Uncovering the "ooh-ah" factor of fall leaves

How It's Done looks at secrets and curiosities behind Northwest icons, traditions

and natural phenomena.

Outside my window is a tree, a maple of some kind. I'm always taken by surprise when, out of nowhere, one leaf turns suddenly lipstick red, garish and outnumbered by its green comrades. A few days later, another red leaf pops up, and another until, by late September, the reds hold their own among the greens. Eventually, scarlet triumphs, but even that blaze is soon extinguished and fades away. It's a dramatic metamorphosis that swings from exuberance to melancholy and every year I wonder, why?

Who better to ask than Randall Hitchin, who will lead a November tour called "The Fabulous Shades of Autumn" in the University of Washington's Arboretum. Hitchin is collections manager of the UW Botanic Gardens and holds an advanced degree in forestry. He's also a kid in a candy store when it comes to flora.

Q: Without beating around the bush (sorry, can't resist), why do tree leaves change color every autumn?

R.H.: This is the final stage in the life of a leaf and has to do with four groups of pigments. You know about chlorophyll, the green chemical that does all the heavy lifting spring through summer. Then there are the carotenoids — carotene and a suite of related compounds — that give leaves yellow and orange coloring. Tannins produce the russets and browns. Finally, anthocyanin provides the classic autumn colors of red and purple. During the growing season, chlorophyll dominates. It's constantly being made to replace what's degraded by ultraviolet light, and the leaf appears green. Depending on tree species, yellow, orange or brown pigments have been there all along, concealed by green chlorophyll. In autumn, as chlorophyll production slows and ultimately stops, the yellows, oranges or browns emerge.

Q: What about red?

R.H.: Anthocyanin is genetically controlled, so some tree species get red and some can't. Unlike the carotenoids, anthocyanin isn't usually present during the growing season, but is produced in leaves with fall weather conditions. In trees that can go red, the amount of it depends on environmental factors. In ideal conditions, you see vivid pinks, reds and purples — that ooh-and-ah factor.

Q: What are those ideal conditions?

R.H.: Clear weather with warm days and cool nights within a certain range below 45 degrees but above freezing.

Q: OK. Wonky but interesting. How do leaves know it's time to turn?

R.H.: The basic triggers are shortening hours of daylight, the photoperiod is reduced and temperatures start to drop.

Q: It seems that the East Coast has more vivid colors than the West Coast. How come?

R.H.: They have that perfect suite of conditions I mentioned. Southeast Canada, a good chunk of the Northeastern U.S., and northeast Asia are justly famous as the best fall-color regions in the world. That's no coincidence. They have similar climates with clear skies, warm days, crisp nights, and they're close enough to oceans so there's no early killing frost.

In contrast, the Northwest autumn is often overcast or rainy with only minimal cooling at night; not optimal for fall color. Another reason has to do with numbers. In the Northwest, the effect of color is diluted because of species composition in our forest canopy, which is typically dominated by evergreens. With the evergreens outnumbering the changing deciduous trees, you don't get those huge sweeps of colors you see back east.

Q: When it's all over, why do leaves fall, thus forcing multitudes out with rakes?

R.H.: Well, that's another cool phenomenon. As autumn approaches, the tree prepares for leaf shed by moving nutrients in its leaves to winter storage in roots and stems. At this point, a hormone is produced that causes something called an abscission layer to form at the base of the leaf stalk, where it meets the stem. Think of it as the belly button of a leaf. Before the leaf falls, the layer blocks sugars in the leaf from escaping to the tree, although water still gets through. If it's a bright autumn, there's still a little bit of photosynthesis going on, creating more sugars that are trapped. They accumulate and contribute to the intense anthocyanin flashes of red and purple. Eventually, the layer forms a weak point that allows the leaf to break away cleanly without sap loss or leaving the stem open to disease.

Q: Where's the best viewing for fall foliage in our area?

R.H.: I don't say this because I work here, but in Seattle, I think the Arboretum is the best because of the tremendous diversity. In the native landscape, a great place to go in late October is north of Cle Elum in the Teanaway Basin to see the Lyall larches that offer large swaths of gold and yellow. This is one of the classic fall hikes in the Northwest.

Q: You're busy this time of year, especially with the recent opening of the Pacific Connections Garden on the southeast end of the Arboretum. What is that?

R.H.: The first phase, just completed, is five gardens that showcase five floras from around the Pacific Rim — Chile, China, New Zealand, Australia and Cascadia. Phase two develops the remaining 12 acres with forests that are typical for those regions.

Q: Clearly a long-term goal?

R.H.: As the saying goes, "the best time to plant a tree was 20 years ago and the next best is today."

Freelancer Connie McDougall of Seattle is a regular contributor to NWWeekend. Contact her: conniemcdougall@yahoo.com.