IIT Fall Newsletter: Fall Foliage - How Do Trees Decide

By Chloe Joule - From October 2024 IINews Trust Topic

On the island, we are reaching that season of full Autumnal glory. The canopy is a riot of scarlet, ocher and umber, which fall to blanket the dulling greens of the forest floor, so that everywhere you look the land is ablaze with a scene which passes almost as soon as it arrives. It is a time of year I find myself reveling in annually, both for the beauty it offers and the-often-overlooked, biological wonder of it all.

Why do some trees drop their leaves? What determines the color? Where do those colors come from? Why do some tree species seem to change color and lose their leaves right away and others retain their leaves long into the Fall and even Winter?

Like so much in life, the answer is a combination of genetics and environmental influences. Where we live, in the Northeast, our tree species have adapted to the dramatic changes our seasons bring. Deciduous trees, referring to all trees that annually drop their leaves and go into a state of dormancy for the Winter, have adapted to do so mostly because of the lack of sunlight in our Winter months. It is the leaf’s job to absorb sunlight and turn that into energy in the form of sugar through the process of photosynthesis. But, it takes a fair amount of energy to sustain these leaves - so when the amount of energy needed to sustain the leaves outpaces the amount of energy they will be able to provide with shorter daylight hours, the answer is to stop trying to photosynthesize and take a well-deserved rest for the winter. Before doing that, trees remove the pigment in leaves which gives them their green color and facilitates photosynthesis, Chlorophyll. By drawing chlorophyll out of the leaves and storing it in the root system, the tree stops the process of photosynthesis and stores energy from the pigment for the Winter. When this happens, we get to see the other pigments that are always in the leaves, but overpowered by the color of chlorophyll through the Summer season: Carotenoids (which give plants their yellow-orange colors) and Anthocyanins (which provide the variety of reds we see in plants).

Now comes into play the specific genetics of individual tree species. After chlorophyll, the next pigment we often see is Carotenoids which is always present in a leaf, though typically masked by chlorophyll. This is where we see our aspen, birches and certain maple species (striped and some sugar maple) stop with their pigment withdrawal. Their leaves take on their glorious golden hues and then seem to shed almost all at once.

Other tree species which retain their leaves a bit longer into the fall have the opportunity to produce higher amounts of sugar with access to more sunlight from the clearing canopy. This spurs the production of Anthocyanins (red pigments) - giving us the bright reds and crimsons we see in red maple, some sugar maple and red oak trees. The abundance of sugar that gets produced during the warmer days, then gets trapped in the leaves when the temperatures drop at night and the vascular system of the leaf slowly closes off, creating a build up of red color.

Then there are the trees that hang on to their leaves until all pigment and sugar resources have been drained and stored and the leaves have fully turned brown. In the Northeast, this includes most oak and beech tree species.

So, how do trees decide when to drop their leaves? Mostly, it has to do with the particular species. Some trees - like our oaks and beeches - are willing to risk hanging on to leaves,  which could get weighed down with an early snow storm and do damage to the overall tree, in order to extract all the resources available in their leaves. Others, like aspen and birch, have adapted to let go of their leaves (and any resources left in them) in the beginning of Fall and leaf out earlier than other trees in the Spring.

Of course, there are always exceptions and this is where some of the environmental influences can impact fall foliage. For optimal Autumnal colors, consistently warm days, followed by cold (but not freezing) nights is a necessity. Maine has traditionally been able to rely on that pattern for its spectacular foliage. However, due to climate change, the consistency of this weather pattern is not a given and we often see trees that remain green further into the season and do not follow the sequence of color change we have come to expect. Similarly, disease, pests, drought or flood can impact groups or individual trees and prompt premature coloring or loss of leaves.

Whatever colors you find outside your window, I hope you are enjoying them with perhaps a deeper understanding of the wonder they inspire.