Nature News: Here’s to hoping for vibrant fall colors

This year’s autumnal equinox arrives on Tuesday, Sept. 22, 2020, at 9:31 A.M. The equinox occurs at the same moment worldwide, the big difference being that if you are in the northern hemisphere, from this point onward, the nights are going to be longer than the days and will keep getting longer until the winter solstice in December. If you are in the southern hemisphere, this is the spring equinox when the reverse happens: days are going to get longer, while nights get shorter.

One of the most well-known harbingers of autumn in New England is our fall foliage. My son moved back to New England from California this summer and was so excited to see just one early, brilliant red or mottled orange maple leaf lying tattered on the ground that I was reminded of how lucky we are to live among all this color. I can’t wait to see how excited he is going to be when the foliage peaks.

Contrary to popular belief, it isn’t the cold or an early frost that starts off the biochemical cascade that results in fall color. Instead, it is a growth process triggered by day length. As days get shorter and nights get longer (nights have been growing longer since the summer solstice), in particular when nights reach a certain threshold length, the cells along the border between the leaf and stem form a corky layer (the abscission layer) that begins to block the flow of things like water and sugars between the leaf and the rest of the tree. This is in preparation for leaf drop. After the abscission layer forms, bright, sunny days and low temperatures at night will cause the chlorophyll to be rapidly destroyed and the fall colors will start to emerge.

Where do the fall colors come from? The yellows and oranges were already there. The reds, however, are usually produced later in the season. The primary pigment that gives a leaf its green color is, of course, chlorophyll — the green photosynthesizing pigment. It gets help with photosynthesis from other pigments — carotene (the pigment that also makes carrots and egg yolks orange) and xanthophyll, a yellow pigment. Those pigments are always present in the leaf. As fall approaches and leaves start to die, the green chlorophyll is broken down first and absorbed by the tree, uncovering the yellow and orange pigments — they take longer to break down.

Brilliant red and purple hues are produced by anthocyanins. Anthocyanins are typically not present all season (except in trees with red/purple leaves, like Japanese maples). The current theory for their presence is that they protect the leaf from intense sunlight-prolonging the photosynthetic season for a leaf by protecting the leaves from photodamage. A dry autumn with lots of sunlight triggers a tree to produce more anthocyanins. Sunny days means that more photosynthesis is happening in the leaf, producing more glucose. Cool, crisp nights and the formation of that abscission layer trap the glucose in the leaf. Anthocyanins are the product of a chemical reaction involving glucose, so, the more glucose, the more anthocyanins can be produced, leading to more brilliant reds.

So, for peak color you want a growing season with plenty of rain followed by a cool, dry, sunny autumn. According to the U.S. Forest Service, a succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. What you don’t want are windy, heavy rains and/or hard frosts — both can cause leaves to drop too soon.

The big question now is how our drought conditions are going to affect fall color. We did have a fairly good start to the growing season, but then came the severe summer drought. Dry conditions concentrate sugars in the leaves which can lead to more intense color. However if the conditions are too dry, the leaves might just shrivel up and fall off (sadly, this happened to a lot of the shrubbery I planted last year).

I’m hesitant to try to predict fall foliage; however, it seems to me that, if we continue to have these crisp nights and sunny days, we could get some intense reds and, hopefully, some spirit-lifting fall color.