Taking the Awe Out of Autumn

Global warming may ravage the sugar maples that set New England woods ablaze with color each fall and that provide the basic ingredient for America's maple syrup industry

• Jennifer Weeks
• Oct 01, 2007

WHEN DAYS SHORTEN and nights grow cooler in late summer and fall, trees such as maple, oak and beech cloak themselves in the bright shades of autumn. No colors are more central to this palette than the yellows, oranges and reds of sugar maples, which range from eastern Canada through the mid-Atlantic states, west to Minnesota and south along the Appalachians into Tennessee. Also prized for timber and maple syrup, sugar maples highlight the foliage displays that draw millions of people outdoors for nature's autumn fling.

But fall is gradually becoming less colorful in the Northeast, thanks to rising temperatures, and the next 100 years of global warming could wipe out much of the region's fall color and the tourist trade based on it, as well as its sugar maple industry. Researchers predict that global warming could cause sugar maples and other eastern forest species to abandon New England and move north as their ideal temperature ranges shift.

"We can expect truly dramatic climate change over the coming century, depending on the emissions pathway we choose to follow," says Erika Spanger-Siegfried, Northeast Climate Project manager at the Union of Concerned Scientists (UCS). "If emissions stay high, the character of our forests may change dramatically." In 2006, a team of climate experts convened by UCS projected that annual temperatures across nine Northeast states will increase 3.5 to 12.5 degrees F by 2100, depending on whether nations reduce greenhouse gas emissions or stay on their current course. In the low-emission case, a summer day in New Hampshire in 2100 would feel like northern Virginia today. Under the hotter scenario, summers in New Hampshire would feel like summers in South Carolina. Under either scenario, New England's fall fabric could fade permanently.

With warming will come other changes. Researchers expect annual precipitation to increase, but they also predict that summer droughts will worsen. These changes bode poorly for sugar maples, which grow in cool, moist climates where soils are neither too dry nor too wet. An online "Climate Change Tree Atlas" developed by the U.S. Forest Service (www.nrs.fs.fed.us/atlas) shows that if emissions remain high, areas outside of New York, New England and Michigan's Upper Peninsula could lose virtually all of their suitable habitat for sugar maples. Conditions also would worsen in the remaining zones, so sugar maples will be much sparser there than they are today.

Climate change will shift sugar maples' optimal habitat north, but it's not automatic that maples (or beeches and birches, which grow with them) will follow. "We don't know what the change in suitable habitat will actually do to forests," Spanger-Siegfried says. "Maple/ beech/birch forests aren't likely to migrate northward as a distinct forest type. They're more likely to disassemble, with different impacts on different trees, and we don't know how they will reassemble."

Many animal species rely on sugar maples. Deer, moose and snowshoe hares browse on seedlings, porcupines gnaw their bark, and mice and squirrels eat maple seeds. Mature trees provide excellent cover and nesting sites for woodpeckers, grosbeaks and other songbirds. As sugar maples decline, these species will have to feed and shelter in other trees, such as oaks, hickories and shortleaf pines that move north, presuming the trees move rapidly enough and can provide food and shelter for species adapted to maples.

Global warming also is undermining the region's $40-million maple syrup industry by affecting springtime production of maple sap, the raw material for syrup. Half a century ago, New England and New York produced about 80 percent of the world's maple syrup and Canada about 20 percent. Today, the figures are reversed. Sugar maples need several weeks of very cold weather followed by a period of warm days and cold nights to trigger sap flow. Traditionally, producers in northern New England tapped their trees in early March, but that calendar is changing. According to researchers at the University of Vermont's Proctor Maple Research Center, sap production starts about one week earlier in the calendar year today than it did in the early 1960s and ends about 10 days earlier--a net loss of 3 days from a 30-day season.

Technological improvements, such as the replacement of passive collecting buckets by vacuum systems that pump sap out of trees, have probably offset that loss so far, but continued warming will further curtail the season and could drive syrup producers out of business. "You don't have to get to the point where production drops to zero to lose the industry, because people will only keep doing it as long as it's economically viable," says Timothy Perkins, director of the Proctor Center.

Although they are relatively hardy, sugar maples are vulnerable to other stresses, including acid rain, soil compaction from nearby development and such pests as gypsy moths and forest tent caterpillars. Trees subjected to multiple pressures can go into "maple decline," a syndrome that alters the trees' internal chemistry, making them more vulnerable to potentially fatal stresses. Symptoms include reduced growth, dead branches in upper canopies and fall color that develops early, in July or August. "Climate change will increase stress on trees that are already outside of their climate optimum," says John O'Keefe, coordinator of the Fisher Museum at Harvard Forest in Petersham, Massachusetts.

The news isn't all bad for foliage lovers. Longer growing seasons could favor sugar maples in the northern half of their range, from upper Vermont into Quebec. Moreover, forests constantly evolve through a process called "succession," in which distinct plant communities replace each other over time, starting with shrubs that fill open spaces and moving over decades toward larger, slower-growing species that can sprout and grow under shady canopies. Because sugar maples are slow growing and shade tolerant, they are characteristic of mature forests. In many Midwest forests logged during the past century, maple/ beech/birch forest cover is taking over from species that came earlier in the succession process. However, global warming could interrupt this natural sequence.

What will fall look like in 2100, if maple/beech/birch forest coverage declines? With fewer accents from sugar maples, the overall mix will be duller. And global warming could mute fall displays in other ways. Rising temperatures will delay the onset of cool nights, which help produce bright leaf colors, and heavy rains may wash leaves down prematurely.

States that generate revenue from leaf peeping take these prospects seriously. New Hampshire reaps about $1 billion yearly from fall tourism, with nearly $100 million coming in over Columbus Day weekend, often a peak time for fall colors. Other natural resource impacts of climate change, such as shorter snow seasons and increased beach erosion from rising seas, also threaten the Northeast's natural beauty and the tourism revenue that it draws. In response, northeastern states have moved to cap greenhouse gas emissions from power plants and are scaling up their use of renewable fuels that do not generate greenhouse gases.

Major cuts in greenhouse gas emissions could avert serious impacts for the hardwood trees that produce autumn foliage, so brown falls are not inevitable yet. But without rapid action to stem global warming, autumn, in the words of 19th century poet William Cullen Bryant, could fade from "the year's last, loveliest smile" to a more sullen, solemn season.

Jennifer Weeks is an environmental writer based in Watertown, Massachusetts.