How a Changing Climate Is Shaping a Leaf Peeper’s Paradise
Fall colors were beginning to show in Cape Breton, Nova Scotia, on Oct. 12. Scientists say that the glorious colors are lasting longer because of climate change. Credit Ian Willms for The New York Times
IONA, Nova Scotia — A century ago, the flaming fall foliage in Nova Scotia would have long faded by early November. But today, some of the hills are still as nubbly with color as an aunt’s embroidered pillow.
Climate change is responsible, scientists say. As the seasonal change creeps later into the year, not only here but all across the northern United States and Canada, the glorious colors will last longer, they predict — a rare instance where global warming is giving us something to look forward to.
“If climate change makes eastern North America drier, then autumn colors will be spectacular, as they are on the Canadian Shield in dry summers, especially the red maples,” said Root Gorelick, a biology professor at Carleton University in Ottawa. The Canadian Shield is a broad ring of forests and ancient bedrock that extends hundreds of miles from the shores of Hudson Bay.
Over the very long term, the warming planet may have a negative effect on fall foliage, but even then any adverse impact is uncertain. It is not just an aesthetic question, but an economic one as well: The changing colors drive billions of dollars in “leaf peeping” tourism in Canada and the United States.
“From a peeper’s point of view, it’s good news,” said Marco Archetti, the lead author of a 2013 paper at Harvard on predicting climate change impacts on autumn colors in New England.
Gary Greene and his wife, Nancy, enjoying the scenery in Cape Breton last month.Credit Ian Willms for The New York Times
We only have to read Henry David Thoreau to know that climate change is pushing the changing colors later into the year. He spent a lot of time tramping around his native Concord, Mass., making notes on how plants changed with the seasons.
In his 1862 essay “Autumnal Tints,” the naturalist wrote: “By the twenty-fifth of September, the Red Maples generally are beginning to be ripe. Some large ones have been conspicuously changing for a week, and some single trees are now very brilliant.”
He goes on to say that sugar maples, whose change generally follows red maples in short order, “are most brilliant” about the second of October.
Anyone sensitive to the onset of autumn in New England these days knows that most trees, including the maples, are still bottle green on those dates.
“In general, peak leaf color in Concord and the surrounding Boston area for these maples is now more typically a week or two later” than what Thoreau observed, said Richard Primack, a biology professor at Boston University. He has been using Thoreau’s records and satellite images to track the effect of climate change on local plant cycles.
The Harvard study, which looked at the percentage and duration of autumn color in Harvard Forest in central Massachusetts from 1993 to 2010, predicted that with current climate change forecasts, the duration of the fall display would increase about one day for every 10 years. Look at it this way: Children born this year could have an extra week to enjoy the colors by the time they are 70.
The study further analyzed data for trees that turn red: red maple, sugar maple, black gum, white oak, red oak, black oak, black cherry and white ash. Only in white ash trees did the duration and full display of color decrease. In the others, the amount and duration of red leaves increased over the course of 18 years.
The Harvard study used data collected by John O’Keefe, the museum coordinator, now emeritus, at Harvard Forest, who made his observations by eye — estimating the percentage of colored leaves for each species and the duration from when 10 percent of a tree’s leaves turned color to when 90 percent had turned.
Those observations have been validated by Andrew Richardson, a professor of evolutionary biology at Harvard, who has since set up a network of 350 “phenocams,” cameras that quantify the duration and intensity of autumn colors in locations from Alaska to Hawaii, Arizona to Maine and up into Canada.
“John’s direct observations on the ground line up pretty well with the camera data,” Professor Richardson said. “In the shorter term, autumn colors may get better before they get worse.”
The changing colors drive billions of dollars in “leaf peeping” tourism in Canada and the United States.Credit Ian Willms for The New York Times
Worse? Scientists say that in the longer term the warming temperatures could threaten cold-weather hardwoods like the blazing maples, pushing their southern border north and narrowing the band in which they can survive between the temperate and circumpolar boreal forests.
More southern — and less colorful — species like oaks and hickories may march north, eventually replacing the maples and other exhibitionists. Some scientists also say that the mechanism that makes leaves red may not work as efficiently in much warmer weather, eventually dulling those colors.
The scientific term for the color change is leaf senescence, when deciduous trees pack up their summer clothes and prepare to sleep naked through the long frigid winter. The green chlorophyll in the leaves breaks down, disappearing to reveal the yellow carotenoid pigments underneath. Those pigments break down more slowly, until the leaves eventually turn brown.
The real magic comes from the trees, maples among them, that produce a compound called anthocyanin as the chlorophyll disappears.
Anthocyanin is the pigment that makes cranberries red and blueberries blue, among other things. Its role in autumn leaves is not well understood, but current theories suggest that some trees have evolved to produce it to protect their leaves from the damaging effects of intense sunlight while the chlorophyll breaks down — the red pigment absorbs wavelengths in the green region of the spectrum that would otherwise be reflected by the disappearing chlorophyll.