During this time of year, many people enjoy the spectacle of fall color change. Raking leaves also marks the beginning of fall.
Often overlooked is the reason why leaves change color and leaves are shed during the fall. Before exploring these questions, there must be a basic understanding of the seasonal changes within the leaf and their function in the plant itself.
According to the U.S. Forest Service, leaves are nature’s food factories. Plants take water from the ground through their roots. Plants take in carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose, a kind of sugar. Plants use glucose as a building block for growing. The process of using water and carbon dioxide combined with sunlight for the purpose of making glucose is known as photosynthesis. Photosynthesis means “putting together with light.” Chlorophyll captures light and makes photosynthesis possible. Chlorophyll is what gives plants their green color.
During the spring and summer, trees are constantly producing sugars through photosynthesis. The sugars are stored in the tree and saved as an energy source for winter. To facilitate this production, trees are actively producing and breaking down chlorophyll. Other pigments in leaves will be masked at this time because chlorophyll is so prevalent, and the green hue prevails.
During fall and winter, because of changes in the length of daylight and in temperature, the leaves stop their food-making factories. As chlorophyll breaks down, the green color begins to disappear. Also during this time, the veins that transmit fluids from the tree branches to the leaves begin to close, and a layer of cells form to clog the veins.
This action traps different pigments in the leaves, resulting in the brilliant fall color changes. These pigments are Anthocyanins, which are pigment compounds with red and purple hues, and Carotenoids, which exhibit orange, yellow and brown colors.
Timing and color variation of leaf change vary with the species. Sourwood leaves often change color and fall off while most other species are still green. Oaks tend to be the last to change color. The exact color the tree species will show is dependent on the amount and types of pigment contained in its leaves. Oaks usually turn red or brown; poplars turn golden yellow; dogwoods turn purple-red; and maples have varying colors depending on the type. Temperature, light, and water are the primary factors that influence the duration of fall color and how vibrant the colors appear. For example, low temperatures will produce red hues in maple due to Anthocyanin production. Decreased sunlight, from overcast days, can actually increase the intensity of fall color. Environmental conditions can also have an influence on the intensity of leaf color change. Adequate soil moisture seems to have an effect on color change. Drought conditions throughout the summer can delay the onset of fall color and lessen its intensity. Early frosts tend to decrease the amount of colors displayed. Rainy days and cool nights tend to produce the best fall colors.
As described above, the veins that supply nutrients to the leaves begin to close in the fall. For most deciduous trees this process results in the aforementioned changes in color but also leaf drop. Deciduous trees drop their leaves as a protection against cold temperatures. Leaves are the tender parts of the tree and are susceptible to freeze damage. Most deciduous leaves are fairly delicate, so dropping them for the winter minimizes cold damage to the trees.
An exception to the deciduous-tree rule is the oak tree. Oak leaves do not detach until new growth emerges in the spring. Evergreens, like pines and spruce, do not drop their leaves or needles because their leaves have a wax-like coating that can survive for several seasons before being shed.