"Nature Guide Journal"

In many regions of North America, people are preparing for a favorite herald of autumn: the change of colors.

The forest's swift change from cool greens to brilliant oranges and reds plays a significant role in the tourism industry in the northeastern US. This seasonal sunset is less exuberant in our part of the continent because of our climate.

In the first place, our western forests tend to have more conifers, since conifers better tolerate our dry summers. Secondly, the climate of the eastern deciduous forests sets a better stage for bringing fall color out in the tree leaves. And, our tree species are somewhat less inclined to produce bright color.

Plant leaves are green in spring and summer because of chlorophyll, the green chemical that plants produce to make food from raw materials and sunlight. But green leaves also have many other colored chemicals, including yellow (xanthophyll) and orange (carotene).

The much anticipated autumn colors occur when well-watered, robust leaves experience dropping temperatures. When the temperature drops far enough, the leaves of certain species can no longer make chlorophyll. The chlorophyll continues to be broken down by the sunlight until it reveals the other chemicals. In addition, the lower temperature also increases the plant's production of the red pigment, anthocyanin. These several colors are always present: The loss of chlorophyll allows the others to shine.

Good fall color is favored by a good, wet growing year that ends in a decided and lasting chill with lots of sunshine.

In the west, our dry-summers usually stress the trees to where there is little of any color chemicals left in the leaves. Further, by the time we get cold temperatures our rainy season has begun, giving less light to break down the remaining chlorophyll.

Tans and browns are more common autumn colors for our region. The browning leaves of red alder signifies the death of leaf tissues. As the cast-off leaves drop into creeks and streams, their tannins stain the water the color of tea.

The Pacific Northwest forests can sparkle in autumn, however. Under the right conditions, our native vine maple can produce brilliant yellows to reds–giving quite a show when nestled among dark evergreens.