In Colorado, the fall season harkens shorter days, cold nights, and the promise of the first snows. While many eagerly anticipate the upcoming ski season, many too revel in this time of year enjoying the famous and often spectacular display of color put on by the leaves of aspen trees throughout the high country. While appreciating the fall foliage, stop and consider why the leaves of aspens are turning yellow, orange and red and being shed for the winter.
Aspens, like many deciduous trees in North America, go dormant in the winter to conserve energy and protect themselves from freezing temperatures. An important part of this process is shedding their leaves. Day length, temperature and moisture all play a role in signaling the tree to go into dormancy. As the tree prepares to shed its leaves, it breaks down and reabsorbs the pigments that give it color, starting with the green-colored molecule chlorophyll. Through this process, the less abundant pigments become visible including those that are yellow and orange. As the chlorophyll breaks down, trees also produce new pigments, including those responsible for the color red. As the season continues, the leaves eventually become brown and fall from the tree.
One of the scientific competencies at the Denver Museum of Nature and Science is paleobotany, or the study of fossil plants. It has been long hypothesized that many deciduous trees that live in our backyard today, have ancestors that first evolved to shed their leaves in response to changing light conditions, as opposed to either changing temperature or moisture. Ancestors of these trees can be traced back to the time of the dinosaurs, when the Earth was, for the most part, a greenhouse. As a result, both the north and south poles would have been free of ice and completely covered by forests. Trees living near the north or south pole would have flourished in a season of light, when they leafed out and grew as much as possible, and they would have ceased to grow through a season of darkness, when they adapted to lose their leaves and lay dormant.
Research at the Denver Museum of Nature and Science focused on the extinction of the dinosaurs suggests that many plants also went extinct too. Caused by the impact of an asteroid 10 kilometers in diameter that hit the Yucatan Peninsula, the first minutes to hours of the extinction saw a massive shockwave, an intense firestorm, towering tsunamis, and an atmospheric thermal pulse equivalent to temperatures in an oven that would be hot enough to bake cookies. As these effects subsided, dust in the air persisted and blotted out much of the sun for months to even a year or two. Many plants were in trouble, since they need abundant light to photosynthesize. However, those plants in the polar regions that had evolved to be deciduous were already adapted to a season of darkness, and, as a result, fared better. Today, many trees that lose their leaves in the fall, have ancestors that survived the darkness that followed the impact of the asteroid that killed the dinosaurs.
Today, while the fall climate and changing day length in Colorado signals the aspens to shed their leaves, changes in weather from year to year determine the intensity and length of the fall foliage display. The best conditions include cool night air and sunny days proceeded by plenty of growing season moisture.
(Images by Kate Cummings)