autumn woods
October 6, 2020

 Autumn leaf color is a phenomenon that affects the normal green leaves of many deciduous trees and shrubs, in which they take on various shades of yellow, orange, red, purple, and brown.  Leaves change color during the autumn because the amounts of their various pigments change as the leaves prepare to fall from the trees.

 All leaves gradually lose chlorophyll throughout the growing season, and this loss speeds up before leaf fall.  Chlorophyll captures solar rays and uses the resulting energy in the manufacture of the plant's food – simple sugars which are produced from water and carbon dioxide.  These sugars are the basis of the plant's nourishment but in their food-manufacturing process, the chlorophylls break down and are being continually "used up"; still, during the growing season the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.

Chlorophyll is the most abundant membrane protein on earth and as it degrades, hidden pigments of yellow (xanthophylls) and orange (beta-carotene) are revealed. These pigments are present throughout the year, but the red pigments, the (anthocyanins) do not show until roughly half of chlorophyll has been degraded. The amino acids released from the process are stored all winter in the tree's roots, branches, stems, and trunk until the next spring, when they are recycled to grow new leaves for the tree.

As autumn approaches, daylight hours shorten, and temperatures drop, the veins that carry fluids into and out of the leaf are gradually closed off by a layer of special cork cells at its base.  Water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly, and during this time, the amount of chlorophyll in the leaf begins to decrease. Often, the veins are still green after the tissues between them have almost completely changed color.

Sugar maple is one of our most spectacularly colorful trees during the autumn.  Its abundance in some areas is a result of extensive planting along roadsides during the eighteenth and nineteenth centuries to provide a source of sap for maple sugar.  It is likely that its natural distribution will move towards the north over the next century, with the anticipated increase in temperatures due to the accumulation of greenhouse gases. Over time, the autumn colors of our forests may diminish as conditions become less favorable for this tree.

Although always impressive, autumn leaf colors vary from year to year, and seem to be more intense in some regions where it is economically interesting because the colors are a huge tourist draw, worth hundreds of millions of dollars in revenues every year..

The right weather during the autumn can promote more intense color production. The reds (anthocyanins), which require sunlight for production, are enhanced by cold and sunny days. Rainy and windy weather during the autumn can knock leaves down prematurely thereby shortening the color display at its peak.  Summer drought conditions stress trees and they may thus lose their leaves earlier or start color production prematurely. The result is a reduction of color during the peak of the season. Adequate summer rains promote good tree health, leaf retention and, therefore, color production during the autumn.

Thus, it has been determined that three factors seem to influence the autumn leaf color-show.  The timing of color change and leaf fall are primarily regulated by the calendar; that is, the increasing length of night. None of the other environmental influences--temperature, rainfall, food supply, and so on--are as unvarying as the steadily increasing length of night during autumn. As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin.

A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays.  Lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions--lots of sugar and lots of light--spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson.  Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.

The amount of moisture in the soil also affects autumn colors and as soil moisture varies greatly from year to year so does the color.   A late spring, or a severe summer drought, can also delay the onset of fall color by a few weeks.  A warm wet spring, good summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors.

We are told that autumn colors were different in the past, and they will likely continue to change during this century due to land-use changes, introduced pests and diseases, and climate change from fossil fuel emissions.  We should enjoy them while we have them.