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Mountain plant communities moving down despite climate change, study finds

By Bettina Boxall, Los Angeles Times

January 21, 2011

Predictions that climate change will drive trees and plants uphill, potentially slashing their range to perilous levels, may be wrong, suggests a new study that found vegetation in California actually crept downhill during the 20th century.

The research, published in the Jan. 21 issue of the journal Science, challenges widely held assumptions about the effect of rising temperatures on shrubs and trees that play a critical role in mountain environments.

Various studies in recent years have predicted that to survive global warming, mountain plant communities will march to higher elevations in search of cooler temperatures -- and if they are unable to do so quickly enough -- could perish.

But comparing data from the early and late 20th century, authors of the Science paper found that despite warming, many plant species in California mountain ranges are growing at lower elevations than they did 80 years ago. The scientists attributed the shift to a wetter climate in Central and Northern California, which offset the effect of higher temperatures.

The lesson, said co-author John Abatzoglou, a University of Idaho assistant geography professor, is that "we'd be remiss if we just focus on temperature," in forecasting the influence of climate change on plant life. "This might mean species extinction rates may not be as dire as predicted."

Climate warming models have consistently indicated that California will get hotter. But modeling has been less certain about the effect on total precipitation. Some models suggest the state will grow wetter -- if less snowy. Some suggest it will grow drier.

The researchers were careful to say that the rise in precipitation in much of California over the last century could be a function of natural variability, and have no link to the effect of greenhouse gases accumulating in the atmosphere.

For whatever reason, Abatzoglou said the Sierra was 5% to 10% wetter in the final half of the 1900s than in the first half, allowing trees and shrub species to take hold at lower elevations.

Comparing historic vegetation data from 1905 to 1935 to information gathered from 1975 to 2005 by researchers and federal agencies, the study found that about five dozen species had on the whole migrated downhill an average of about 264 feet.

Researchers relied in part on a treasure trove of botanical information collected in the 1920s and 1930s as part of a broad-ranging survey of California wild lands directed by U.S. Forest Service silviculturist Albert Wieslander. Partly funded by New Deal programs, it includes records from about 14,000 plots, hand-drawn maps and several thousand photographs that document timber stand conditions.

"These data sets provide us with an unprecedented view" of the large-scale changes in plant distribution that have occurred the last 75 years in California, said co-author Solomon Dobrowski, an assistant forest management professor at the University of Montana.

Those shifts, he added, were driven by changes in water availability rather than in average annual temperature, which rose about 1 degree across the state during that period.

Implications of the findings extend beyond California. Globally, "many locations north of the 45-degree latitude have experienced increased precipitation over the last century," Dobrowski said. "And global climate models generally predict these locations [will] become wetter over the next century."

If it turns out California does grow drier with global warming, "We would expect things to turn a corner and start moving uphill," he added.

Even if they don't, the effect of climate change on mountain environments could be complex. Insects are more sensitive to temperature and are likely to move uphill, Dobrowski said. And if the plants they eat and pollinate are shifting downhill, that could be an issue.

"We can't oversimplify the problem in terms of biological communities," he said.

bettina.boxall@latimes.com

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Calif. Plants Put A Wrinkle In Climate Change Plans

by Richard Harris

January 21, 2011

As the globe warms up, many plants and animals are moving uphill to keep their cool. Conservationists are anticipating much more of this as they make plans to help natural systems adapt to a warming planet. But a new study in Science has found that plants in northern California are bucking this uphill trend in preference for wetter, lower areas.

The simple message — that things are going to move uphill and toward the poles — may not be the answer in all cases.

- Solomon Dobrowski, University of Montana

Usually, coping with climate change is an uphill struggle for ecosystems — literally. Plants and animals want to be in a temperature zone where they can survive best.

"We see it consistently for mobile species such as insects and animals," says Solomon Dobrowski, an assistant professor of forest landscape ecology at the University of Montana. "A lot of the real foundation studies of this have come out of studies of butterflies, for example."

Dobrowski expected he'd see the same trend when he looked into historical movements of plants in a vast area of northern California. He dug through a remarkable record of the region's vegetation, collected back in the 1930s thanks to a federal project started during the Great Depression. He and his colleagues from the University of Idaho and the University of California, Davis then compared that with modern vegetation surveys.

"What we found was counter to our expectations," he says. "We found that in fact the preponderance of plants in our study area had actually moved downhill 80 meters, or roughly 240 feet." The United States Forest service set out to document the flora in California in the 1920s and 1930s. Photos like this one, showing vegetation, were used in a new study to measure how plant species have responded to a changing climate.

The United States Forest service set out to document the flora in California in the 1920s and 1930s. Photos like this one, showing vegetation, were used in a new study to measure how plant species have responded to a changing climate. The United States Forest service set out to document the flora in California in the 1920s and 1930s. Photos like this one, showing vegetation, were used in a new study to measure how plant species have responded to a changing climate.

The United States Forest service set out to document the flora in California in the 1920s and 1930s. Photos like this one, showing vegetation, were used in a new study to measure how plant species have responded to a changing climate.

Water Availability Is Key

Individual plants don't move of course, but the optimal range of many different species in the area studied has been creeping downhill. That means more new seeds sprouted downhill, and more new plants took root. This was true not just for annual plants but also for bushes and even trees.

Why would that be, Dobrowski wondered, considering that the area has warmed up. He and his colleagues say the answer lies not in the temperature, but in the amount of life-giving rain and snow. It turns out this region has been getting wetter.

"These plants are tracking water availability more so than temperature," he says.

That's not that big a surprise when you come to think of it. But until now, ecologists doing this kind of study had mostly noticed a trend linked with temperature. Dobrowski says that still holds in many cases. But, "the simple message — that things are going to move uphill and toward the poles — may not be the answer in all cases."

Disruptions In Ecosystems

This adds some pretty big wrinkles to conservation plans. For example: It's not always a good assumption that protecting areas upslope from plants will help protect their future habitat as the climate changes.

Dobrowski says if ecosystems see their optimal temperature range moving uphill and their water supply moving downhill, that could be quite awkward at times. What if plants move downhill to better moisture conditions but the butterflies that rely on those plants are driven uphill to avoid the heat?

"You could have situations in which plant and animal communities are even disrupted further," he says.

This is quite a sobering finding for ecologists trying to anticipate what will happen to natural systems in the coming century.

Scott Loarie, a researcher at the Carnegie Institution of Science on the Stanford University campus, says most of the research into this topic has been focused on temperature trends, "because we know the most about temperature. Pretty much all our evidence shows the earth will get hotter, and there's very little dispute about that. We don't know very much about precipitation."

Loarie says the new study underlines just how important precipitation can be. Unfortunately in many parts of the world, scientists simply can't say whether climate change in the long run will bring more moisture or more drought. Loarie says California is a case in point —the various climate forecasts disagree.

"So it's really a crapshoot in California whether we're facing a drier or a wetter future," he says.

That means it's entirely possible that the plant communities that have been marching downhill for the past 80 years will eventually reverse course, and head back up the slopes.