Time may not be the only thing falling back in the autumn: That signature fall change in leaf colors may be drifting further down the calendar.
Scientists don’t quite know if global warming is changing the signs of fall as it already has with an earlier-arriving spring. Studies in Europe and Japan indicate leaves are changing color and dropping later, so it stands to reason that it’s happening here as well, said Richard Primack, professor of biology at Boston University. And while this year's tourist numbers are normal, in the future the delayed or absence of vivid fall leaf colors could mean fewer dollars from leaf-peeping tourists in New England.
Phenology, the study of periodic plant and animal life cycle events and how these are influenced by seasonal and inter-annual variations in climate, is a tricky business when it comes to fall foliage. The budding of plants each spring is tied almost exclusively to warming temperatures, while fall’s changing colors are linked to cooling temperatures, decreasing sunlight and soil moisture.
The brilliant colors associated with fall happen when production of chlorophyll, the green pigment in plants that’s crucial to photosynthesis, slows down as the days get shorter and nights grow longer. That exposes the leaves’ yellow, red and orange pigments that are normally hidden.
How and when that happens depends on temperatures and moisture levels. In some years, the colors are more vibrant than others. Further complicating matters: A tree that’s stressed may simply drop its leaves with no color change, or turn brown.
"Fall is still an enigma," said Jake Weltzin, executive director of the National Phenology Network in Arizona and an ecologist with the U.S. Geological Survey.
Heavy rain, drought-like conditions or temperature extremes can cause dramatic year-to-year fluctuations that don’t establish a long-term trend. For example, heavy rainfall in New England this spring, followed by a deluge caused by Irene, is causing fungal growth that’s causing some trees’ leaves to turn brown and drop earlier than normal.
William Ostrofsky, forest pathologist with the Maine Forest Service, is skeptical about whether there’s a proven link between fall foliage and climate change.
“I just don’t know that there’s any evidence to indicate there’s a trend one way or the other,” said Ostrofsky, who points out that year-to-year fluctuations make it difficult to discern long-term trends.
But some data point to a later leaf drop:
• Researchers at the NASA Goddard Space Flight Center and at Seoul National University in South Korea used satellites to show the end of the growing season was delayed by 6 1/2 days from 1982 to 2008 in the Northern Hemisphere. (At right: fall foliage peaks across the US)
• In Massachusetts, the leaves are changing about three days later than they were two decades ago at the Harvard Forest 65 miles west of Boston, according to data collected by John O’Keefe, a retired Harvard professor and museum coordinator.
• In New Hampshire, data collected at the federal Hubbard Brook Experimental Forest in Woodstock suggest sugar maples are going dormant two to five days later than they were two decades ago.
• In Vermont, state foresters studying sugar maples at the Proctor Maple Research Center in Underhill found that the growing season ended later than the statistical average in seven out of the last 10 years.
And there are regular folks like 83-year-old Nancy Aldrich at Polly’s Pancake Parlor in Sugar Hill, New Hampshire, who has been keeping her own records since 1975. Her numbers show that color change is a moving target.
“I’m know I’m vague about it, but so is nature,” she said.
Providence Journal,"Autumn falling later ", by David Sharp, accessed October 16, 2011