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​Fall is upon us once again. Although it may not feel quite fall-like to us, deciduous trees and shrubs are starting to reflect the change. Forests throughout the state are already emblazoned with shades or red, yellow, and orange, with more plants transitioning all the time. But some observant individuals may have noticed the fall flush is a bit more ephemeral now compared to years past, with some trees shedding their leaves practically the same instant they change color. What's the cause of these short-lived displays? Is the lack of rainfall in our area to blame? To answer these questions and others, we'll need to take a trip back to middle school biology class and examine the science of why leaves change color in fall. 

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Deciduous trees and shrubs begin their annual growth in spring, when days become long enough and temperatures become warm enough to support the emergence of overwintering buds into new stems. This growth is typically completed by late June for most trees in the Northern Hemisphere. At this time, trees also begin to set next year's buds, which won't open themselves until they experience the chill of winter followed by the warmth of spring. Once this year's leaves are expanded and next year's buds are set, the tree will begin to manufacture carbohydrates through photosynthesis and store them to support next year's growth.

In late summer or early fall, days become shorter and nights longer. Deciduous trees and shrubs, like most plants, are very sensitive to night length. Once a certain threshold is reached and nights become long enough, these plants respond by forming what's known as an "abscission layer" where the leaves connect with the stems. This corky layer blocks the passage of material into and out of the leaves, including the green pigment chlorophyll. In the absence of chlorophyll, two new pigments become visible, the yellow pigments known as xanthophylls and the orange pigments called carotenoids. These are always present in leaves throughout the growing season, but are normally masked by the green of chlorophyll. Because these pigments are always present in leaves, yellows and golds tend to be fairly constant from year to year. One other group of pigments, the anthocyanins responsible for red and purple fall colors, are manufactured during fall from carbohydrates stored in leaves. These alone are not typically present during the growing season. The amount of these pigments produced depends on the amount of carbohydrates stored in leaves, and so the intensity of reds and purples is more variable in a given year.

Eventually, the abscission layer will thicken and the leaves will drop. 

A number of environmental factors impact the intensity and duration of fall colors. Lots of sunlight and low temperatures after the time the abscission layer forms cause the chlorophyll to be destroyed more rapidly, resulting in earlier fall colors. Cool night temperatures combined with abundant sunlight promote the formation of more anthocyanins, and thus more intense reds and purples. Freezing conditions halt the production of anthocyanins, so early frost means an early end to colorful foliage. A growing season with ample moisture that is followed by a dry, cool, sunny fall with warm days and cool (but frostless) nights provides the best weather conditions for development of the brightest fall colors. Lack of wind and rain in the fall prolongs the display; wind or heavy rain may cause the leaves to be lost before they develop their full color potential. 

Drought, in particular, can have a severe impact on fall colors. Drought conditions can slow the process of color change, and lead to less colorful, muted displays. Drought stress during the growing season can also sometimes trigger the early formation of the abscission layer, and leaves may drop before they even have a chance to develop fall coloration. Lack of moisture should have less of an impact on yellows and oranges than reds and purples, but these leaves also won't be hanging around too long.

So there you have it. Enjoy the splash of colors while you can, because like fall in the South, it won't last long. 

Mitchell Vaughan, Auburn University Department or Horticulture and Extension Graduate Student, Fall 2016


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