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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RESTON, Virginia, January 30, 2008 (ENS) – Nine states in the Mississippi River Basin contribute the majority of nutrients to the northern Gulf of Mexico, threatening the economic and ecological health of one of the nation’s largest and most productive fisheries, according to a federal government report released Wednesday.

The states of Illinois, Iowa, Indiana, Missouri, Arkansas, Kentucky, Tennessee, Ohio and Mississippi make up only one-third of the 31-state Mississippi River drainage area, but contribute more than 75 percent of nitrogen and phosphorus to the Gulf, U.S. Geological Survey, USGS, researchers report.

Excess amounts of these nutrients have resulted in a zone of low dissolved oxygen or hypoxia, caused by the growth of large amounts of algae. This can stress and cause death in bottom-dwelling organisms in the Gulf, so it is commonly called the dead zone.

The study found that agricultural nonpoint sources contribute more than 70 percent of the nitrogen and phosphorus delivered to the Gulf. By comparison, only about nine to 12 percent originates from urban sources.

Corn and soybean cultivation is the largest contributor of nitrogen to the Gulf. Animal manure on pasture and rangelands and crop cultivation are the largest contributors of phosphorus.

These are among the new findings in the report, “Differences in Phosphorus and Nitrogen Delivery to the Gulf of Mexico from the Mississippi River Basin,” by the U.S. Geological Survey, USGS, published in the journal “Environmental Science and Technology. “

“This study is important because it reveals new details about sources of phosphorus,” said Richard Alexander, USGS scientist and lead investigator. “The report shows that animal manure on pasture and range lands contribute nearly as much phosphorus as cultivated crops, 37 versus 43 percent.”

The study suggests that phosphorus associated with the wastes of unconfined animals is a much larger source of phosphorus in the Mississippi River Basin than previously recognized. Current animal manure management emphasizes controlling nutrients primarily from confined animal facilities.

Delivery of nutrients to the Gulf was found to be highest from watersheds in the central and eastern portions of the Mississippi River Basin that are drained by large, fast flowing rivers with very little natural removal.

Runoff entering Fall Creek in
Indianapolis, Indiana (Photo
by Charles Crawford courtesy
USGS)

Alexander concludes that nutrient reductions in the Gulf may be most efficiently achieved by managing nutrients in watersheds drained by large rivers.

Reservoirs, particularly common in the Tennessee Valley and along the Missouri River, are effective at removing phosphorus from watersheds in the Mississippi Basin, but Alexander and his team found that this creates water quality issues in the reservoirs themselves.

The joint federal-state Gulf of Mexico Hypoxia Task Force is evaluating recommendations by the U.S. Environmental Protection Agency’s Science Advisory Board to set reduction targets of at least 45 percent for both nitrogen and phosphorus.

The targets would be set in an effort to shrink the size of the dead zone in half to 5,000 square kilometers by 2015.

USGS Associate Director for Water Dr. Robert Hirsch says “on-the-ground” water monitoring is essential to provide credible, comparable data to verify computer modeling predictions across large regions, such as the Mississippi River Basin.

Yet surface water quality monitoring stations are disappearing. Only 35 stations exist today, down from about 425 stations in the early 1990s, Hirsch said.

Hirsch says this study shows that nutrient issues are complex and management of animal and crop production, control of nutrient sources in close proximity to large rivers, and consideration of reservoir effects on phosphorus all are necessary to reduce the nutrient burden flowing in the Mississippi River Basin.

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