'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?