Though this summer was one of the driest on record for southern New Hampshire, it’s been a top-notch foliage season — the weather in the summer actually has little to do with the foliage in the fall — but we’re not out of the woods yet.
Long-term climate change and volatile weather variations may hurt our foliage seasons in the future.
“[Good foliage] has much more to do with the weather during the foliage season. If we have cool nights and we have warm sunny days, that tends to make the colors show,” said Dave Anderson, a naturalist with the Society for the Protection of New Hampshire Forests.
He said while some individual trees living in particularly dry soils and urban areas with limited hydration might have suffered to the point of dropping their leaves early, they are generally the exception to the rule this year.
In addition to that, the tree species that inhabit different parts of the state play a significant role. The hardwood trees with the best foliage, such as the beech, red maple and yellow birch — what Anderson calls the “holy trinity” — are predominant in the radial swath that extends from the Monadnock region up to the Upper Valley, through the Lakes Region and White Mountains.
“The part of the state that is well-known for its foliage and famous for its foliage wasn’t nearly as affected by the drought as coastal, southeastern and southern New Hampshire,” Anderson said.
North of that is populated mostly by spruce and fir and the southern tier most affected by the drought is largely populated by oak and pine, which are better adapted to sandier, drier soil.
“The area that got hit hardest by the drought is probably the most drought-adapted, tough, tenacious forest in New Hampshire for that condition,” Anderson said.
The dryness doesn’t affect the color of the leaves as much as whether they are still hanging come autumn.
“A tree that’s distressed is going to change early, it’s going to shut down early. The trees that … are under stress because of the site that they’re on are the last to leaf out in the spring and the first to lose leaves in the fall,” Anderson said.
In fact, having too much rainfall can have a greater effect on leaf color as a surplus of moisture can invite molds and fungi that can cause the leaves to be duller or browner.
Karen Bennett, a professor and forest resource specialist at the UNH Cooperative Extension, said heavy rainfall during autumn can also cause the leaves to fall off faster. So, in a sense, the continuing lack of rain might in some way extend peak foliage season later than usual.
This year, Bennett said the foliage season started late because the cold nights came later than usual and is lasting longer. Peak foliage in southern New Hampshire is typically around Columbus Day weekend, but it’s closer to late October this year.
But so long as fall continues to provide longer, colder nights and shorter warm, sunny days, the chemical process that changes leaf color will remain unchanged.
That’s because, for the leaves that turn yellow and orange, those pigments are already in the leaf before the green pigment, the chlorophyll, breaks down. This change is triggered more by the reduction in daylight than temperature. Once the chlorophyll is gone, the xanthophyll (for yellow) and carotenoids (for orange) remain.
The process differs for the leaves that turn red. The pigment that makes leaves red or purple, known as anthocyanins, are produced when trees like red maple have stores of sugar trapped in the leaves and sunlight reacts with the sugar to produce the pigment. While daylight patterns start off this change, colder temperatures trap the sugars.
So how might climate change have an effect in the long term?
“As weather changes over time, we might see changes with our foliage,” Bennett said.
She said increasingly warmer climates may mean fewer reds or less bright oranges.
And, over the course of centuries, the northern species that provide the best displays may push further northward.
“When these trees die and are regenerated, they may be replaced by trees that have less brilliant foliage,” Bennett said. “So if those species are replaced by those that are more southern … you’re going to have a different suite of colors out there.”
In the short term, one year of extreme drought is not likely to trigger such a massive change. But Anderson said if the weather becomes more extreme and variable, that could cause other problems.
“These wild fluctuations in weather … stress trees out,” Anderson said.
In New Hampshire, foliage season attracts about 9.5 million tourists and $1.42 billion in tourist spending each year.