Fall Colors: Naturalist Notes
Let's start by discussing the leaf's primary role before delving into the factors influencing their colors. The primary function of a leaf is to generate nourishment for the plant through the process of photosynthesis. This process involves chlorophyll, the pigment responsible for the characteristic green hue of plants, absorbing sunlight. Alongside carbon dioxide and water, leaves manufacture glucose or sugar, which serves as sustenance for the plant. This vital role gives rise to two distinct types of leaves: sun leaves and shade leaves. Leaves thriving in the shade, known as 'shade leaves,' are generally larger in size but thinner compared to leaves exposed to full sunlight, commonly referred to as 'sun leaves.' Sun leaves, as a result, grow thicker than shade leaves. In addition to chlorophyll, some tree leaves, like birches and cottonwoods, contain carotene, which contributes to their brighter green appearance by absorbing blue-green light.
The amount of sunlight received by a tree acts as a signal, instructing the tree when and how to proceed with its functions. As the days grow shorter, the tree conserves energy by ceasing the production of chlorophyll in its leaves. Throughout the summer, chlorophyll continually breaks down when exposed to sunlight, necessitating constant replacement, which consumes energy. In response to decreasing daylight, the leaves begin to utilize the stored glucose for sustenance. As the stored food (glucose) is consumed, a layer of cells forms at the base of each leaf, known as the abscission layer. These cells have a spongy texture similar to cork and serve as a slow-closing door separating the leaf from the rest of the tree. This door only closes completely once all the leaf's food is depleted.
During this gradual closure, the other colors present in the leaves become visible. These colors always exist within the leaf but are typically masked by the dominance of chlorophyll. The unique colors of a particular tree are the result of interactions between carotenoids, anthocyanins, and any remaining chlorophyll. The formation and quantity of these chemicals are influenced by temperature, moisture, and sunlight. Consequently, each foliage season is distinct due to variations in the internal chemical balance of the leaf. Carotenoids, responsible for orange and yellow pigments, persist in the leaf throughout the summer. Red and purple hues are produced by anthocyanins, which form as a result of the glucose created by the remaining, fading chlorophyll. This glucose becomes trapped within the leaf by the abscission layer, leading to anthocyanin production. Anthocyanin plays a protective role against diseases and drought, repairs leaf damage, and acts as a natural sunscreen. The brown color in leaves results from the presence of tannins.
Specific colors are associated with particular tree species:
Oaks: red, brown, or russet
Hickories: golden bronze
Aspen and yellow-poplar: golden yellow
Dogwood: purplish red
Beech: light tan
Sourwood and black tupelo: crimson
The color of maple leaves varies by species:
Red maple: brilliant scarlet
Sugar maple: orange-red
Black maple: glowing yellow
Striped maple: almost colorless
The intensity and diversity of colors in any given autumn season are closely tied to weather conditions that occur before and during the reduction of chlorophyll in the leaves. Temperature and moisture are the primary influencers. An ideal scenario for vibrant colors includes a series of warm, sunny days followed by cool, crisp, but not freezing nights. During such days, ample sugars are produced in the leaf, but the cool nights and gradual closure of veins prevent these sugars from moving out. Here is an estimated map of the peak colors in Michigan for this year:
…and we couldn’t let you go without this…
Submitted by naturalist Cathy Wesley, September 2023