Climate Change and Fall Foliage: Not a Good Match

By Michael D. Lemonick

It’s admittedly not on a par with the direct threats posed by rising seas or melting icecaps or extreme weather, but with autumn now upon us, it’s worth noting that climate change could also affect the brilliant foliage that paints forests from the Ozarks to the Appalachians with vibrant color every fall.

Fall foliage in New England. Click on the image for a larger version.
Credit:  BrtinBoston/flickr.

The damage isn’t just esthetic, either: national statistics are hard to come by, but officials in New Hampshire estimate that leaf-peeping tourists pump up the state economy by about $1 billion each year. The estimate is about the same for North Carolina, and if you project those revenues onto New York, Massachusetts, Vermont, Maine and other prime fall-foliage destinations, you’re talking about real money.

This year, at least in some places, the money may not be flowing in. “I hope I’m wrong,” said Karl Niklas, a professor of plant biology at Cornell, in an interview, “but I just think it’s not going to be a great year in central New York.”

The primary reason, he said, is the double-whammy of drought and heat that afflicted the Empire State along with much of the nation this past summer — both of which are tied, at least in a statistical way, to global warming.

Drought puts enormous stress on trees, and while it’s much harder to kill a tree than it is, say, a corn or soybean field, arid conditions will make leaves turn brown and drop to the ground before they can flare into yellow or red for the tourists.

That’s bad enough, but, Niklas said, “high temperatures make things even worse,” and the combined stress of dryness and heat make leaves more prone to bacterial and fungal infection. So it’s more of a triple-whammy. “One response of a tree that’s infected is to drop more leaves,” he said. “And if leaves are dropping now, there are fewer left on the canopy to display color.”

Precipitation departures from normal for the month of August, showing the widespread below average rainfall amounts (yellow and orange). Click on the image for a larger version.
Credit: NOAA.

In western Pennsylvania, meanwhile, forest ecologist Marc Abrams is worried about another climate-related foliage threat. “The drought affected us in July, but we had enough rain in August to largely make up for the deficit,” he said. The problem here is that the normal drop in nighttime temperatures that happens at this time of year isn’t kicking in. “We’re only dropping into the mid-40’s at night,” he said, “ when we’d normally be in the mid-30’s. We really want a light frost by the first week in October, which really triggers the onset of peak coloration.”

Thanks to a strong cold front that passed through the region on Thursday, such a frost may soon occur, but it has been unusually mild up until that point.

Unusually warm overnight temperatures are a hallmark of climate change, and even though you’d get occasional spells of overnight warmth without climate change, they’re likely to increase as the planet heats up. Thousands of warm overnight temperature records were set across the lower 48 states this summer, from the South to the Canadian border.

Both scientists agree that the leaf-color deficit this year will be most severe in the areas hardest hit by drought and heat. New York State and Pennsylvania, for all their climate troubles, aren’t very high on that list. “There’s going to be a very big effect in the Midwest, the Plains and the Great Lakes states,” said Abrams, all of which recorded punishing heat and parching drought through most of the summer and into the first half of September.

All of these factors could improve in some parts of the nation by next fall, and one major influence on the turning of autumn leaves — the shortening of days that signals trees it’s time to go dormant — won’t be affected at all by climate change.

But since heat, drought and an increase in overnight temperatures are all expected to become more prevalent in a warming world — to say nothing of the fact that some species known for their fall brilliance could begin to die off wholesale in some of their traditional range — means that, the brilliant foliage of autumn could be put at risk by the heat-trapping greenhouse gases we’re pumping into the atmosphere.

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