A blustery gust rattled the branches of the red oak near our house, tearing off handfuls of worn out leaves and sending them skittering across the street.
Their seasonal work coming to a close, the chlorophyll had already started to break down and drain from the leaves. With the green chlorophyll gone, the other colored chemicals in the leaves were starting to shine through in brilliant reds and oranges and yellows. The color and intensity of autumn leaves on deciduous trees and shrubs is determined by how much of which of those chemicals is in the leaf — a mix that varies by species and by the productivity of the season.
So why didn’t these leaves fall during the stiff winds of summer?
It’s not just a weakness in the stem (called a “petiole”) of a worn-out leaf: the plants systematically prune themselves through a process of “abscission.” (“Abscission” is from Latin for “to cut away”—think “scissors.”) Abscission starts with the plant salvaging the chlorophyll and, later, other chemicals from the leaf. Once the valuable chemicals are resorbed, the cells in the “abscission zone” at the end of the petiole will build up a corky layer where the leaf will disconnect. On either side of where the break will be, other cells produce chemicals to water-proof the upcoming raw end.
When ready, nearby cells will either create enzymes to digest the connections between cells in the abscission zone or nearby cells will swell with water until they burst to break the abscission zone. The cutting methods can be complex and apparently vary by species, but either way, the end of the petiole is cut and the leaf is cast off.
Plants will also cast off fruit, spent blossoms, and damaged leaves through abscission, and they’ll sometimes abscise otherwise healthy leaves during drought or insect attack. Evergreen trees and shrubs abscise their old leaves, too, but gradually enough that we usually don’t notice. (And, yes, some animals abscise body parts, such as certain lizards that cast off their tails when trying to escape.)
That’s not the end of the leaf’s role, however.
When leaves reach the ground, they become “litter” — in a biological sense rather than a garbage sense. As decay progresses, such litter on the forest floor becomes “duff,” loose, partially rotted organic material. “Humus,” the more-fully-rotted, organic-rich section of the soil, will develop over time from the duff.
Far from being a nuisance, natural litter and duff play valuable roles in woodland ecology.
Litter and duff insulate the soil from both winter’s cold and summer’s heat. A heavy layer of duff can even shelter the soil and its inhabitants from fire.
When winter finally hits full-bore, the mantle of fallen leaves will help protect the soil from the eroding effects of wind and rain. The litter absorbs the impact of the pummeling raindrops; the slowed water trickles down toward the soil, soaking into the litter and duff as it goes. The layers of wet leaves then hold in the moisture all season, shielding the soil from drying.
The harder, denser parts of the leaves — the petioles and veins — add texture and woodier material to the developing soil. Such amendment improves both soil drainage and water storage, and diversifies the microhabitats, as well. Many seedlings sprout in the moist, rich fluff, getting a head start as the roots make way to sturdier soil.
Litter, duff, and humus are homes for an astounding variety of organisms. Arthropods, such as millipedes and sowbugs, work over the fallen leaves, shredding them into more manageable chunks. Earthworms grab loose leaves and pull them down into their burrows for leisurely consumption, effectively tilling the soil and furthering decomposition. Bacteria and fungi work to decay the leaves, breaking the sheets down into still smaller bits useable by plants.
The tiniest animals, from nematodes to mites, plow through the litter and duff, consuming both the leaves and smaller leaf-decomposers. Spiders, carnivorous beetles, centipedes, salamanders, lizards, snakes, and shrews hunt through the litter and duff while hidden and protected from their own predators.
In an interesting circle, salamanders eat earthworms… but earthworms eat salamanders’ hiding places. In fact, several researchers have noted European earthworms spreading through some northeastern American forests have denuded the soil of valuable litter and duff, resulting in a less-protective habitat and decreased population of woodland salamanders and other duff-dependent species.
Over time, the fallen leaves’ decomposition slowly frees the stored nutrients, returning about 70 percent of their original nutrients to the soil. And, of course, those nutrients are recycled into — among other things — new leaves.
How many leaves? Of course, that depends on the kind of plant (and the size of its leaves), as well as its age and condition, but numbers for trees range from 2,000 to 200,000 leaves. Over 60 years, a large tree could drop 3,600 pounds of leaves to the forest floor, meadow, stream, or yard.
Unless they blow too far away, most of those leaves will return to the plant that dropped them, providing critical nutrients and habitat protection for other forest dwellers in the meantime. Though some rush to rake and haul off fallen leaves, I find this natural recycling and habitat enhancement abundant excuse to leave them be.