Fall colors

Posted: Sunday, December 21, 2008

As I was driving into Douglas one day last fall, I spotted a gaudy maple tree, all red and gold-but only on one side. The other side was still bright green. Near this half-and-half tree were some Sitka alders. No gaudiness for them! Their leaves just turn brown and fall off.

Those simple observations started me thinking about autumn leaf colors, prompting a number of questions - and precious few answers in the mental archives.

Why do leaves of some species turn red and gold in the fall? This is really two questions: Why red and gold? And why those species?

So, why do leaves turn gaudy colors in fall? For this one, I vaguely remembered approximately how it works. As temperatures drop, leaves start breaking down their green pigments (chlorophyll). Loss of the green pigments unmasks yellow and orange pigments present in the leaf. The leaves may also increase production of reddish or purple pigments (anthocyanins), using carbohydrates stored in the leaf, and these reds are also exposed by the loss of the greens.

Environmental factors increase formation of the red pigments: Lower temperatures (but not freezing), bright light, dry weather. The green leaves on my half-and-half maple were on the shaded side of the tree, the bright colors on the unshaded side. So light exposure helps explain the two-sided maple. But dry weather was something pretty rare around here last summer and fall, so apparently the effect of light (even our grayness) outweighed the general dampness.

If you drive up through the Yukon in early fall, you might notice the hillsides of glowing aspen trees. Most of them are yellow or gold, a few are orange, and fewer still might be red. Aspens can spread vegetatively, making extensive clones, so there might be large patches of different shades on a slope. There is individual genetic variation in the ability to make pigments and variation in environmental conditions, which combine to produce the array of color we see there.

The second part of the question is Why are the bright colors produced only by some species? For instance, why don't alders put on a color show? They don't, apparently, have the genetics that allow them to make the colors, but why not? Thus, the difference between alders and maples presumably lies in some evolutionary process.

That means we should consider what the bright pigments actually do for the plant. Most of the suggested functions relate to the red pigments. Physiologists have claimed that the red pigments help protect the aging leaf from 'excess' light, so it can retrieve and store more nitrogen from the breakdown of the green pigments. Left unexplained (so far) is why old leaves should be more sensitive to light than younger leaves and how much light is "excessive." And further - how significant is this amount of nitrogen to the well-being of the plant?

Ecologists have suggested that bright autumn leaf colors in some way protect the leaf against herbivores. Maybe they indicate that old leaves offer little food to the herbivores or that the leaves are ready to be shed. However, brown color might signal these conditions to potential herbivores just as well as red or yellow. Alternatively, reds and yellows could make insect herbivores conspicuous to their predators and thus diminish their effects on leaves.

Other ecologists have proposed that red or yellow leaves make edible fruits more conspicuous to birds that eat fruit and disperse the seeds. Evidence for this idea is not good and, in any case, cannot apply to maples, which make hard, dry fruit with wings for dispersal by the wind.

Thus, we find ourselves with at least as many questions as we had in the beginning. The evolutionary reasons for the ability of leaves to make gaudy fall colors (or not) remains to be explored. In the meantime, there are many questions for the curious naturalist to contemplate.

Fortunately, Juneau will never have the hordes of autumn "leaf peepers" that flood into New England and the upper Midwest to gawk at the color show of native trees and shrubs. In our own, more subtle, way, we can enjoy the multi-hued blueberry shrubs in the alpine, the golden cottonwoods, or the pink leaves of highbush cranberry-both aesthetically and intellectually.

• Mary F. Willson is a retired professor of ecology.

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