The Science Behind Grandfather Mountain’s Fall Leaf Season

In autumn, Grandfather Mountain beautifully blends a landscape of rock faces and granite cliffs, the dark-green hue of evergreens and a kaleidoscope of fall colors.

The mountain’s wide range in elevation provides vast biodiversity of plant species and an extended fall season, often starting in late September in shrubs like blueberry, huckleberry and azalea at the highest elevations. In October, trees like oak, birch, poplar and maple turn. Standouts for vibrant color include species such as sourwood, black gum, sassafras and cherry.

In late October and into November, Grandfather Mountain turns into a unique vantage point for fall. Even as autumnal tones are leaving the mountainside, the nature preserve’s lofty heights provide a spectacular view of the leaf change when it creeps down to the North Carolina Piedmont below.

Just like Grandfather’s synchronous firefly display each summer, science and a number of factors go into the phenomenon of fall. Read on for a primer on the science behind the famous leaf season.

Why do leaves change color?

Weather, leaf pigments and the length of night are three important ingredients in the process. In the summer, most trees have green leaves because they contain the pigment chlorophyll. This pigment is used to convert sunlight, carbon dioxide and water into energy for the tree. During the warmer months, chlorophyll is constantly replaced in the leaves. As temperatures drop, days get shorter and the intensity of sunlight declines, deciduous (leaf-shedding) trees stop making chlorophyll (the process of photosynthesis reduces) and begin to break it down. The tree can then absorb the nutrients making up the chlorophyll before the leaves fall off.

The change in leaf color is more dependent on light than temperatures; therefore, leaves start changing color at the same time each year. As this daylight threshold is reached, most of the stored sugars (food) are transferred from the leaf to the branch. No new nutrients enter the leaves, and the trees prepare to separate from their leaves.

The green color of chlorophyll masks other pigments in the leaves. These other pigments include the yellow of xanthophyll (like in corn) and the orange of carotene (like in carrots). As the chlorophyll breaks down, the xanthophyll and carotene become visible.

Reddish-purple anthocyanin (like in apples) are leaf pigments that are produced in the fall. Sugars remaining in the leaves after the tree stops sending nutrients to the leaves are used in the formation of anthocyanin. Anthocyanin not only creates the reddish colors, but it also is used in plant defense, likely to protect the leaves from the sun and other potential environmental stressors.

How does weather affect autumn foliage?

The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences.

Low temperatures still above freezing can help to produce anthocyanin and brilliant red color; however, an early frost weakens the color by reducing anthocyanin production. Drought can also cause leaves to fall off without changing color.

In addition, high winds during the actual leaf change can drastically affect a season. As autumn progresses, the veins that carry fluids into and out of the leaf gradually close, sealing off connecting tissue and eventually causing the leaf to fall. While not a regular occurrence at Grandfather Mountain, remnants of a tropical storm sometimes make their way into the region, and if timed just right (or wrong), a brilliant fall show could be sent to the ground prematurely.

What is the recipe for the best fall color?

The perfect recipe for the best fall color is a warm, wet spring and warm, sunny, dry fall days with cool nights. In September, a succession of warm, sunny days and cool, crisp (but not freezing) nights brings about the most spectacular color displays. During these days, lots of sugars are produced in the leaf, but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions – lots of sugar and light – spur production of the brilliant anthocyanin pigments (reds, purples and crimson).

The amount of moisture in the soil also affects autumn colors. A summer drought can delay the onset of fall color. See our Fall Color Gallery for daily photo updates each October.

In the end, no two fall seasons are ever alike. Some years may bring more reds, some years may start a few weeks late and some years may see certain species really outshining others. The good news for leaf peepers is that every fall is unique and special at a place as biologically diverse as Grandfather Mountain.

More info: Fall Insider Tips!