Climate change has doubled forest mortality

The death rate of the most stable and resilient forests in western North America has doubled during the past few decades as the climate has warmed, according to research to be published Friday.

The increased mortality suggests future landscapes will be thinner, sparser and far more susceptible to widespread diebacks.

The new data from a team of 11 scientists provide more evidence that climate change is having a broad and significant impact, independent of other human activities such as logging and development.

And while the study focused on Western North America, scientists say the global temperature rise is likely affecting all the world’s forests – from the Northern boreal to the Eastern hardwoods to the tropics – to some degree.

In North America, scientists say, the trend is clear: Western forests are becoming more susceptible to wildfire, disease and invaders such as bark beetles. Average tree size is shrinking; creatures dependent on large, old-growth trees will increasingly find themselves out of a home. 

!ADVERTISEMENT!

And as temperature and mortality climb, these forests will store less and less carbon – and could potentially flip from being carbon sinks to carbon sources, further speeding global warming.

"The important message here is that wherever we looked, mortality rates are increasing," said Nathan Stephenson, a scientist with the U.S. Geological Survey’s Western Ecological Research Center in California and a co-author on the study, published in the journalScience.

"It’s very likely that mortality rates will continue to rise."

The research tracked growth rates and tree mortality from 1955 to the present in 76 plots of old-growth forest across the West.

Tree death increased in every plot and every region, at every elevation, in trees of every size and every type, scientists said. And the change is happening fast, with estimated doubling periods ranging from 17 years in the Pacific Northwest to 29 years in the Rocky Mountains. The birth rate for new trees remained unchanged.

Temperature alone is driving this decline, researchers found. From the 1970s to 2006, temperatures increased 0.3ºC to 0.4ºC per decade across the region, drying out the snowpack, triggering an earlier spring melt and lengthening the summer dry season.

"We do see clear evidence that climate change is resulting in an increase in stand replacement," said Jerry Franklin, a professor of forest resources at the University of Washington and another co-author. "These are what we consider to be our most stable, most resilient stands."

The study's scale allowed researchers to discount other factors: Spreading disease has increasingly afflicted vast swathes of North America’s pine and fir forests, but when scientists excluded afflicted patches, the remaining healthy stands saw the same increasing mortality rates.

Similarly, another blight for forest health, smog, could be discounted after researchers found the death rate for trees growing in the relatively pure air of Washington's Olympic Peninsula no different from those of trees growing in California's smog-afflicted Sierra Nevada.

The findings, researchers said, have broad implications for land managers and policy makers. For starters, Franklin said, it underscores the need to preserve the remaining stands of old growth forests throughout the West.

Old growth stands are "extraordinary carbon stores," Franklin said, sequestering a large mass of carbon in very stable conditions.

Greenhouse gas emissions associated with logging old growth - to say nothing of habitat loss - cannot be mitigated by new growth, Franklin said.

"One lesson in it for me is ... we probably do not want to get into these forests and mess around. Because we aren’t going to help. If anything, we could potentially mess it up," he said.

New regulations will likely be needed, to both help the forest and keep people safe, particularly as fire risk rises, said Thomas Veblen, a professor of geography at the University of Colorado and another co-author.

"This is further evidence that we're really seeing continental-scale effects of the warming," he said. "We have to start thinking outside the box in terms of how as a society we adapt to the change that's under way."

Land managers say they are getting that message.

In California the Nature Conservancy is calling for the establishment of large protected areas that permit species migration, promoting more resilient ecosystems and sponsoring legislation in Sacramento requiring wildlife corridors as condition for highway development, said Louis Blumberg, director of the organization's California climate change team.

"And we're doing science," he said. "We're trying to figure it out, too."

But it is likely this latest data, from one of the largest-ever surveys of North American forests, underestimates the true impact of climate change on forest health. 

The researchers did not include any Western forest stands hit by massive trauma that is at least partially linked to climate, such as wildfires or the bark beetle epidemic.

British Columbia has lost 40 million acres of forest to the bark beetle; Colorado is approaching 2 million acres of dead forest;  Wyoming just recently crested the 1-million-acre mark, said Mary Ann Chambers, spokeswoman for the U.S. Forest Service's Bark Beetle Incident Management Team for the Rocky Mountain region.

Last summer the Forest Service closed a quarter of its 120 campgrounds to remove dead trees, and more are dying: The beetle epidemic has recently crested the continental divide, jumping from the predominantly lodgepole pine forest on the Western Slope to a forest of mixed conifers to the east. "The research says if the beetle is in lodgepole, it pretty much stays in lodgepole," Chambers said. "But that isn't the case any more."

All this has an impact, said Kyle Patterson, a spokeswoman for the Rocky Mountain National Park, where a popular camping spot will be closed this spring so crews can essentially clear cut the dead trees. 

"We held on for as long as we could," Patterson said. "There are a lot of people who have been going to that campground for years. They saw the (hills) getting redder and redder. They knew it was coming. It’s still very hard to take."

"The forest is changing," she added. "It's very upsetting to those of us who won't see it regenerate in our lifetimes."