Senescence can be traced back to Latin senex, meaning “old.” There’s also the much rarer senectitude, which, like senescence, refers to the state of being old (specifically, to the final stage of the normal life span). This event happens every year – senescence – to the deciduous trees in Colorado, including the beautiful stands of Aspen at higher altitudes. The leaves have grown “old,” and are at the end of their life span. Throughout the spring and summer the leaves have served as mini-chemical factories where most of the foods necessary for the tree’s growth are manufactured. This food-making process takes place in the leaf cells containing chlorophyll, which gives the leaf its green color. Chlorophyll absorbs from sunlight the energy that is used in transforming carbon dioxide and water to carbohydrates starches and sugars, also known as photosynthesis. The gold and yellow colors visible in autumn are always there; in the fall, they simply are revealed when the green hues from chlorophyll production fade. If sugars are trapped in the leaf, they are responsible for producing anthocyanins, which reveal the deep reds and purples. A color change is not the only transformation occurring in trees this time of the year.
As the fall colors appear, other changes are taking place. At the point where the stem of the leaf is attached to the tree, a special layer of cells develop into a ‘corky tissue’ which gradually severs the tissues that support the leaf. At the same time, the tree seals the cut, so that when the leaf is finally blown off by the wind or falls from its own weight, it leaves behind a leaf scar. There are other variables at work before the leaves eventually fall to the ground. These have the potential to affect the intensity of fall color.
Sunlight, temperature and moisture are the variables which play an important role in the decrease of photosynthetic activity as the days grow shorter in the fall. According to the United States National Arboretum, a wetter growing season followed by a dry, sunny autumn with cool but frost-free nights result in the brightest fall colors. Low temperatures above freezing will favor anthocyanin formation producing bright reds and purples. However, early frost will weaken the brilliant red/purple colors. Rainy and/or overcast days tend to increase the intensity of fall colors. Time is drawing to a close for many plants – their days are truly numbered.
Local weather stations abound with daily reports and sightings of the inevitable end of life for the deciduous leaves of our Colorado trees. Many agree the best time to enjoy the autumn color would be on a clear, dry, and cool day. So as Miss Elliott aptly wrote, before the [season] is done “catch the leaves dancing in the autumn sun.”
“I see the turning of a leaf dancing in the autumn sun, and brilliant shades of crimson glowing when a day is done.”
Hazelmarie Mattie Elliott
