'Tis the season


    In the Rocky Mountains, wedged between Summer Tourist Season and Fall Big-Game Hunting Season, is a relatively brief interval of crowded highways known as Aspen Season. It has nothing to do with the Colorado resort town, and everything to do with the tree, whose leaves change color.
    Technically, the leaves don't exactly change color. The yellow (or red, in some stands) was there all along, but was masked by the green chlorophyll during the summer. As summer fades, so does the chlorophyll, and the submerged colors emerge for a few days before the leaves fall.
    But even without leaves, aspen continue to practice photosynthesis. You might have noticed a greenish tinge in aspen bark -- that's cholorphyll, and it allows the aspen to continue to respire, and absorb atmospheric carbon, through the winter.
    Aspen are odd trees in other respects, too. They generally grow from root suckers, rather than seeds, so all the trees in a grove are genetically identical, and could be considered one giant organism.
    Their leaves move in the slightest breeze because their stems are almost flat, and the stem surface is perpendicular to the leaf surface, so there's something to catch the breeze, no matter which way the wind blows.
    That's why they're often called "quaking aspen," frequently shortened to "quakers" or "quakies."
    Botanists tell us that the color change is not a response to colder weather, but instead to reduced sunlight from shorter days.
    Now, this is just a personal impression, but it seems to me that the peak color comes later now than it did a few decades ago. Back in the 1970s, the peak seemed to come about Sept. 10, and now it's more like Sept. 20. (Bear in mind that some years, there's no peak at all because an early storm brought most of the leaves down.)
    If it's "hours of sunlight" rather than "atmospheric temperature" that produces golden leaves, then global warming doesn't explain this later peak aspen display. Is there some other factor? Or is my impression of a later peak just plain wrong?

T'was The Season
Mickey D
Mickey D
Sep 20, 2009 06:37 PM
I think you are on the right track, and the coloring of the leaves is probably a combination effect.

My wife and I were married on Sept 29th, 30 years ago in VT. We chose the date because it was a pretty good bet to be a peak foliage weekend. And it was, as usual, spectacular. The peak period in the past several years has been no earlier than Oct 12 and often close to month's end.

This year, State Environmental experts predict a peak on or about Oct 20. But, they do hedge by saying foliage season is mid Sept to Late Oct. thereby selling more hotel rooms.

It seems to me the leaves are turning later, and although sun-time is the dominant factor, doesn't freezing play a role in hastening the process. How else would one explain how I can sit on top of Mount Mansfield and see every progression of color from peak foliage, up-high, to mid-turn in the foothills, and still complete green in the Valleys? In any case, maybe after our 60th anniversary we can enjoy the most brilliant days of Autumn during Thanksgiving Dinner.