Mother Nature's art gallery

Among the many signs of seasonality, one of the most spectacular is the display of beautifully colored leaves that tells us that the season must be autumn. This painting spree by Mother Nature lasts for one or two months and then is gone. When a child asks why the leaves change color in the fall, the question ranks right up there with "why is the sky blue?" It's one of those things of the world around us that we accept without questioning, unless we're a child or a scientist set on finding the answer.

Botanists have figured out how this works. As the days become shorter, plants with deciduous leaves stop producing the green pigment chlorophyll, and the chlorophyll already present in the leaves begins to break down. Carotenoids that have been present as insoluble pigments in the plant's chloroplasts are then exposed as the bright yellows that we see in cottonwoods and aspens and bigleaf maples.

The brilliant reds are produced quite differently. The autumn drop in temperature reduces the transport of sugars out of the leaves, and in some trees the excess sugar is used to synthesize anthocyanins, the same pigments found in red flowers. This happens in our vine maples, our blueberries, and our mountain ashes, among others. A combination of anthocyanins and carotenoids produces the range of colors that we see in many species.

Even knowing the "how," have we answered the "why?" Is there any advantage to a plant in having these colors in its leaves in the fall? That question remains to be answered. Perhaps this particular phenomenon has no adaptive significance. But perhaps it does!

Autumn is a time during which nature can be enjoyed at a distance; just look at forests or tree groves or individual trees in a park. In New England, the whole world changes color. In the Pacific Northwest, the brilliant reds and yellows of the deciduous trees are scattered through landscapes of evergreen conifers.

The color change is very different in different groups of plants. Red alders are prominent in our Northwest forests, but they contribute nothing to the fall extravaganza, the leaves merely turning brown and then dropping from the tree. This leaves many of our westside forests rather dull except for bright spots of red vine maples and yellow cottonwoods. For total immersion in autumn colors, a resident of the Puget Sound area should head into and east of the Cascades.

Dennis Paulson