It’s the last day of October and probably the last Sunday afternoon in which to soak in the beauty of autumn coloration at our Midwest latitude and elevation. How beautiful the sugar maples and hickory trees were today as the afternoon rays filtered through their rustling yellow and golden leaves. While hikers, tourists, and artists enjoy the beauty of the autumn colors as green gives way to yellow, golden, and red, there is much more here than meets the eye.
Every Autumn season represents a major biological or ecological event in which millions of tons of leaves on deciduous trees and shrubs enter a transition that eventually leads to the familiar fall of autumn leaves. This marvelous phenomenon which is particularly striking in the temperate latitudes of North America, Europe, and Asia, is a great example of how the created order of planet Earth, noted in our previous entry of Oikonomia (see Finding Purpose in the Created Order: Stewardship of Time), includes a provision for the survival and fruitfulness of the forest trees and shrubs.
Leaf coloration is an outward display of the internal physiological processes that are at work preparing broad, thin, delicate leaves to avoid the rigors of the cold, dry, winter air. Some plant species such as the pines and other conifers retain their leaves (needles) throughout the unfavorable season. In these plants, preparation for winter or drought involves more subtle changes such as development of antifreeze compounds in the needles and twigs. These changes enable evergreens to tolerate cold and dry conditions and allow the leaves or needles to gather the rays of sun to a limited extent in winter and then revive to full activity again in Spring.
Unlike the evergreen trees and shrubs, deciduous species prepare for the harshness of winter by more drastic measures. Each leaf is subject to something like an amputation, more commonly called leaf abscission by physiologists. But, what many observers do not know is that this preparation for winter in the temperate zone begins long before the cold temperatures of October and November arrive.
There are two reasons why cold temperatures themselves are inadequate to prepare trees and shrubs for winter. First, if trees and shrubs are not preconditioned or acclimated to freezing temperatures before they arrive, the freezing conditions will kill the leaves right on their branches. Second, as we can see from the unseasonably warm temperatures of this month, daily temperatures are too variable to be a reliable predictor of closeness to winter. Therefore, our questions must become, “How are trees and shrubs “alerted” to the fact that winter is coming, and how are the leaves prepared for abscission?
Although cold temperature can influence the processes of leaf coloration and abscission, the plants have a more reliable signal–the shortening daylength. Thus, the consistent laws of creation and especially the predictable relationships of planetary motion– the revolution of the Earth around the Sun, and the Earth’s rotation on its tilted axis–afford the trees and shrubs with a predictable pattern of decreasing daylengths such that leaves can be acclimated before cold temperatures arrive.
The fact that deciduous woody plants are programmed to respond to this “warning in advance” of the cold temperatures is a wonderful illustration of how our Creator uses the light of His Word to warn us of danger in advance. Indeed, by His grace, we are often warned repeatedly. Hopefully, we eventually will experience the tender but firm pulling of His Spirit to turn from our wandering tendencies to the safety of godly paths.
Another picture or metaphor of God’s created order is evident through what physiologists have revealed about the changes leaves undergo during their acclimation in response to shortening days of late summer. When we observe the first hint of leaf coloration, it is evidence that the green chlorophyll pigments are being broken down. Along with chlorophyll, many of the leaf proteins are also digested into soluble amino acids. Did you know that leaves can digest their own proteins? The resulting amino acids, soluble nitrogen (e.g. from chlorophyll), and other nutrient constituents of the summer leaf now become transportable. Guess what their destination will be?
Soluble products of pigment and protein digestion are transported from the now coloring leaf to the nearby woody twigs and branches where they will be stored in readiness for growth of new shoots and leaves when the buds begin growing in the spring. So, instead of simply allowing each leaf to abscise from its place on the branch, many of its valuable nutrients are “rescued” from the leaf and retained for use by the parent plant before leaves are lost. Some nutrients, including lots of carbon remain in the falling leaves and become nourishment for arthropods, earthworms, and microbes of the soil below.
We can see in this a marvelous “economy of creation” in which the woody plants retain access to nutrient building blocks while also preparing their leaves for abscission and leaf fall. At the same time the “rescue” of these nutrients is not so efficient or extreme that the leaves are of no food value for consumers in the soil below.
Thus, in response to predictable daylengths which are based on predictable laws of planetary motion, deciduous woody plants are prepared for the onset of cold temperatures through changes in their branches and leaves while at the same time allowing for the conservation of nutrients for future growth.
Thanks be to our Creator for His wisdom as evidenced through His provision of a mechanism to “forewarn” creatures of damaging temperatures as well as to allow conservation of valuable nutrients otherwise lost during leaf fall. May we as God's image-bearers exercise good stewardship of the light of His truth and wisely steward the resources He has loaned to us–our time, opportunities, talents, and natural resources.