Tree mortality rates have doubled in old-growth forests across the Sierra Nevada and western United States because of rising temperatures associated with climate change, a new study has found.
The study, published today in the journal Science, suggests that if the trend continues, the region's majestic conifer forests may become younger, smaller and less healthy -- making them vulnerable to massive die-offs.
"Everywhere we looked, we saw mortality rates increasing," said Nathan Stephenson, co-leader of the study and a research ecologist with the U.S. Geological Survey in Three Rivers near Sequoia and Kings Canyon National Parks. "That to me is a red flag that says there is potentially something pretty big going on." While the actual increase may not appear large -- annual mortality rates have risen from less than 1% to between 1% and 2% across the region over a quarter-century -- it is nonetheless significant, Stephenson said.
"You can imagine if [mortality rates] doubled in the human population," he said. "We'd sure take notice and go, 'Oh, my gosh. What's happening that's causing twice as many people to die every year?' " The study, which builds upon previous research, was based on data gathered from 76 research plots across the western United States and southern British Columbia over the past half-century. In California, most of those plots were in the southern Sierra, with a handful in Yosemite National Park and near Lassen Volcanic National Park.
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"It's almost like giving the forest a physical," Stephenson said. "We return to individual plots and go up to trees and effectively ask: 'Are you alive or dead? And if you're dead, what killed you?' " Stephenson and 10 other scientists who worked on the study found that mortality rates have increased across the map -- from damp coastal forests in the Pacific Northwest to windswept high-elevation stands in the Rockies.
"What's eerie is it's happening in the major kinds of trees -- it's Douglas firs, hemlocks, pines," Stephenson said. "And it's happening in trees of all sizes, at every elevation." Trees die of multiple causes, of course. But researchers found that common suspects -- such as insect infestations and overcrowding caused by fire suppression -- could not explain the widespread, sustained increase in mortality.
But climate change could, in part because Western forests thrive in cold-adapted landscapes where even small changes in temperature can translate into big differences on the ground.
"Average temperature in the West rose by more than one degree over the last few decades," said Phil van Mantgem, a U.S. Geological Survey scientist and co-leader of the study, in a news release. "While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt and lengthen the summer drought."
The findings jibe with those of many Sierra residents who say they are seeing more trees die these days, although not in dramatic numbers.
"Sometimes the slowness fools you," said Jan van Wagtendonk, a retired fire scientist who has watched ponderosa pines slowly fade from the foothills west of Yosemite.
"It doesn't seem to be very different this year than it was last year," van Wagtendonk said. "But if you look at this year vs. 30 years ago, then the change is dramatic."
As they examined their data, the scientists found that past droughts -- often implicated in tree deaths -- didn't fully explain the mortality increase. But today's droughts -- seasoned by temperatures warming through climate change -- may be contributing.
"It puts increased evaporative demand on the plants," Stephenson said. "If you're walking through the desert with a bottle of water and [the temperature] goes up 10 degrees, you're more likely to run out of water because you're sweating faster. The same thing happens with plants." As an example, he pointed to stands of hardy pinyon pine in the American Southwest that have survived previous dry spells but now are dying across vast areas.
Rising temperatures don't just sap moisture from trees, the scientists found. They also give insects a chance to wreak havoc. In British Columbia, for example, bark beetles once held in check by winter cold snaps have devastated a swath of forest one-third the size of California.
Similar die-offs are occurring across the northern Rockies, the researchers reported.
"In any of these cases, there are other [factors] going on," Stephenson said. "But driving them is a climatic change." Overall, scientists found that 1% to 2% of the trees in old-growth stands die each year, depending on the location -- and that the death rate had doubled over the past 17 to 29 years.
Such stands are natural resource treasure troves. Not only are they home to many rare species, from salamanders to spotted owls, they soak up and store water -- and heat-trapping carbon dioxide -- and provide recreational solace and inspiration to millions.
All that may be in danger.
"If a doubling of background mortality rates persists for a couple hundred years, the average age of a tree in the forest would be about cut in half," Stephenson said.
"Consequently the average tree would be smaller. Even if it just stopped at a doubling, that would pretty drastically change the forest."
The study also found that trees were dying faster than they were being replaced -- another discouraging trend.
"I view this as a canary in the coal mine for potentially larger changes in the future," Stephenson said. "It could be an indication that the forests are stressed and, if warming continues, they might get pushed over a tipping point and suddenly you lose vast swaths of forest to die-off."