The Heat Is Online

Scientists Startled By Speed of Change in New England Forests

Trees are responding to warming in New England much faster than expected

HUNTINGTON, Vt. - After about a mile of snowshoeing up Camels Hump mountain, University of Vermont ecologist Brian Beckage paused to take in the trees around him - not just for their beauty, but for the dramatic change they appear to be undergoing because of climate change.

Forty years ago, this part of the mountain would have been blanketed with cold-loving red spruce and balsam fir trees. But today, warmer-loving northern hardwoods such as sugar maples and American beech are taking over.

Scientists have long thought it would take generations if not centuries for tree populations to shift in response to a warming world. But Beckage and his colleagues' work on Camels Hump and two other nearby mountains suggests that climate change might affect New England forests far sooner than scientists thought.

"The fact that we found shifts here may be indicative that forests are changing throughout the region," he said.

Beckage led a study published earlier this month in the Proceedings of the National Academy of Sciences that found that the boundary between northern hardwoods and colder-loving trees shifted about 350 feet uphill in the last 40 years in response to warming temperatures. Climate change is likely only one factor in the forest transformation, he said, and he is studying other potential influences.

New England has warmed 2 to 4 degrees Fahrenheit in the last 40 years and it's the consensus of scientists that part of the warming is due to the release of heat-trapping gases from power plants, factories, and vehicles.

Rising temperatures have pushed spring to begin at least a week earlier and the growing season to expand more than 10 days in some places. While some plants are clearly beginning to react, it's been difficult for scientists to see changes on a larger scale because of the lack of long-term data.

But Beckage has such a data set. In the 1960s, University of Vermont graduate student Thomas Siccama began vegetation surveys on the 4,083-foot-high Camels Hump and other nearby mountains. His survey work helped scientists to later conclude that sulfur dioxide and nitrogen oxides released from power plants reacted in the atmosphere to create acid rain that harmed trees, especially those at high altitudes.

In 2004, one of Beckage's graduate students, Ben Osborne, returned to the mountains to painstakingly resurvey the plots up the mountains' slopes. Osborne measured types of trees and density every 200 feet.

Then, Beckage and colleagues compared the results to the past.

Tree communities at low and high elevations were relatively unchanged. But in the transition zone, where northern hardwoods quickly give way to the dark spruce-fir forest, the change was dramatic.

In this zone, about 2,600 feet up, Beckage and his students found that cold-loving trees had declined from 43 percent to 18 percent. Northern hardwoods increased from 57 percent to 82 percent. Overall, the entire zone shifted upward several hundred feet.

Such changes in this transition zone make sense, he and other scientists say. They expect to see little change at the bottom of the mountain because northern hardwoods already out-compete cold tree species. At the top, where cold-loving or boreal species dominate, there are no northern hardwoods nearby to seed the area as it warms. But in the transition zone, northern hardwoods and boreal species are at their climatic boundaries. Warmer temperatures likely helped make it more inhospitable for colder-loving trees to survive. And the abundance of northern hardwood seeds nearby allow those warmer-loving species to take hold.

"And it's not in one place - we are seeing it across the area," said Carrie Pucko, a co-author of the report and a graduate student who works with Beckage.

Still, many questions remain, Beckage and scientists who have read the paper say. Trees on mountains don't only respond to temperature; precipitation, cloud cover, and wind also determine everything from height to health to the location of the tree line. Beckage only looked at temperature and, to a lesser degree, precipitation.

The rate of change he found - in just a few decades - is also surprising because trees live for hundreds of years. Even if they were responding to climate change, it might take a century or more to see it, some scientists say.

"How do you account for the [lack of a] lag?" asked Charlie Cogbill, a visiting fellow at Harvard Forest, an experimental forest run by Harvard University. He said Beckage's work was interesting, but wonders if some of the forest change is a natural trend or unconnected to global warming. And just because the 1960s is the baseline researchers have, doesn't mean that it is the best one to use.

"We also need to take into account what was happening before the first plots were surveyed," he said.

Beckage agrees the answers are complicated. Other factors such as beech bark disease may have killed off enough trees to trigger some of the changes he found in forest composition. Acid rain also likely contributed to the decline of red spruce trees at high elevations.

Beckage is now studying how climate change combined with acid rain may be hastening change in forests in New England.

"Acid rain may be killing off trees and creating opportunity for other species," he said.

The long-term prognosis for New England's iconic sugar maples is mixed. While the change Beckage saw on the mountains indicates that maple tree boundaries might be extending up Vermont's Green Mountains, maples will also have more trouble thriving here if the region continues to warm.

But it may be centuries before farmers see any dramatic change in species composition in their carefully managed maple forests.

Farmers "are not going to allow trees to change," said Tim Perkins, director of the University of Vermont Proctor Maple Research Center and a co-author of the report. "But the work shows that change is probably happening on a scale we haven't really been aware of."

(c ) The Boston Globe, 2008