|Rusty Blackhaw Viburnum|
Fall is the time when the quiet, green palette of summer gives way to the crisp reds, vibrant oranges, and mellow yellows that paint the natural landscape. During the growing seasons of spring and summer, our trees and shrubs use sunlight to convert water and carbon dioxide from the air into sugar. Called photosynthesis, this process begins to wane in November in Central Texas, and the leaves on some plants begin to change color in preparation for winter’s rest.
Pigments are natural substances formed by the cells of leaves that provide the basis for leaf color. Most familiar is chlorophyll, which produces the color green, and is vitally important as it is required for photosynthesis. Carotenoid, which produces the colors yellow, orange, and brown, is a common pigment in many fruits and vegetables, as are anthocyanins, which produce the color red. Both chlorophyll and carotenoid are present at the same time in leaf cells, but the chlorophyll covers the carotenoid and hence the leaves appear green in the spring and summer. Not all trees can make anthocyanins, however, and most are produced under certain conditions and only in the fall.
As the days grow shorter, the decreasing amount of sunlight eventually causes trees to stop producing chlorophyll. When this happens, the carotenoid in a leaf can finally show through, turning the leaves into a myriad of yellows, oranges, and browns. Red, on the other hand, is an entirely different matter. Affected by temperature and cloud cover, red fall colors can vary greatly from year to year. A lively showing of reds depends upon warm, sunny autumn days and cool, but not cold autumn nights. This type of weather pattern triggers the production of anthocyanins, which the tree produces as a form of protection. Anthocyanins allow trees to recover any sugar or nutrients left in the leaves, moving them through the leaf veins and down into the branches and trunk, and its presence generates the red color before the leaves fall off. Rainfall during the year can also affect fall color, with too much lowering the overall color intensity, and too little delaying the arrival of color.
Fall leaf color can easily be used to help identify local tree and shrub species. The most notable reds and oranges in our area are produced by Texas Red Oak, Flameleaf Sumac, and Rusty Blackhaw Viburnum. Dotting the hillsides, roadsides, and upper reaches of wooded canyons, they contrast well with the surrounding greens of Ashe Junipers and Live Oaks. Golds and yellows are represented by Eastern Cottonwood, Escarpment Black Cherry, Mexican Buckeye, Bald Cypress, and Little Walnut, whose colors transform the low-lying areas near creeks and streams.
While a tree’s trunk and branches can survive the colder winter temperatures, many leaves cannot. Made up of cells filled with water and sap, these tissues are unable to live throughout the winter, and the tree must shed them to ensure its survival. As the days grow shorter, the veins that carry sap to the rest of the tree eventually close. A separation layer forms at the base of each leaf stem, and when complete, the leaf falls. Some oak trees are the exception, with this layer never fully detaching and the dead leaves remain on the tree until new spring growth pushes them off to the ground. Once on the ground, the leaves slowly decompose with the help of earthworms, beneficial bacteria, and fungi, creating the soil necessary for the continuation of the cycle of life.