“Those falling leaves drift by your window, those autumn leaves of red and gold….” So goes the old song. Here in Colorado, those autumn leaves are mostly gold: aspens and cottonwoods. For red, maples are the best but there are none of those in our native flora. Nonetheless, few sights equal groves of golden aspens against crisp blue skies in our mountains. In any event, why the beautiful colors as autumn arrives? And, why are those leaves falling?
Botanists have studied for years to understand the color changes in trees and shrubs in autumn. It turns out that three factors are involved: pigments in the leaves, the increasing length of the nights, and changes in the weather. Among these, the increasing length of the nights principally regulates both the color change and leaf fall. As days grow shorter and nights grow longer and cooler, biochemical processes begin in leaves that change their colors from green to gold, orange, or red.
There are three pigments in leaves: chlorophyll, carotenoids, and anthocyanins. Chlorophyll gives leaves their basic green color and permits photosynthesis, the chemical reaction that enables them to use sunlight to manufacture sugars for their food. Carotenoids produce the yellow and orange colors familiar in flowers, fruits, and vegetables. Anthocyanins produce the red and purple colors of flowers and fruits. Different species have different proportions of carotenoids and anthocyanins, but chlorophyll predominates, and during the growing season, its green masks other pigments. When the season begins to change, chlorophyll production slows and finally stops. Then the carotenoids and anthocyanins are unmasked and the leaves change to characteristic autumn colors.
Weather conditions do have an influence. When a succession of warm, sunny days is followed by cool, crisp (but not freezing) nights, we can expect the most spectacular color displays. This year, we experienced more spring rain than in the past several years, but they have had little effect on the fall colors. However, rain (and in the mountains, snow) followed by winds in the early part of the fall this year unfortunately knocked many of the golden leaves from the aspens. Until recently, the days in town were warm enough to keep chlorophyll in the leaves, but a nice change should come soon if freezing nights do not interfere.
In the temperate zone, leaves fall from trees in autumn to ensure the continued survival of the trees. Leaves of deciduous trees (those that shed their leaves each autumn) are thin and fragile and contain watery sap that freezes readily. In response to gradually declining intensity of sunlight in early autumn, leaves begin the process leading up to their falling from trees. Veins that carry fluids into and out of the leaves gradually close off as a layer of cells forms at the base of each leaf. Once the separation layer is complete and connecting tissues are sealed off, the leaf will fall. Thus, the tree can endure through the winter, its vital fluids protected within the trunk and branches, and produce new leaves next spring.
Of course, not all trees shed their leaves in autumn. Evergreens (pines, spruces, firs) retain their leaves, which we know as needles. Those needles have protective waxy coatings and the fluid inside their cells contains substances that resist freezing. The needle-like leaves of evergreens can withstand all but the most severe of winter conditions. Evergreen needles survive for some years but eventually fall because of old age and are replaced by young ones.
Doug Nichols was a Scientist Emeritus with the U.S. Geological Survey and a Research Associate with the Denver Museum of Nature & Science. He was a resident of Berthoud. We mourn Doug’s untimely passing in Jan. 2010.