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.
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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 coauthor 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 effects 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 tree 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 '30s 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 over the last 75 years in California, said coauthor Solomon Dobrowski, an assistant forest management professor at the University of Montana.
Those shifts, he said, 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 that California does grow drier with global warming, "We would expect things to turn a corner and start moving uphill," he said.
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.
Connie Millar, a U.S. Forest Service research ecologist who is studying climate change's effects on alpine ecosystems, said the Science paper demonstrated that global modeling results can't just be uniformly applied. "There are surprises at regional and local scales," she said, adding that land managers needed to take that into account in planning how to deal with climate change.
For instance, if valued plant communities are moving out of public lands at higher elevations into private property downhill, they will be more vulnerable to development and more in need of open space corridors connecting them to protected areas.
Millar's research in the Eastern Sierra and the Great Basin has also found that tree lines are moving down rather than up, although for slightly different reasons. They are shifting down slope into drainages that are cooler, and coincidentally, moister.