New York State Ornithological Association

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ConservationPosted 3/3/10
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Projected Effects of Climate Change on High Elevation Forests
Joan Collins, NYSOA Conservation Committee
Published in the January 2010 issue of NY Birders

High Elevation Forest, Bicknell's Thrush Habitat

There are already many observable changes rapidly occurring in the North Country of New York as a result of climate change: ice cover occurs much later in the fall and melts earlier in the spring; average winter temperatures are increasing (The Adirondack Atlas, by Jerry Jenkins, documents a 3.5 degree Fahrenheit winter warm-up in the Adirondacks between 1895 and 1999); there is less snow and more rain; spring temperatures are warming (black fly populations appear to be declining as a result); several bird species’ breeding ranges have expanded north; and with more open running water in the winter, species such as Bald Eagles and Belted Kingfishers are now often observed.

In the early part of the past decade, biologists at the Vermont Institute of Natural Science (now the spun-off Vermont Center for Ecostudies) wondered what would happen to the high elevation spruce-fir forests of the Northeastern United States as the climate continues to warm. In their Mountain Birdwatch 2003: Final Report to the U.S. Fish and Wildlife Service, J. Daniel Lambert and Kent P. McFarland included an alarming addendum titled, Projecting Effects of Climate Change on Bicknell’s Thrush Habitat in the Northeastern United States. Their findings will be published this spring in the journal Mitigation and Adaptation Strategies for Global Change, in an article titled, The Potential Effects of Climate Change on Birds of the Northeast. The journal article is the result of collaboration among many scientists to assess the potential effects of climate change on as many as 150 bird species.

The high elevation “islands” of spruce-fir habitat in the northeastern U.S. exist only at certain latitudes, elevations, and temperatures. Since mountains do not move or change their height, temperature is the critical variable and factor in determining the montane spruce-fir zone. These habitat islands provide breeding areas for the range-restricted Bicknell’s Thrush and other high elevation bird species.

Using the lowest elevation detections of Bicknell’s Thrush, Lambert and McFarland came up with a GIS model for the species habitat in the northeastern U.S. after finding that the threshold of suitable habitat drops 83 meters for every degree increase in latitude. For instance, to find potential Bicknell’s Thrush habitat, you would need to be at a higher elevation in the lower-latitude Catskill Mountains than in the higher-latitude areas of Maine.

To project what might happen to Bicknell’s Thrush habitat with warming temperatures, Lambert and McFarland used two GIS modeling approaches that are both built on known relationships between temperature, elevation, and forest type. Using the “Temperature Lapse Rate” (TLR) we know that the temperature changes 1 degree Celsius for every 154 meter change in elevation. The “Mean July Temperature” model shows that boundaries between Alpine, spruce-fir, mixed woods, and hardwoods correspond well with mean July isotherms. Lambert and McFarland hypothesized that every 1 degree Celsius increase in mean July temperature will elevate the lower distributional limit of Bicknell’s Thrush by 154 meters (or about 500 feet). They projected climate change effects on Bicknell’s Thrush habitat in 1 degree increments.

Graph showing loss of Bicknell's Thrush habitat

Warming of only 1 degree Celsius will result in a loss of over half the potential habitat for Bicknell’s Thrush. A 2 degree warm up will be enough to eliminate the breeding habitat in the Catskills and most of Vermont. With a 3 degree increase, almost all of the breeding habitat for Bicknell’s Thrush will be gone from the northeast. Only remnant patches will exist in the Presidential Range of New Hampshire and on Mount Katahdin in Maine with a warm up of 5 degrees. Predicted temperature increases by the end of this century range from 2.8 to 5.9 degrees Celsius. Although the first 1 degree Celsius increase eliminates more than half the Bicknell’s Thrush habitat, the effect on population will be muted since these most vulnerable areas are also the most sparsely inhabited by the species. At left is a disturbing graph showing habitat loss by state as the temperature warms up by 4 degrees Celsius.

Has warming already had an effect? Extirpations of Bicknell’s Thrush at lower elevations and along the southern edge of the breeding range have already been documented. Bicknell’s Thrush is also extirpated from Mount Greylock - the only peak in Massachusetts where the species was known to historically breed.

Additional climate related factors will impact Bicknell’s Thrush and other high elevation species. The frequency and severity of natural disturbances could increase; availability of invertebrate prey may play a role; breeding and migration phenology may be influenced; competition from species like Swainson’s Thrush (NYSOA Conservation Column...Continued from page 5) (which may be physiologically restricted from cool summits) may have an impact; and possible infestation of northern balsam fir from the balsam woolly adelgid, which has devastated highelevation stands in the southern Appalachians, could play a role. There may also be climate change “wild cards,” currently unknown, that may have an effect on high elevation spruce-fir habitat.

While the montane spruce-fir zone in the northeastern U.S. appears destined to disappear by the end of this century, there are still many unknowns. It is unclear how the forest communities will respond, and how long it will take for the spruce-fir zone to retreat up the mountains from the encroaching zone of mixed woods and hardwoods. There is a also a question of what will happen to the Bicknell’s Thrush population when there are remnant patches of suitable habitat left on only the highest peaks in the northeastern U.S.

Lambert and McFarland concluded their 2003 climate change projection research with the following suggestion: “Given the vulnerability of current Bicknell’s Thrush habitat, we propose that experimental manipulations of high-elevation forests are warranted in order to identify management options for counteracting or delaying effects of climate change.” “Delaying effects” is an appropriate phrase given the dire predictions for the next century as a result of human-induced climate change. After all, there is no more “up” when the sky island habitat is gone.

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