During this time of year, many people enjoy the spectacle of fall color change. Often overlooked is the reason leaves change color and fall.
Before exploring these questions, there must be a basic understanding of the changes within the leaf and their function.
According to the U.S. Forest Service, leaves are nature’s food factories. Plants take water from the ground through their roots and carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose, a kind of sugar used as food and as a building block for growing.
The process of using water and carbon dioxide combined with sunlight for the purpose of making glucose is photosynthesis. Chlorophyll, which captures light and makes photosynthesis possible, is what gives leaves their green color.
During the spring and summer, trees are constantly producing sugars through photosynthesis. The sugars are saved as an energy source for winter. To facilitate this production, trees are actively producing and breaking down chlorophyll. Other pigments in leaves are masked because chlorophyll is prevalent and the green hue prevails.
During fall and winter, because of changes in temperature and the length of daylight, the leaves stop their food-making. 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. This action traps different pigments in the leaves, resulting in the brilliant fall color changes. The newly visible 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 is dependent on 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 type of pigment 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 as a result of Anthocyanin production. Decreased sunlight, from overcast days, can actually increase the intensity of fall color. Environmental conditions also influence the intensity of leaf color. Adequate soil moisture seems to affect color. 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 produce the best fall colors.
As the veins that supply nutrients to the leaves begin to close resulting in the aforementioned changes in color, leaves begin to drop.
Deciduous trees drop their leaves as a protection against cold temperatures. Leaves are the tender parts of the tree and susceptible to freeze damage. Dropping them for the winter minimizes cold damage to the trees.
An exception to the deciduous tree rule is the oak. Oak leaves do not detach until new growth emerges in the spring.
Evergreen trees, such as pines and spruce, do not drop their leaves or needles because their leaves have a wax-like coating that can survive through several seasons before being shed for new growth.