What Determines Brilliance of Fall Colors?
Many of us have heard that this fall should be a banner season for leaf color in our area. We are looking forward to fall outings with the family and bracing for tourists to pour in. Like with most things in nature, a combination of factors culminate to produce the brilliant fall colors we love.
Most trees in our area that change color in the fall are deciduous- that is, they lose their leaves every winter and grow new ones each year. In contrast, evergreen trees, including many conifers, stay green all winter. The Eastern white pine and a few other conifers lose some of their needles each year and so some of them turn yellow or brown during the fall, leaving the deep green of the younger needles to overwinter.
The green color in leaves is generated by the compound chlorophyll. Chlorophyll is central to a plants ability to perform photosynthesis, the process by which plants utilize energy from the sun, plus water and carbon dioxide, to create their own energy. In the fall, deciduous trees slowly cut off the movement of water and nutrients between stem and leaves. This reduces the amount of chlorophyll that can be made, and gradually, the leaf loses its green cast.
A few primary compounds –carotenoids, xanthophylls, tannins, and anthocyanins- remain in the leaves and create either yellow-orange colors or red-pink-purple colors. Carotenoids and xanthophylls lend yellow and orange colors. Tannins are a plant defense compound produced in leaves to resist herbivory and bacterial infection. They can create some golden colors and contribute to brown leaf color in late fall. Anthocyanins are antioxidant compounds that form from sugars in the leaves. The anthocyanins actually help protect the leaves from light exposure. Sunny days enable photosynthesis to occur, so more sugars and anthocyanins are created, and there is greater potential for red, pink, and purple pigments to form in leaves.
Weather patterns earlier in the year and during the fall also affect the color brilliance. The best displays of fall color occur in years when there is adequate moisture throughout the year, including in the fall. Very dry periods in the fall can reduce the amount of photosynthesis, reducing sugar and anthocyanin production, and cause leaves to drop before colors develop fully. Wet periods also reduce photosynthesis. The ideal fall weather for leaf color display is a combination of sunny days and cool nights. The cool nights reduce the transfer of sugars from the leaves into branches, so more anthocyanins form in the leaves. An early frost can cause leaves to drop before colors are fully developed. Other stresses, such as disease or root damage, can also cause leaves to turn or drop prematurely.
Each plant creates different levels of these compounds, leading to different shades and hues. Trees and shrubs with predominately red and purple fall color include sassafrass, sugar maple, black, white, red, and scarlet oak, dogwood, hawthorn, serviceberry, and others. Trees and shrubs that display mostly yellow and orange color include willow, birch, silver maple, white ash, tulip poplar, honey locust, gingkos, linden, and others. Some colors vary based on where the tree is growing, and of course, weather conditions.
We are fortunate to live in an area with a wide mixture of tree species, and variable elevations. Trees will turn on the distant mountaintops before they do at the lower elevations. Sometimes, in our immediate surroundings, they may turn earlier in the slight dips, where cool air settles. Hopefully, the current weather patterns will hold, and we will indeed get to witness some brilliant hues this season. It should be a picturesque time for family outings, hikes, photography, or simply enjoying the beauty of our surroundings.
For more information on choosing plants for fall color, or with gardening questions, please contact the NC Cooperative Extension-Caldwell County Center at 828-757-1290, or visit our website at https://caldwell.ces.ncsu.edu.