Death Rates Have Doubled, Researchers Find
The death rates of trees in western
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
Nathan L. 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 home town, you'd be concerned."
The study was conducted by a team of 11 researchers from institutions including the USGS and the Forest Service; the
They examined a variety of tree types including pine, fir and hemlock, documenting major die-offs in
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 region's 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
"There's a large array of organisms that depend on large trees," he said.
Thomas T. Veblen, a geography professor at the
Mountain pine bark beetles have killed roughly 3.5 million acres of lodgepole pine forests in northwestern
"Our society needs to devise policies that will help us to adapt to the changes that are underway," 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
"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."