Tree death rates rising in West

WASHINGTON – The death rates of trees in western U.S. forests have doubled over the past two to three decades, according to a new study spearheaded by the U.S. Geological Survey, driven in large part by warmer temperatures and water scarcity linked to climate change.

The findings, published today in the online journal Science Express, examined changes in 76 long-term forest plots in three broad regions across the West, and found similar shifts regardless of the areas’ elevation, fire history, dominant species and tree sizes. It is the largest research project based on old-growth forests in North America.

Nathan Stephenson, one of the lead authors, said summers are getting longer and hotter in the West, subjecting trees to greater stress from droughts and attacks by insect infestations, all factors that contribute to greater tree die-offs.

“It’s very likely that mortality rates will continue to rise,” said Stephenson, a scientist at the Geological Survey’s Western Ecological Research Center, adding that the death of older trees is rapidly exceeding the growth of new ones, akin to a town where deaths of old people are outpacing the number of babies being born. “If you saw that going on in your hometown, you’d be concerned.”

The study was conducted by a team of 11 researchers from institutions including the USGS and the Forest Service; the University of British Columbia in Vancouver; the University of Washington, Seattle; Northern Arizona University; Oregon State University; the University of Colorado, Boulder; and Pennsylvania State University.

They examined a variety of tree types including pine, fir and hemlock, documenting major die-offs in northern California, Oregon, Washington and southern British Columbia along with interior Western states like Colorado and Arizona. In the Pacific Northwest the researchers found that tree death rates had doubled in just 17 years, compared to 29 years for interior Western forests, but the researchers cautioned against making too much of these differences.

The recent warming in the West “has contributed to widespread hydrologic changes, such as a declining fraction of precipitation falling as snow, declining water snow pack content, earlier spring snowmelt and runoff, and a consequent lengthening of the summer drought,” they wrote.

The scientists said it was hard to predict how the changes would transform the Western landscape, although they anticipated that in the future the West will boast sparser forests that cannot store as much carbon as they do now, which could contribute to even more warming in the future.

“In the end, the forest will tend to equilibrate at a lower level of stored carbon,” said Jerry F. Franklin, at UW-Seattle’s College of Forest Resources, noting this will occur “over a very long time period.”

Franklin added that some of the West’s most imperiled animal species, such as marbled murrelets and the northern spotted owl, depend on old growth trees for critical habitat.

“There’s a large array of organisms that depend on large trees,” he said.

Thomas T. Veblen, a geography professor at the University of Colorado-Boulder, said the combination of increased wildfires, drought and bark beetles has devastated some of his state’s forests. Temperatures in Colorado’s subalpine forests, which are between 8,500 and 10,000 feet in altitude, have risen markedly over the past 50 years during all seasons, he said.

Mountain pine bark beetles have killed roughly 3.5 million acres of lodgepole pine forests in northwestern Colorado over the past decade, wiping out 90 percent of pine forests in that area, Veblen said. During the same time period, spruce bark beetles also killed large areas of spruce forest in northern and southwestern Colorado.

“Our society needs to devise policies that will help us to adapt to the changes that are under way,” Veblen said. “This is further evidence that we’re seeing continued effects of the warming in increased fire risk.”

If current tree mortality rates continue and even accelerate, the paper’s authors warned, there is a chance that U.S. forests could shift from being a carbon sink that takes greenhouse gases out of the atmosphere to becoming a net emitter of carbon dioxide.

Franklin said policymakers should keep that in mind when negotiating a new international climate pact.

“One of the things that should absolutely be on the table in terms of any global agreement is the notion of avoided carbon releases,” he said, adding that when you lose older trees in a forest, “there’s no way you can make up for that.”