Once we come to some understanding — however rudimentary — of this ongoing process and its wide-ranging effects in both the plant and animal kingdoms, we’ll have gone a ways toward comprehending the natural world here in the northern temperate zone. Before we start poking around in the leaf litter, however, let’s take a quick look at the processes that produce the raw material for that rich and distinct zonal layer we so often stride across without a second thought.
In winter, next year’s growth is already contained in buds at the branch tips of the numerous deciduous tree, shrub, and vine species native to the Smokies region. These “end buds” are so distinctive they can often be used to identify species when no foliage is present. Smaller “side buds” may also be produced between the “bud-scale scar” (marking last year’s twig tip) and this season’s “end bud.”
During the spring growing season, the buds elongate out of their protective winter wrappings and produce new twigs, leaves, and flowers as if by magic. To a great extent, this growth pattern is keyed and controlled by growth-regulatory hormones called “auxins.”
We all learned in grade school that leaves are green because they contain a dominant pigment called chlorophyll that is responsible for photosynthesis, the process by which plants combine water and carbon dioxide with the sun’s energy to create high energy sugar compounds. Only plants can directly convert solar energy into chemical energy; and, of course, life as we know it on this planet would not exist without their ability to do so.
The green pigment produced by chlorophyll masks other pigments naturally present in leaves. Cool temperatures and decreasing daylight in fall activate a decrease in “auxin” levels, causing the formation of corky layers between leaf stems and twigs. This hardened “abscission layer” cuts off the flow of water and minerals to the leaf so that the chlorophyll breaks down; thereby revealing accessory underlying yellowish pigments such as those produced by tulip tree, birch, and hickory.
Other fall colors are direct byproducts of hardened abscission layers. The reds in maples (especially sugar maple), black gum, Virginia creeper, poison ivy, cherry, dogwood, and many other species are produced by chemical reaction that takes place in leaves with high sugar content. Dry weather (which reduces sugar content) and the absence of early cold snaps (which activate abscission layer formation) are conditions not conducive to truly dramatic fall color seasons.
Eventually the connection between the leaf stems and twigs at the abscission point weakens and severs so that the leaves float to the ground, creating a mixed-species leaf-litter layer that is renewed each fall. This upper layer is the bulk source of humus formation, which in turn produces the underlying soil that is the reservoir of calcium, nitrogen, phosphorus, sodium, magnesium, potassium, and other essential elements that sustain the ongoing forest and those of us dependent upon it; that is, all of us, in one way or another.
Go ahead and get your hand dirty. Delve into the leaf-litter. There you will find distinctive layers of decomposition. First, there is the relatively intact layer of newly fallen material in which the leaves are still recognizable as to species. Next come skeletonized, sometimes slimy leaf layers that can be difficult to identify in regard to species. But note that some leaves with high carbon to nitrogen rates (beech and sugar maple are examples) may take three years to fully decompose.
And finally, there is the gritty, dark, moist humus zone. This layer marks the point of departure between formerly organic living systems and those to come. Ecologists recognize two basic sorts of humus: “mor” (which is relatively acidic and consists of the thick litter characteristic of conifer dominated forests) and “mull” (which is relatively alkaline and characteristic of the rich, diverse litter created by deciduous forests).
The decomposition of leaf-litter is carried on by a variety of plant and animal mechanisms including bacteria, slime molds, fungi, protozoans, and various ground-dwellers. Poke around in the litter before hard winter really sets in and you’ll turn up various critters — slugs and snails, earthworms and roundworms, millipedes and centipedes, beetles, ants, termites, spiders, and many more — still busily going about their eternal recycling chores.
Some litter dwellers feed directly on the fallen leaves, needles, twigs, branches, and trunks, while others prey upon the feeders. But all are part of the same litter-food-soil web, and, in death, all contribute their carcasses — along with larger animal carrion and feces — to the total complex that becomes humus and the enduring and life-sustaining earth.