Rate of tree death found to double in the western US

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

The findings, published yesterday 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, 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 landscape.