HOLLOWS, PEEPERS, AND HIGHLANDERS: AN APPALACHIAN MOUNTAIN ECOLOGY

23. Autumn Leaves

165 23 Autumn Leaves I t happens every fall. I’m hell-bent on splitting a pile of wood, winterizing the old truck, or jogging from the house to the lab when I feel the tug. Resist as I might, my concentration wanes. I give in, and indulge in inspecting the colorful leaves. The meander takes me farther and farther from my task. During these rambles I am reminded of a favorite yet deceptively complex question, “Why do leaves change color and drop?” When biologists confront “why” questions, two camps usually emerge and commence battle. One group argues that each feature of every living thing has a purpose, that in some way every trait is an adaptation that contributes to the survival or reproduction of the individual . The other school counters that some life processes have no reason for being, are adaptively neutral, just happen. (As you know by now, I sit in the former camp.) The recent history of evolutionary biology is punctuated with the ebb and flow of this debate. With these conflicting possibilities in mind, I’ll weigh in with my interpretation of the meaning of autumn leaves. Leaves change color, then fall. Let’s consider these events in their natural sequence. We all know that leaves are green in the summer; what may be news is that summer leaves also harbor other pigments. July’s dogwoods certainly contain chlorophyll, a molecule that reflects green light and plays a crucial role in photosynthesis, but they also contain carotene and xanthophyll, molecules that reflect orange and yellow light, respectively. Together, these three pigments allow a plant to absorb light of a wider range of wavelengths, and 166 thereby enable the plant to capture more energy from sunshine, than if it had only one pigment. Because summer leaves contain all three pigments and yet are green, the green of chlorophyll must overpower the other colors. The process of photosynthesis continually breaks down chlorophyll , but as long as the weather is warm, plants continue to produce it. Through the summer, plants maintain a fairly constant chlorophyll concentration and thereby go on reflecting green wavelengths. As the temperature drops, leaves absorb less water and minerals, and chlorophyll synthesis decreases. By late summer, chlorophyll is breaking down faster than it is being replaced. What little photosynthesis still occurs in the fall completely exhausts any remaining chlorophyll molecules. Suddenly, the other pigments are unmasked. Zooming up to the continental scale, the fall color wave advances southward at about forty miles per day. Though the yellow of October’s hickories seems to appear spontaneously, it was there all along. From this perspective , the yellow and orange shades of autumn leaves, leftovers of a previously functioning biological mechanism, are manifestations of adaptive traits. Red autumn leaves, on the other hand, develop in a different way. During the cool nights and sunny days of fall, leaf cells begin to synthesize a new kind of molecule called anthocyanin, which reflects red wavelengths, instead of chlorophyll. Anthocyanin accumulates in the sap of leaf cells. Anthocyanin’s formation depends on (1) the breakdown of sugars in the presence of bright light and (2) a decrease in phosphate due to the plant’s diminishing water supply. The brighter the light during this period, the greater the production of anthocyanin, the more brilliant the colors. The most flaming reds develop when autumn days are bright and cool, and the nights chilly but not freezing. In contrast to orange and yellow, we know of no purpose for the trait “red leaves.” Thus, the redness of tree leaves in the autumn appears to be adaptively neutral — they convey neither advantages nor disadvantages to the reproductive success of the individual tree. Hollows, Peepers, and Highlanders 167 Since publication of the original Hollows, a tentative idea has surfaced on why leaves turn red in the autumn. Anthocyanin, a molecule the leaf makes fresh in the fall, screens out blue wavelengths that normally power photosynthesis in the chlorophyll-laden layers of the leaf. This observation suggests that by filtering out blue, the red pigment turns off the photosynthesis reaction, and this in turn facilitates the tree’s recycling of nutrients from its leaves to seeds or roots. Thus, it is possible that red leaves may not represent, after all, a caveat to the neo-Darwinian dogma. The second feature of a tree’s seasonal change is its loss of...