ARCATA, California, January 22, 2009 (ENS) – Death rates of old-growth trees in western U.S. forests have more than doubled over the past few decades, and the most likely cause of the trend is regional warming, finds new research to be published Friday in the journal “Science.”

Led by scientists with the U.S. Geological Survey, the research team found that the increase in dying trees has been pervasive. Tree death rates have increased across a wide variety of forest types, at all elevations, in trees of all sizes, and in pines, firs, hemlocks, and other kinds of trees.

Increasing tree mortality rates mean that western forests could become net sources of carbon dioxide to the atmosphere, further speeding up the pace of global warming, the scientists indicate.

Regardless of the cause, higher tree death rates could lead to substantial changes in western forests, said Phil van Mantgem, a USGS scientist and co-leader of the research team.

“The same way that in any group of people a small number will die each year, in any forest a small number of trees die each year,” said van Mantgem. “But our long-term monitoring shows that tree mortality has been climbing, while the establishment of replacement trees has not.”

The result is that forests have begun to lose trees faster than they are gaining them, said van Mantgem, a research ecologist with the USGS Redwood Field Station in Arcata, California.

Dead tree in Sequoia National Park, California (Photo by Nate Stephenson courtesy USGS)

These changes could change the suitability of forests for wildlife species, the scientists suggest.

They 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.

Instead, 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.

“Tree death rates are like interest on a bank account – the effects compound over time,” said Nate Stephenson, also with the U.S. Geological Survey and research team co-leader.

“A doubling of death rates eventually could reduce average tree age in a forest by half, thus reducing average tree size,” said Stephenson, director of the USGS Sierra Nevada Global Change Research Program.

In some cases, increasing tree deaths could indicate forests vulnerable to sudden, extensive die-back, similar to forest die-back seen over the last few years in parts of the southwestern states, Colorado, and British Columbia.

“That may be our biggest concern,” said Stephenson. He worries that the trend observed by the research team is a prelude to bigger, more abrupt forest changes.

Complete findings appear in the article, “Widespread increase of tree mortality rates in the western United States.” Scientists with the U.S. Forest Service, University of Colorado, University of Washington, Oregon State University, Pennsylvania State University, Northern Arizona University, and the University of British Columbia contributed to this research.

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WASHINGTON, DC, May 28, 2008 (ENS) – Forests in the interior western United States, the southwest, and Alaska are already being affected by climate change with increases in the size and frequency of forest fires, insect outbreaks and tree mortality. These changes are expected to continue, according to a new report issued Thursday by the U.S. Climate Change Science Program.

The Climate Change Science Program integrates the federal research efforts of 13 agencies on global change change. The U.S. Department of Agriculture, USDA, is the lead agency for this report and coordinated its production.

The report describes the effects of climate change on agriculture, land resources, water resources, and biodiversity in the United States.

“The report issued today provides practical information that will help land owners and resource managers make better decisions to address the risks of climate change,” said Joe Glauber, chief economist with the the USDA.

The report was written by 38 authors from universities, national laboratories, nongovernmental organizations, and federal agencies. It was subject to peer review by 14 scientists through a Federal Advisory Committee formed by the USDA. The National Center for Atmospheric Research assisted in coordinating production of the report.

The report concludes that climate change is already affecting U.S. water resources, agriculture, land resources, and biodiversity, and will continue to do so.

Much of the United States has experienced higher precipitation and streamflow, with decreased drought severity and duration, over the 20th century. The West and Southwest, however, are notable exceptions, and increased drought conditions have occurred in these regions.

There is a trend toward reduced mountain snowpack and earlier spring snowmelt runoff in the western United States, the report finds.

A continuation of the trend toward increased water use efficiency could help mitigate the impacts of climate change on water resources, the authors advise.

The growing season has increased by 10 to 14 days over the last 19 years across the temperate latitudes. Species’ distributions have also shifted.

Barley harvest in Washington state’s
Palouse Hills. (Photo courtesy USDA)

Grain and oilseed crops will mature more rapidly, but increasing temperatures will increase the risk of crop failures, particularly if precipitation decreases or becomes more variable.

Higher temperatures will negatively affect livestock. Warmer winters will reduce mortality but this will be more than offset by greater mortality in hotter summers. Hotter temperatures will also result in reduced productivity of livestock and dairy animals.

Weeds grow more rapidly under conditions of elevated atmospheric carbon dioxide, CO2, the main greenhouse gas emitted by burning fossil fuels. Under projections reported in the assessment, weeds migrate northward and are less sensitive to herbicide applications.

Horticultural crops such as tomato, onion, and fruit are more sensitive to climate change than grains and oilseed crops.

Invasion by exotic grass species into arid lands will result from climate change, causing an increased fire frequency. Rivers and riparian systems in arid lands will be negatively impacted.

But not all plants will experience negative effects as the climate warms. Young forests on fertile soils will achieve higher productivity from elevated atmospheric CO2 concentrations. Nitrogen deposition and warmer temperatures will increase productivity in other types of forests where water is available.

The rapid rates of warming in the Arctic observed in recent decades, and projected for at least the next century, are dramatically reducing the snow and ice covers that provide denning and foraging habitat for polar bears, the report finds. The U.S. Department of the Interior earlier this month declared the polar bear to be Threatened under the Endangered Species Act.

Agencies within the USDA already are responding to the risks of climate change, the report states, saying, “For example, the Forest Service is incorporating climate change risks into National Forest Management Plans and is providing guidance to forest managers on how to respond and adapt to climate change.”

The Natural Resources Conservation Service and Farm Services Agency are encouraging actions to reduce greenhouse gas emissions and increase carbon sequestration through conservation programs.

The USDA’s Risk Management Agency has prepared tools to manage drought risks and is conducting an assessment of the risks of climate change on the crop insurance program.

The report, “Synthesis and Assessment Product 4.3 (SAP 4.3): The Effects of Climate Change on Agriculture, Land Resources, Water Resources, and Biodiversity in the United States,” is posted on the Climate Change Science Program website. To access it, click here [www.climatescience.gov].

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