Think of them as America's giant lungs, soaking up masses of carbon dioxide from the atmosphere. But just as human lungs can become sick, so the forests of the western U.S. have fallen ill. At least that's the finding of an extensive, thirty-year study published on Thursday, which found that trees in the American west are dying at a quickening and alarming rate. The most probable cause, according to the study: global warming, the very trend trees should be helping to slow.
The study, led by authors from the United States Geological Survey and published in the journal Science, found the rate of tree deaths has more than doubled in the last few decades even in apparently healthy, well-established forests. Death rates have increased at all elevations, and for trees of all sizes and types, leading the researchers to worry that the U.S. may soon suffer massive and sudden die-backs of its seemingly healthy forests, a cascading effect that could release carbon dioxide into the air, further speeding global warming. (See pictures of trees.)
Scientists have already witnessed mass tree deaths in American forests due to beetle infestations. Periodic outbreaks of Mountain Pine Beetle in Colorado, for example, has killed an estimated 7.4 million trees in the past decade. But the Science study, titled "Widespread Increase of Tree Mortality Rates in the Western United States," is the first to show a creeping death rate in ancient, well-established coniferous forests with no evidence of epidemic infestations.
Over the course of thirty years, scientists from the U.S. Geological Survey (USGS), U.S. Forest Service, National Park Service and various academic institutions counted trees in 76 plots, each the size of two football fields, in forests across six western states and British Columbia. When they looked at the data last year, they found that mortality rates had increased significantly in almost every plot, with the rate doubling over periods ranging from 17 years to 29 years. Crucially, the increasing deaths were not matched by higher rates of "recruitment" the technical term for a forest's birthrate.
Nathan Stephenson, a Research Ecologist for the USGS and one of the study's authors, said the scope of the research revealed trends that would otherwise have gone unnoticed. "The changes have been subtle. If you were walking around these forests you'd think everything was fine. But when you have the long view, it's very worrying. There's a real concern that if mortality rates continue to double the forests could reach a tipping point where they begin to actually be net emitters of carbon. Or, worse, that this preludes mass die-backs like the ones we've already seen in the Southwestern U.S. and British Columbia, as the forests grow weaker and less able to resist [beetle] infestations."
Forests act as huge carbon sinks, capturing and storing carbon dioxide. Along with oceans and other plant life, trees removed approximately 54% of all carbon dioxide created by human activities globally during the period 2000-2007, according to research group The Global Carbon Project. But when they die, trees release their sequestered carbon as they decompose, leading some scientists to the theory that mass tree deaths from global warming could lead to a worsening cycle in which each stage of warming sets off another. Such a cycle could hasten climate change from the predicted timescale accepted by the U.N.'s Intergovernmental Panel on Climate Change.
The Science study suggests that such a troubling cycle might befall U.S. forests, as warming was listed as the most likely cause and result of the increase in tree mortality. The study ruled out increasing competition among trees, changes in the composition of tree species, pests, fires, air pollution or logging as the reason for the increase. The likely culprit, the researchers said, was stress from warming temperatures. (See pictures of a heat wave.)
"We know that the temperature has gone up about .4 C per decade in the elevations where forests are found [in the western U.S.]. That means that the snow-pack is melting earlier and less snow is falling. So the trees have seen a lengthening of the summer drought even without a change in precipitation levels. This leaves them stressed and vulnerable," Stephenson told reporters.
Stephenson added that the Science study could help the U.S. Forest service improve its husbandry measures by, for example, responding differently following forest fires. "We might want to think about planting a different species of trees perhaps one from further south or lower elevations that are better adapted to warmer temperatures," he said. But even that would be a band-aid solution. According to Stephenson, the best chance of saving American forests is to slow and eventually cease the cause of their distress. "Anyway you cut it, the best solution is to get a lid on humanity's carbon output," he says.