Written by: Naturalist at The Inn at Honey Run, Rome Marinelli
Leaves change color in autumn due to leaf pigments, weather, and length of night.
Plants possess pigment molecules which are responsible for its colorful blooms to the colorful fall foliage. These pigments are appreciated by the human eye but are primarily used to attract specific animals which, in turn, benefits the plant (pollinates flowers and/or disperses seed).
The primary pigments in the leaves of trees consist of chlorophyll, carotenoids, and anthocyanin. Briefly, carotenoids absorb blue wavelengths which allow colors like yellow, orange, and even reds to be produced. Chlorophyll is the pigment that allows for photosynthesis (conversion of sunlight into sugars/energy) and gives us that green plant color. Finally, anthocyanins, absorb blue-green wavelengths and allow pinks and reds to be showcased in both spring blooms and all red tree leaves in the fall.
As nights grow longer and days shorten, plants begin the biochemical process that showcases brilliant fall colors. Plants detect the declining availability and intensity of sunlight which signals the plant to overhaul the resorption of nutrients by breaking down chlorophyll. Veins carry these fluids out of the leaf. but in time, the veins shrivel and clog, trapping sugars within the leaf structure. In the case of brilliant fall color, we see warm, moist days followed by cool, but not freezing, nights. This allows the plant to produce sugars, but the nearly- or already-closed-off veins prevent the nutrients from being resorbed. This results in surge in anthocyanin production and produces intense fall foliage.
Leaves that turn orange, consist of anthocyanin and carotenoids. Carotenoids have been present in the leaf structure, but are only revealed once chlorophyll breaks down. When carotenoids are revealed without much or any anthocyanin, we’ll see those golden yellow leaves. In the case of leaves turning shades of red, are those leaves that produce anthocyanin. Higher concentrations of anthocyanin with some carotenoids will result in reds while chlorophyll and anthocyanin will produce browns. These color variations vary from species.
There are a few exciting hypotheses as to why these colors occur. Leaf color changes don’t just affect us, but also animals. Evolutionary biologist, William Hamilton, brought the idea that red and yellow leaves may cause animals to associate the plant to be toxic, poor edibility and/or poor palatability, dissuading them from consuming or hosting off the plant, thus saving the tree/plant from future damage (i.e. aphids that host on trees). Lastly, it could be as simple as the need for nutrients, which decomposing leaves add to soils.
The colors of this world never cease to amaze me. Rather, I should correct myself to say, the function of life in this world never ceases to amaze me. There are some glorious native trees for those interested in adding fall interest to their own landscapes. Oaks, Red Maples, Dogwoods, and Black Tupelo showcase brilliant reds in the fall and Sugar Maples, Hickories, and Serviceberries show off dazzling oranges and golds.
Until next time,