Climate is shaping a leaf peeper's paradise

bright hues

A century ago, the flaming fall foliage in Nova Scotia, Canada would have long faded by early November. But today, some of the hills are still as nubbly with colour 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 colours 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 colours 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, Canada. The Canadian Shield is a broad ring of forests and ancient bedrock that extends hundreds of miles from the shores of Hudson Bay, Canada.

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 colours 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 colours in New England.

We only have to read Henry David Thoreau to know that climate change is pushing the changing colours later into the year. He spent a lot of time tramping around his native Concord, Massachusetts, USA 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.”

Thoreau & climate change

Anyone sensitive to the onset of autumn in New England, USA these days knows that most trees, including the maples, are still bottle green on those dates. “In general, peak leaf colour 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, USA. 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 colour in Harvard Forest in central Massachusetts, USA 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.

The study further analysed 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 colour 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 coloured leaves for each species and the duration from when 10% of a tree’s leaves turned colour to when 90% 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 colours 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,” Andrew said. “In the shorter term, autumn colours may get better before they get worse.”

Worse? Scientists say that in the long  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. More southern — and less colourful — 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 colours.

Changing colours

The scientific term for the colour change is leaf senescence, when deciduous trees pack up their summer clothes and prepare to sleep naked through the long 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.

Anthocyanin’s 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. Leaves that contain roughly equal amounts of yellow carotenoids and red anthocyanin appear bright orange in the fall. The higher the proportion of anthocyanin, the brighter red the tree will be.

Sunlight, particularly in late summer and fall, sets off the production of anthocyanin. Cloudy weather dampens production and leads to less colourful displays. Many scientists argue that warming temperatures do not have much to do with the intensity of colour, only with its timing: when it appears and how long it lasts. But Howard Neufeld, a professor of biology at Appalachian State University in North Carolina, USA said climate change could eventually affect the complex processes in leaf senescence and lower anthocyanin production, dulling the autumn reds.

To produce anthocyanin, leaves need energy, which they get from the sun and from sugar in the leaves — sugar that otherwise would be metabolised by the tree. Warmer temperatures, Howard said, may speed up enzymes involved in night time respiration, when leaves exhale carbon dioxide and absorb oxygen, as humans do. Respiration requires sugar for fuel, and burning it faster would leave less available for anthocyanin production. The result: drearier leaves.
So far, however, warmer temperatures do not seem to have had this effect. John  noted that despite this year’s record warmth and one of the latest onsets of fall colour that he had seen in 27 years, in his neck of the woods it had been “a brilliant red year.”