January 23, 2009
Weekly News UpdateTree Mortality in West Caused by Warming Climate
Research published today in the journal Science indicates that the major contributing factors to tree mortality in the West is regional warming and drought stress. There has been a rapid increase in the number of dead trees found in old-growth forests over the past 50 years. Forests in the Pacific Northwest (Oregon, Washington in the US; British Columbia in Canada) have been particularly negatively-affected by regional warming.
The results from the study indicate that a continued increase in mortality rate could eventually lead to a 50 percent decrease in the average age of trees, a potential decrease in average tree size, and an increased susceptibility to a sudden die-back of trees throughout western forests.
Another concern is the development of a "feedback loop" within the forest system. As regional warming causes trees to die, forests become smaller. The smaller forests absorb less carbon dioxide from the air than the once-larger forests. This means that more carbon dioxide remains in the atmosphere, which in turn fuels an even higher level of atmospheric warming.
The scientists also took a look at other factors that affect forest health, such as insect attack, fire suppression, forest overcrowding, forest fragmentation, and air pollution. However, even after taking these factors into consideration, they found that the most significant mechanism affecting forests was indeed atmospheric warming. Over the period of the study, average temperatures in the western United States have increased by less than 1 degree. Though this amount may seem insignificant, this increase in temperature is enough to cause a number of changes in the region's water cycle. For example, less precipitation falls as snow, snowmelt occurs earlier in the year, and the summer drought occurs for a longer period than before.
The researchers studied 50 years of data gathered by a number of different scientists in forest stands containing trees 200 years old or more. The areas they studied included sites in Oregon, Washington, California, Arizona, Colorado, New Mexico, and southwestern British Columbia. The main method used in the study was fairly simple—counting trees.
The study was led by Phillip J. van Mantgem and Nathan L. Stephenson of the US Geological Survey (USGS) Western Ecological Research Center. They worked in collaboration with researchers from the University of British Columbia, University of Washington, Oregon State University, Northern Arizona University, University of Colorado, Pennsylvania State University, and US Forest Service. Funding for the study was provided by the National Science Foundation, US Department of Agriculture, and USGS.