Global Warming: Tree Deaths Have Doubled Across The Western U.S.

The study, published in the Jan. 23 issue of 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.

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.

USGS researchers Phil van Mantgem and Nathan Stephenson led the study. Co-authors included Veblen and Jeremy Smith of CU-Boulder, John Byrne of the U.S. Forest Service Rocky Mountain Research Station, Lori Daniels of the University of British Columbia, Jerry Franklin and Andrew Larson of the University of Washington, Peter Fule of Northern Arizona University and Mark Harmon of Oregon State University.

"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.

"Average temperature in the West rose by more than 1° F over the last few decades," said van Mantgem. "While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought."

The lengthening summer drought could be stressing trees, leading to higher death rates, he said. Warmer temperatures also might favor insects and diseases that attack trees. Some recent outbreaks of tree-killing bark beetles in the West have already been linked to warming temperatures.

In the Science study, the tree deaths measured in Colorado are all from stands re-measured prior to any stands being attacked in the current bark beetle outbreak, said Veblen. "The previous elevated rates of tree mortality in these forests may have been harbingers of the abrupt increase in tree mortality due to the current bark beetle outbreaks in Colorado."

During the past decade, mountain pine bark beetles have killed roughly 3.5 million acres of lodgepole pine forests in northwestern Colorado, and the outbreak has spread to the study's forest plots on the state's Front Range only within the last year, Veblen said. During the same time period, spruce bark beetles also killed large areas of spruce forest in northern and southwestern Colorado, he said.

"Forest entomologists and ecologists agree that warming temperatures are highly favorable to the population growth and survival of these beetles," said Veblen. "Moisture-stress induced by both warming and reduced snowpack increases tree susceptibility to bark beetle attack."

Veblen said the study suggests increased tree mortality rates may be indicators of climate-induced stress that could increase tree susceptibility to more abrupt causes of tree deaths like bark beetle outbreaks. "Recent events in subalpine forests in Colorado fit that pattern quite well," he said.

Given the evidence that recent climate-induced ecosystem changes are now so abundant, society needs to discuss policies that will help humans adapt to the changes under way, said Veblen. In the context of wildfire management, land managers need to reconsider the effectiveness of both fire suppression and fire mitigation efforts, including fuel reduction projects like timber thinning, he said.

"Instead, we need to consider developing land-use policies that reduce the vulnerability of people and resources to wildfires," Veblen said. "Activities include reducing residential development in or near wildland areas that are naturally fire-prone and where we expect fire risk to increase with continued warming."

Another significant part of the concern, Mark Harmon, professor of forest ecology at Oregon State University said, is that a "feedback loop" appears to be developing. As regional warming causes some trees to die, the diminished forests will absorb less carbon dioxide and then inject more greenhouse gases back to the atmosphere. This in turn could cause even higher levels of atmospheric warming.

"In ecology there's a bias toward understanding how things grow," Harmon said. "But my studies are mostly on how things die and decompose, and that's what's happening here. When trees across the West appear to be dying at twice the rate they used to, that's not a good sign."

The 76 western forest study plots harbored nearly 59,000 living trees. The research team studied the plots during two periods -- from 1955 to 1994 and again from 1998 to 2007, said Veblen. The permanent study plots on Colorado's Front Range were part of a study funded by the Colorado Commission on Higher Education in 1982-83 during a short-lived program when CCHE had a research grant program, Veblen said.