Submitted by Celina Tuttle
10th October 2007
Autumn: a time of idyll observation, or is it?
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Fall. It’s marked by spectacular bursts of color and periods of idyll observation. It’s a time to watch leaves spiral lazily to the ground, one where sunlight filters through canopies of orange, yellow and vivid reds. The smooth, silver bark of beech trees contrasts sharply with the craggy black and grey of maples and the corky bark of the hackberry. Lacy fronds of balsam, cedar and hemlock sway gracefully above carpets of red and gold pine needles; gardens and streets are littered with crisp brown leaves. The aromatic scent of cedar and the damp, rich smell of wet leaves carries on the slightest of breezes.
Well, perhaps it’s not such idle observation. There’s always a burning question. What causes the change from the verdant greens of spring and summer foliage to shades of brown and yellow and the brilliant oranges and reds that sometimes takes our breath away? What determines whether a leaf will be purplish in color, a mellow yellow, brilliant red or dull brown, and what causes trees to shed their leaves?
The leaves of trees provide most of its food and nutrients through a process called photosynthesis. Chlorophyll, a molecule that gives most plants their green color, is an important ingredient in photosynthesis, as is sunlight. Chlorophyll uses the sun’s energy to transform carbon dioxide and water into sugars and starch.
Along with the green color of chlorophyll, there are yellow and orange pigments in leaves. These pigments, the carotenoids and the xanthophylls, are in the leaves for most of the year but are overpowered by the large amounts of green chlorophyll produced during the growing season. In the fall, with fewer hours of sunlight, leaves are unable to produce as much chlorophyll, their green color begins to fade and the yellow and orange pigments are revealed. The hackberry, birches, elms and poplars change color in this way.
Another cell pigment in leaves, anthocyanin, causes the red and purple colors in maples, sumacs, dogwoods and ashes. Sugar is trapped in the leaves after photosynthesis stops in the fall and is changed chemically into anthocyanin. The pH levels of the tree’s sap will determine whether its leaves turn red (acidic sap) or purple (alkaline sap).
In other trees, the anthocyanins and the carotenoids combine to create the deep oranges, reds and bronze colors that mark our local hardwood forests. The brown leaves of the oak and some other trees are caused when tannic acid, an astringent found in tree bark and pine cones, combines with the carotenoids.
Another process, occurring at the same time as the colors change, causes the leaves to fall. Cooler temperatures and less daylight stimulate the growth of a layer of cells, called the “abscission” or separation layer, between the leaf stalk and the woody branch. These cells slow the exchange of water and carbohydrates between the leaves and the tree’s roots, as the tree begins its winter dormancy. Food trapped in the leaves becomes the pigment for red and purple-colored leaves.
Other factors influence fall colors. Early frosts will cause leaves to die before the abscission process begins and they may drop before the fall pigments develop. Photosynthesis, the production of green chlorophyll, will continue with warm weather and the colors won’t be as vivid. Low light and water levels will diminish the color of the leaves.
Celina Tuttle is a member of the Urban Forest/River Corridor Advisory Committee, the aim of which is to ensure our urban forests and river corridor are healthy, attractive and available for future generations. Questions may be emailed to the Committee’s Chair, Jim McCready, jmccready@cyberus.ca, and will be considered as topics for future articles.
FUN FACT: What do autumn leaves and ripening bananas have in common?
The green color in unripe bananas comes from chlorophyll, the same pigment that gives green leaves their color. As bananas ripen, the chlorophyll breaks down and disappears, revealing the yellow color, which has been there all along. The yellows and oranges of autumn leaves are also revealed as their chlorophyll breaks down. Other changes also occur as bananas ripen: the starches change to sugar and the flesh softens as pectin (a carbohydrate) breaks down. Source: www.sciencemadesimple.com
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