Fall Foliage: An Educated Leaf Peeper’s Primer

October 6, 2021 · 4 minute read
Fall Foliage: An Educated Leaf Peeper’s Primer

It’s almost time for that favorite fall pastime: leaf-peeping. Soon there will be countless photos of glowing, multicolored trees posted all over social media. But most people don’t really know why trees do this or what color change can indicate if it shows up a little too early. This won’t be a class on biology but will hopefully give you a new insight into what those colors mean and if it is actually a sign of hidden tree issues. With a little knowledge and a little bit of looking up, it may even be possible to determine dying branches, declining trees, or maybe just a tree that’s a bit thirsty.

Most of us learned in school that trees change color in the fall as a way to conserve resources, as pigments such as Chlorophyll, Carotenoid, and Anthocyanins are broken down into simpler chemicals and pulled back into the tree before leaves fall to the ground. These chemicals are recycled in the coming year as the tree puts leaves on in the spring. Changing temperatures and reduced light levels trigger the trees to begin the process of reabsorbing the nutrients. The first pigment to be broken down in the leaf is the green Chlorophyll, leaving behind the less prevalent orange-colored Carotenoid and the reddish to almost blue Anthocyanins. The concentration of these pigments in the leaves of different trees will dictate what color you see.  

As fall approaches many people love to discuss whether or not it will be a year with good fall color which can be heavily dictated by the weather in the months preceding. A dry and stressful year for the trees, for instance, usually creates a lackluster fall display. This is because the trees either move through their color change much more quickly or simply drop their leaves without much of a color change. This is a survival adaptation to simply go dormant before the tree has to suffer more stress. A lack of good fall color can also be caused by the inverse: too much rain in the late fall, which floods the leaf tissue with water and essentially dilutes the colors. These stress-induced changes can help us diagnose issues in individual trees.

I have a city sugar maple planted in front of my house, and as with many open-grown maples, it has grown with very poor architecture, with a limb shooting out and trying to become the main leader of the tree. This has left a stunted and somewhat lackluster central stem. Not only is this stem stunted, but it is always the first portion of the tree to change color in the fall while the rest of the tree maintains a deep green background.  While this could be seen as a desirable outcome with extended fall color arising from the issue, it actually acts as a signal that something is wrong in the tree.  Quite often trees will have one or two low branches or sections that prematurely develop color. It’s likely that those limbs or areas will be unhealthy or even dead in the following growing season. The tree senses a lack of vigor in those areas and takes action to get every last bit of energy and nutrients out of it before evening considering shutting down the rest of the tree.  

A similar situation occurs with newly planted trees, which can easily be seen in planted rows of the same species such as those in commercial or community plantings. This time of year the stressed or poorly planted trees will begin to show color much sooner than other adjacent trees as they are hurrying to go dormant before they have to endure further stress. When this occurs, it is almost always a root issue and by identifying these trees, it’s possible to give them the additional support and care they need in the following growing season.  A little work on the root zone to improve soil conditions can quickly turn around a tree that is struggling as long as it is caught before the tree irreversibly declines.

Some trees also use early color and leaf drop as a survival strategy to get through the rest of the growing season with color and leaf fall sporadically occurring as early as July or August. This occurs frequently in river birches and tulip poplars as they are both fast-growing and water-loving trees. In the spring these species put on large amounts of growth when water is plentiful and as the rain reduces in the summer, the trees need to cut back on their water loss from foliage. Leaves scattered throughout the canopy begin to turn yellow and fall, causing a concerning site for the tree owner. This reduces the overall leaf surface area on the tree and thus reduces the loss of water from the tree, allowing it to maintain proper moisture levels even when rain is scarce.

One dissimilar situation is the tree species that simply drop their leaves earlier than most others.  Black Walnuts are a peculiar tree as they are nearly always the first tree to drop their leaves and the last to put new ones on in the following year. This has nothing to do with stress. It’s simply the natural characteristic of the species. Ash trees also drop earlier than many other species but can put on quite the show when they do, creating an almost glowing canopy of many different colors.  

While not all trees provide showy fall colors, we’re lucky in the region to have many that do. But don’t dismiss the colors as just an aesthetic benefit. Pay attention as the show may actually be the tree signaling an issue that needs attention. Here’s to what will hopefully be a fall filled with lots of happy and dazzling trees!