”

Results are based on comparisons of air temperature recorded at meteorological towers located in forested areas in the United States and Canada and, as a proxy for cleared land, nearby surface weather stations operated by the National Weather Service and Environment Canada. In a review of data from Florida to Manitoba, researchers found forested land to be warmer than nearby open land north of 45 degrees latitude and cooler south of 35 degree latitude. Between 35 and 45 degrees latitude, forested and open land had similar temperatures.

North of 45 degrees, approximately the northern border of Vermont, temperatures recorded in forest interiors were warmer than temperatures recorded in open areas. This is largely because year-round night time temperatures in forests remain higher than open areas due to the mixing down of warm air aloft in forests. In addition, snow-covered open areas reflect sunlight while dark forests absorb sunlight and its warmth.

South of 45 degrees, maximum daytime temperatures in forested lands were lower than over nearby open land, but forest nighttime minimum temperatures were still higher. South of 35 degrees, approximately the southern border of Tennessee, the overall effect was reversed, with forests cooler than open land.

Forests represent one of the most extensive land use types, covering approximately 30 percent of the terrestrial surface. “This study makes it clear that at least in southern latitudes, there are important climatic benefits for maintaining or increasing forest cover,” Hollinger said.

The mission of the U.S. Forest Service is to sustain the health, diversity, and productivity of the nation's forests and grasslands to meet the needs of present and future generations. The agency manages 193 million acres of public land, provides assistance to state and private landowners, and maintains the largest forestry research organization in the world. The mission of the Forest Service's Northern Research Station is to improve people's lives and help sustain the natural resources in the Northeast and Midwest through leading-edge science and effective information delivery.