Considering the future of the alpine zone
By Rob Burbank
AMC Outdoors, September/October 2012
Why does the Northeast have some of the world's lowest-elevation alpine habitat in relation to its latitude? Given potential climatic changes, will the alpine zone occupy a higher elevation—and a smaller area—by the end of the century? And, at the heart of it all, will the Northeast's alpine ecosystems survive?
The findings of a three-year AMC-led study supported by a grant from the National Oceanic and Atmospheric Administration answer those questions, and more, and the reasons are sometimes surprising. Cooperators in the study included the Mount Washington Observatory, University of New Hampshire, and Plymouth State University.
To take the last question first, yes, AMC's research has concluded that the Northeast alpine zone will survive, at least through 2100. Given the low-growing stature of most vegetation in high-elevation alpine zones, one might posit that trees could eventually march upslope to crowd out, and outcompete, the smaller alpine plants, especially if climate change leads to warmer temperatures at higher elevations. But diminutive Diapensia and their ground-hugging ilk are specialists that thrive in certain alpine conditions, and taller species aren't about to overtake them anytime soon. That's due in large part to clouds and wind—features that help define the Northeast's alpine summits.
The northeastern United States is home to 13 square miles of alpine habitat, with more than half concentrated in New Hampshire's Presidential Range. The largest of the region's alpine zones, the zone in the Presidentials is also the southernmost. While the relatively long growing season at this latitude can aid in alpine plant propagation, that's not what led to the preponderance of alpine habitat here, says AMC Director of Research Kenneth D. Kimball, PhD. Rather, the physical impacts of clouds, wind, and ice are at play, helping to keep taller plant species at bay.
"This broad range of alpine zone is less driven by the growing season than it is by the inability of the forest to advance because of physical damage," Kimball says. "We do see a very strong alignment between where the alpine zone is and the frequency of clouds coupled with strong winds."
Mount Washington, at 6,288 feet, is the tallest peak in the Presidential Range and is in the clouds about 60 percent of the time, making it one of the cloudiest places in the world. Its relative proximity to the seacoast's moist air contributes to that condition, as does the fact that clouds are formed when moisture-laden air is pushed up over mountains.
In winter, clouds and wind form rime ice, which collects on exposed objects. If that object is a balsam fir tree, for example, the ice loading causes mechanical damage that inhibits the tree's vertical growth, making that exposed location inhospitable for tree species that thrive in the lower, boreal zone. Only in very sheltered areas can you find small patches of stunted krummholz trees in the alpine zone, Kimball notes.
Greenhouse gases reduce nighttime cooling, which is also a factor in alpine plant growth. "Clouds also act as a thermal blanket, so, historically, there has been a thermal blanket on the region's mountains well before these areas were affected by greenhouse gases," Kimball says.
Northeastern alpine species, therefore, appear less threatened by regional warming than do species at lower elevations. While AMC's research into 75 years of local weather data found a warming trend, the trend diminished with elevation. "We are seeing a warming trend at all elevations, but at the upper elevations, it is at a slower rate than reported for lower elevations throughout the region," Kimball says. That finding is attributed not only to high cloud frequency but also the stratification of air masses. "The upper part of the mountain for part of the time is seeing different air masses than the lower parts of the mountain," he explains.
"Climate change has always happened and always will," Kimball says. But, he notes, "Right now, we're hypothesizing that alpine species under the current regime would survive to the end of the century. But if we push today's temperatures well past the last major warming period of several thousand years ago, or alter precipitation patterns, all bets are off."