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Reds and yellows among the tree leaves

The bright red leaves of a sunlit red maple. (Photo by Larry Weber)1 / 2
Yellow leaves on a red maple growing in the shady woods. (Photo by Larry Weber)2 / 2

Living here as we do, we are surrounded by trees. Mostly, we take them for granted and except for a few times during the year, we hardly even take a close look at them. Roughly seven months of the year (mid-October until mid-May) the nearby forests of deciduous trees are devoid of leaves. The green landscape is nearly depleted and only the local conifer species keep this color around us.

As we grew through the month of September, the trees adjusted to the days of earlier darkness and later sunrises to prepare for the coming cold. The leaves of trees are remarkable organs. Thin and variously shaped, they are the food-makers for the trees. Growing on the many branches of the tall trees, the leaves get a lot of exposure to the sun and thus take advantage of the light source. The leaves also absorb moisture. With proper enzymes within the leaf cells, called chloroplasts, the trees produce needed sugars to survive. Chlorophyll, the green substance in the numerous chloroplasts, is why the leaves take on the color of being green.

This arrangement works fine for the trees throughout the warmer weather, but as the days slowly move into the cooling and darker autumn, the food-making system is shutting down. Food already formed but not yet used is stored in the safety of stems and roots. The tree does not need to produce more of the summertime sugars and so the chlorophyll will break up. Soon the trees will drop the food-making organs, the leaves, since they are no longer needed.

We see this change every year. The leaf-drop of autumn is an adaptation to the climate. Besides making food, the leaves also have much water go from them in an evaporation known as transpiration. They deal with this well in summer, but in the arid air of winter this transpiration could cause the trees to desiccate and die of dehydration in the cold. Also the surface of these flat leaves would hold the falling snow, adding much weight to the branches and maybe causing damage to the trees. And so the trees drop their leaves even though they are the sole source of food-making for these large plants.

Fortunately this process is done in a blaze of color. Basically, there are three pigments responsible for this autumn scene: xanthophylls (yellow), carotenes (orange) and anthocyanin (red). As it happens, the yellows and oranges have been present in the leaves all summer, but due to the immense amount of green, we have not been able to see them. When this dominant green is taken from these arboreal organs, the yellows and oranges will appear.

Since yellow was already in the leaves all summer and not produced in fall, yellows are much more common now. As we go through late September and early October, we note an abundance of yellow in sugar and mountain maple, birch, poplar, cottonwood, willow, ash, ironwood, cherry, basswood, elm, mountain-ash, aspen and most oaks. Later we will see more yellow in tamarack, silver maple and weeping willow. (Yellow prevails in roadside plants of dogbane and milkweed as well.) But it is the reds blended with all this yellow that makes us take a closer look, maybe even travel for some leaf-looking.

Reds (anthocyanin) are different. They were not present in the leaves in summer, but are formed now in the fall. This bright pigment is produced from excess sugars within the leaves. Recent research indicates that this may be the trees way of coping with stressful conditions. I have found that with many trees, red is more likely than yellow: red maple, dogwood, sumac, cherry, highbush cranberry, arrow-wood and some red oaks (mostly smaller ones). The vine Virginia creeper climbs up to catch our eye with its red leaves as does its less-appreciated cousin, poison ivy. Raspberry, blueberry, blackberry and bunchberry turn this color as well.

Regardless of the plant, one thing they have in common is that they are in the sunlight. A red maple growing in the light is red; another in the shades of the woods is yellow. And so, most of the bright autumn colors that we are fond of seeing are from small trees at the woods edge or large ones that grow where they get this light exposure.

As much as we may enjoy this fall foliage finale, the colors may have little impact on the trees. But like aurora, rainbows and sunsets, the autumn leaf scene is one of nature's happenings that we are willing to see many times. And this arboreal autumn show is never a disappointment.

Larry Weber

Retired teacher Larry Weber is the author of several books, including “Butterflies of the North Woods,” “Spiders of the North Woods,” “Webwood” and “In a Patch of Goldenrods.” Contact him c/o budgeteer@duluthbudgeteer.com.

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