Lakes of the Clouds hiking hut sits on the southwestern shoulder of Mt. Washington, in New Hampshire’s Presidential Range. At over 5,000 feet elevation, the hut sits about 250 feet above treeline and affords spectacular views of the surrounding peaks.
If you had been visiting the hut (and it had existed) 5,000 years ago, however, you wouldn’t have had any view because the hut would have been surrounded by a balsam fir forest. Scientists have figured this out by examining the pollen, seeds, twigs, and other debris that has settled to the bottom of a nearby lake, accumulating year after year since the end of the last ice age. The sediments form layers, and by taking core samples down through the layers, the scientists have recreated the floral history of the Lakes of the Clouds area since the end of the last ice age.
The record shows that the treeline moved upward on Mt. Washington beginning about 7,500 years ago, as the overall climate warmed. Around 2,500 years ago, the treeline moved back downslope, as a period of neoglacial cooling set in.
The warming period, called the Holocene Climatic Optimum, occurred worldwide, and it appears to have been characterized primarily by warming summer temperatures in the northern hemisphere. On Mount Washington, the treeline responded to the warmer climate by moving up. Given this history, what is likely to happen to the current fir forest on the mountain now that rapid climatic change is underway?
Bicknell’s thrush is a rare mountain bird species with a fluty song and fascinating breeding ecology. It nests only in high-elevation “islands” of balsam fir across the Northeast and eastern Canada. For over 15 years, the thrush and its montane habitat have been studied by the Vermont Center for Ecostudies (VCE).
Since 1992, citizen science volunteers working with the VCE biologists have strapped on their backpacks each June to count birds on mountaintops from the Catskills in New York to Mount Katahdin in Maine and the Green and White Mountains in between. Using this information, coupled with aerial photographs that help determine the acreage of fir forest across New York, Vermont, New Hampshire, and Maine, the biologists created a map of Bicknell’s thrush habitat in the Northeast.
The biologists then compared their habitat map to the mean July temperature map for the region, using a computer model called ClimCalc, which was developed by scientists at the University of New Hampshire from weather station data gathered throughout the Northeast between 1950 and 1980. Nearly all of Bicknell’s habitat was located in a narrow temperature band between 49°F (the average July temperature at the highest elevations near the treeline) down to 60° (the average temperature where the forest begins to change from softwoods to hardwoods).
The researchers then used their computer model to recreate the Holocene Climatic Optimum, slowly warming the mountain slopes and moving the temperature boundaries upward to predict the future location of the fir forest where Bicknell’s thrush nests.
A warming of just 2°F reduced potential Bicknell’s thrush habitat in the Northeast by more than half, and an increase of 4° was enough to eliminate all the habitat from the Catskill Mountains of New York and most of the habitat in Vermont, leaving the highest peaks of New Hampshire’s Presidential Range as the only substantial breeding ground remaining.
A warming of at least 2° to 4°F, meanwhile, is widely accepted as likely to occur in the Northeast before the end of this century. Indeed, summer temperatures locally are projected to rise an average of 5° under a reduced-CO2-emissions scenario and as much as 11° under a business-as-usual, higher-emissions scenario. At this upper end, possible by late in this century, the model shows that suitable habitat for Bicknell’s thrush will disappear entirely from the Northeast.
Fortunately for the thrush, the actual treeline will take longer than this to move upslope because the movement of trees does not take place instantaneously. It is highly likely that shifts in forest composition will lag at least decades behind temperature changes. These lag times may even be as long as centuries. Such lags occur due to slowly changing soil characteristics, interactions among tree species, and the disturbance required for one habitat type to invade and replace an adjacent one under variable climate conditions.
During the Holocene Climatic Optimum, the treeline may have reached an elevation of 5,575 feet on Mount Washington, about 700 feet shy of the summit. But this change took place over a period of 4,000 years. Under our current rate of global warming, the change is predicted to be far faster – perhaps within just a few hundred years – because the temperature change will be so much greater. We may well see drastic changes in mountain forests that are completely unpredictable.