Forest scientists who study global warming in the northeast say that a warmer climate could lead to later and lackluster leaf peeping. Three different things could cause this to happen, acting alone or in combination.
Records kept by the National Oceanographic and Atmospheric Administration (NOAA) reveal that the average annual temperature in the region that includes New England and New York has increased by 0.7°F during the past 100 years. Computer models project that the average annual temperature will rise much faster, from between 6°F to 10°F over the next hundred years.
Barry Rock, a professor of natural resources at the University of New Hampshire (UNH), predicts that, based on two climate models in a regional climate assessment study, “Within the next 100 years, Boston could have a climate similar to either Richmond, Virginia, or Atlanta, Georgia.” At this rate, says Rock, “In 100 years, New England’s cooler regions will no longer promote the growth of sugar maples, which are well adapted to the region’s current climate.”
That’s the first way that global warming could affect leaf peeping – the loss altogether of the sugar maple, whose wide range of yellow, orange, and red leaves makes New England’s foliage so impressive.
David Kittredge, a professor of natural resource conservation at the University of Massachusetts and forest policy analyst at the Harvard Forest, sees several scenarios. “If we get a climate more like that of Pennsylvania, Maryland, or West Virginia, we could still have a sugar maple component in our forests.” Of five computer models created by the U.S. Forest Service to predict the geographic shift in the ranges of forest species, only one foretells that global warming will cause sugar maples to disappear from parts of New England. (To view the range maps yourself, visit: www.fs.fed.us/ne/delaware/atlas/index.html.)
Global warming may also directly affect the hues of autumn leaves. In part, the colors of fall foliage result from the breakdown of chlorophyll, the green pigments in leaves that normally mask the background yellow, orange and brown pigments. Reds and purples, however, are actually created in the leaves each autumn.
Abby van den Berg, a research technician at the University of Vermont’s Proctor Maple Research Center, says that fall leaf colors are initiated by shortening day length and decreasing temperature. “If you change the timing of the onset of cool temperatures, you alter when chlorophyll breakdown starts. Even though we have no good way to predict how climate change will affect the process that creates the colors of foliage season, it will change how the landscape will look over time.”
The first hard frosts, which bring out the most vibrant leaf colors, used to occur around the third week in September. But in recent years, these frosts have arrived later. In 2004, the first hard frost in many parts of the North Country didn’t come until mid-October. Although Columbus Day has traditionally marked the height of foliage season in northern New England, the timing of peak fall leaf color is shifting toward mid- to late October.
Predicting the timing and intensity of fall foliage colors seems to be as much art as it is science. When trees are stressed, perhaps due to a summer drought or even too much rain, they tend to develop particularly bright red fall leaf colors, regardless of a changing climate. Dr. Tim Perkins, director of the Proctor Maple Research Center, says, “It is premature to ascribe recent muted foliage to global warming. Here in Underhill Center, Vermont, we had a spectacular and long foliage season in 2004.”
Barry Rock of UNH predicts that foliage season will gradually come later and the intensity of colors will decrease. Using Earth-orbiting satellite data collected by NOAA and NASA from the 1970s to the present, Rock is analyzing the autumn dates of the onset of red hues in the northern forests. This research will paint a picture of how, and whether, the timing and intensity of the foliage season is shifting.
The third possible link between global warming and a reduced foliage display comes from the ongoing threat to the region’s sugar maples from something that forest scientists are calling “maple decline.” Says Rock: “Hot summer months are likely to be poor air-quality months characterized by acid rain events. Ground-level ozone is another pollutant that may predispose trees to being weakened or killed. Sugar maple, red spruce, and white pine are all struggling. Acid rain and global climate change are two faces of the same beast.”
If global warming does lead to a decline in the beauty of our autumn foliage, the negative economic impact on New England would be substantial. But more than that, it would be a tragedy. Fall foliage is woven into the fabric of New England’s identity and it is an iconic event by which we celebrate the seasons.