Warming doubles tree deaths in western USA

Posted: 9 February 2009

A new study led by the US Geological Survey and other research institutes indicates tree deaths in the US West's old-growth forests have more than doubled in recent decades, probably from regional warming and related drought conditions.

The study, published in Science, documented tree deaths in all tree sizes in the West located at varying elevations, including tree types such as pine, fir and hemlock. Significant die-offs also were documented in the interior West - including Colorado and Arizona - as well as Northwest regions like northern California, Oregon, Washington and southern British Columbia.

Forest researach gondola
Forest researach gondola
Researchers ride in the gondola of the 28-story Wind River Canopy Crane being used to study, among other things, tree mortality in an old growth forest in the Gifford Pinchot National Forest in southwest Washington state. The crane, operated by the University of Washington since 1997, is in a forest plot that was established for long-term study in 1948 by the Forest Service. Credit: University of Washington
The researchers speculated higher tree deaths could lead to substantial ecological changes in the West, including cascading effects affecting wildlife populations. The tree deaths also could lead to possible increases in atmospheric carbon dioxide levels contributing to warming, which could stem from lower CO2 uptake and storage by smaller trees and increased CO2 emissions from more dead trees on the forest floors.

The study shows the establishment of new, replacement trees is not keeping pace with climbing tree mortality in the study plots, said CU-Boulder geography Professor Thomas Veblen, study co-author. The new study is the largest research project based on long-term forest plots ever published on North American forests, said Veblen.

"This regional warming has contributed to widespread hydrologic changes, such as a declining fraction of precipitation falling as snow, declining water snowpack content, earlier spring snowmelt and runoff, and a consequent lengthening of the summer drought," wrote the researchers in Science.

"The increase in tree mortality rates documented in the study is further compelling evidence of ecosystem responses to recent climate warming," said Veblen. "The findings are consistent with other well documented, climate-induced ecological changes, including increased wildfire activity since the mid-1980s and bark beetle outbreaks that are occurring at unprecedented levels in western North America forests, including Alaska."

Climate records from Colorado's subalpine forests, which are roughly 8,500 to 10,000 feet in elevation, show a marked increase in temperatures over the past 50 years during all seasons of the year, Veblen said. Colorado has experienced drought since the mid-1990s, peaking in 2002 and which became the most severe drought of the past century, he said.

The study's authors ruled out a number of possible sources of the increasing tree deaths, including air pollution, long-term effects of fire suppression, and normal forest dynamics. In contrast, increasing regional temperature was correlated with tree deaths.