Vol. 17 Issue 3, Fall 2012
by Emily Bzdyk
Who doesn’t love the fresh, crispy crunch of a fallen leaf? It can’t be just me! Fall is once again here, so let’s turn our attention to the hallmark of the season—the autumn leaf. We can’t help but notice the leaves as they change color on the trees around us this time of year, but what about after they fall to the ground? Many people tidily rake and bag up or blow away these leaves. But the leaves on the ground are actually an important part of our ecosystem, and it is much better to leave them be (pun intended). In temperate zones like ours, deciduous trees drop their leaves in the fall, creating a beneficial layer of dead leaves, or leaf litter, on the ground. This leaf litter is essential for many animals and the temperate forest ecosystem. Leaves are used for food, shelter, and eventually become the new rich topsoil for future plants to thrive on.
Leaves change color primarily due to the influence of day length. As days grow shorter and nights grow longer, this signals the trees that they should slow production of essential compounds, like chlorophyll in the leaves. As chlorophyll disappears, the orange and yellow carotenoids and red anthocyanins are revealed. Different species do this at different times. For example, oaks are often some of the last trees to change color and drop their leaves. The brilliance of the fall palette is also influenced by temperature and moisture. A warm summer with cool nights increases sugar production and retention, which increases the red anthocyanin content in leaves, creating a more dramatic fall leaf color. After the leaves change color, they prepare to drop by shutting off their veins’ connections to the tree. Once fluid movement is stopped, the leaves fall to the ground.
Once on the ground, and away from the nourishment of the tree, the leaf begins to dry out and decompose. It loses even more color as the cells desiccate and die; most leaves become a brownish color. The leaves at this top layer on the ground are known as the litter layer. Below this is the fermenting layer, where leaves are compacted and already rotting. These leaves tend to be moister as water becomes trapped here, and the leaves are broken down. Below this is the humus, which is rich and black and consists of completely rotted plant and animal matter. Small animals and arthropods such as mites, springtails, nematodes, woodlice or pillbugs, and millipede feed on the dead leaves. They are detrivores, meaning they feed on dead material. Earthworms are perhaps one of the better known of these; they eat the leaves and break them down into tiny pieces. Their feces, known as castings, are a site for microorganism activity. Organisms such as bacteria and fungi are the primary agents of this decomposition. These convert the leaf pieces to minerals and nutrients, which can be used by the trees and other plants. All of the small creatures that live in and feed on the leaves, in turn, can be food for small predatory arthropods and other animals. Thus, the fallen leaves form an essential part of many nutrient cycles and food webs.
Besides serving as food, the leaves are an important habitat for many insects and arachnids. Beetles of all sorts roam in the leaves, feeding on the plant material or hunting other small arthropods and mollusks, like slugs and snails. Harvestmen, or daddy longlegs, patrol the leaves for decaying material or other small animals they manage to catch. Wolf spiders and jumping spiders hunt in the cover of the leaves for insects. Web-spinning spiders anchor their webs in the leaves. Crickets that you hear singing in late summer and fall hide in the leaves and construct burrows in the soil beneath. Many moth caterpillars, like Isabella Tiger Moths and Leopard Moths, create their cocoons under the leaf litter, relying on the protection and insulation of the leaves to overwinter. Fly larvae or maggots, including crane flies, fungus gnats, house flies, and hover flies, all can be found in the leaf litter, aiding in decomposition. Salamanders, toads and other small amphibians rely on the shelter and moisture provided by the leaves and eat small invertebrates found there.
Birds such as Common Yellowthroats, juncos, White-crowned Sparrows, towhees, mockingbirds, thrashers, jays, thrushes, quails, pheasants and Wild Turkeys all feed in leaf litter. Ovenbirds forage in the dead leaves, and constructs their nests, the “oven,” on the ground. Jays use the leaves as cover for their cache of nuts and acorns. The numerous insects and other small animals that live in the leaves are a vital food source to many of these birds. In addition, the leaves hold small nuts and berries that fall from many trees, providing another type of forage for birds and animals. The leaves, twigs and plant material in the litter serve as nesting material as well.
Rodents, such as mice, squirrels, and chipmunks, use the leaves as nesting material and rely on leaves for insulation in their burrows, especially throughout hibernation during the winter months. Mice and related rodents also feed on earthworms and insect larvae that live in the leaves. With bunches of dead leaves, squirrels construct nests up in tree branches.
Besides providing animals with essential food and shelter, leaf litter acts as a natural mulch and insulator to fertilize, nourish and protect plants. These are some of many reasons to let leaves remain on the ground instead of attempting to “clean” your yard of them in the fall. Another reason is to avoid using things like leaf blowers, which emit harmful pollution exhaust and waste resources. Raking and bagging leaves is also time consuming, especially when considering the benefits of simply letting leaves lie. As you enjoy the changing leaf color this autumn and watch as they fall gently to the ground, let them stay there. By doing so, you will foster a much more diverse and healthy backyard ecosystem and support wildlife that can be fun and fulfilling to observe.
Johnson, Elizabeth A. & Kefyn M. Catley “Life In The Leaf Litter” (American Museum of Natural History).