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Our trees are dying. Throughout the western United States, cherished and protected forests are dying twice as fast as they did 20 years ago because of climate change, researchers reported in the journal Science. Fire did not kill these trees, nor did some massive insect outbreak. The trees in this wide-ranging study were "undisturbed stands of old growth forests", said Jerry Franklin, a professor of forest resources at the University of Washington and one of 11 co-authors of the report.

"The data in this study is from our most stable, resilient stands of trees," Franklin told IPS.

What this means is that the United States' best forests are getting thinner.

It is like a town where the birth rate is stable but the mortality rate for all ages doubled over the past two decades. "If that was happening in your hometown you'd become very concerned," said Nate Stephenson, an ecologist with the U.S. Geological Survey (USGS).

This dramatic increase of in tree mortality applies to all kinds, sizes, ages and locations of trees. In the Pacific Northwest and southern British Columbia, the rate of tree death in older coniferous forests doubled in 17 years. In California, doubling mortality rates took a little longer at 25 years. For interior states it took 29 years.

Mortality has increased in lock-step with rising temperatures of about 1 degree C in the last 30 years. Air pollution and ground level ozone were investigated and eliminated as the cause of the increased mortality, Stephenson told IPS.

Warmer temperatures in the west have meant the summer drought period is longer. The mountain snow pack contains less snow and melts much earlier in the spring. Warmer temperatures also favour insects like tree-damaging beetles. The combination of trees suffering moisture stress and a few more insect pests appears to be enough to tip the balance, said Tom Veblen of the University of Colorado at Boulder.

"We're seeing continental-scale evidence of warming," Veblen said. "It is very likely tree mortality will increase further as temperatures continue to rise."

Previous research has shown global warming is largely responsible for the enormous increase in forest fires in the west and the massive insect outbreaks like that of the mountain pine beetle, expected to kill 80 percent of the pine forest in Canada's province of British Columbia by 2013.

Forests of all kinds contain more than 80 percent of Earth's terrestrial biodiversity. Not only do they absorb carbon, forests produce 30 percent of the world's oxygen. They are also a key part of the planet's climate regulating system. About half of the world's forests are already gone.

Carbon emissions from burning of fossil fuels is warming temperatures globally but forests play a vital role in capturing carbon from the atmosphere and sequester or trap carbon. As a result, forests around the world store as much carbon as is currently in the atmosphere.

Dead trees release that stored carbon. If the mortality rate of big trees goes up, then North America's forests become a source of carbon emissions, leading to even higher temperatures and still thinner forests in a feedback loop.

"At best they will take up less carbon from the atmosphere," said Franklin. "Older, stable forests should be left alone. We don't want to accelerate this process."

Large old growth trees hold far more carbon than young, fast growing trees and so there is no way to recover the carbon lost from logging old growth, he said. Government policies should reflect this reality. Preserving old growth forests must be part of the international climate agreement that will be negotiated in Copenhagen, he said.

Surprisingly, this is the first large-scale analysis of mortality rates in temperate forests but Franklin believes the increase in mortality is widespread and applies to forests everywhere.

Logging aside, the fact that forests are dying is not new. Scientists have known since the 1980s that temperate forests were suffering from pollutants such as acid rain, nitrogen deposition and increased ground-level ozone, as well as higher ultraviolet radiation levels. While invisible to nearly everyone, the slow decline of U.S. forests was well-documented in a 1995 book "The Dying of the Trees" by science writer Charles E. Little.

Based on the science of the day, Little accurately predicted that the western U.S. would burn and deserts would expand and that sugar maples would largely vanish from the northeast in the near future. And, particularly because of global warming, he regretfully concluded that temperate forests had crossed a threshold. "And the more trees die, the more will die," he wrote.

Scientists working in tropical forests now say these forests are extremely sensitive to increases in temperature. The vast majority of tropical forests exist where the annual average temperature is 25 to 26C. Before the end of this century temperatures in tropical regions are projected to be 3C higher. No forest exists anywhere where the annual average temperature is 28C, Joseph Wright of the Smithsonian Tropical Research Institute (STRI) in Panama told IPS in a previous interview.

"That doesn't mean something else won't replace tropical forests, but we don't know what it will be," said Wright.

Major reductions in carbon emissions and deforestation are urgently needed, the experts all agree. Little said the same thing 14 years ago. But he also said that humanity needed to begin the process of environmental repair: "The trees could save us, if we would save the trees, for they are the threshold."

By Stephen Leahy

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