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By the side of the trail

February 18, 2014

As you wander in a woodland spring,
consider the wonders of the world at your feet

In earliest spring, before flowers and leaves brighten the forest, there’s already a bustling world beneath the brown leaves at the edge of any trail in The Morton Arboretum’s East Woods.

All those leaves are not just litter. This borderland—the few inches of leaves above and the few inches of fine roots just below the soil surface—is home to countless creatures, and every tiny bacterium and bug plays a part in creating the soil that holds and nurtures the forest’s trees.

Nudge aside a tattered oak or maple leaf and you may find one of the most familiar digesters, a roly-poly that can curl up tight as a little brown pill. It is more closely related to a lobster than a beetle. You’ll find beetles too, and earthworms, and snails, and ants, scurrying about now that the soil is warming up, using the leaves and twigs for homes and refuges. But the most abundant leaf-chewers are much tinier: mites and springtails, nearly microscopic arthropods that live in the top layers of the soil.

“When a leaf falls, big shredders start eating it,” says Soil Scientist Bryant Scharenbroch. They pulverize it into pieces that a smaller creature can digest. In each animal’s gut, the leaves support millions of bacteria that break down the leaf tissue to extract sugars for energy. What the animals leave behind is easier for soil bacteria and fungi to feast on. Every bit of dead leaf or root will be food for many organisms, each of which derives a little bit of energy, on its way to becoming soil.

Some parts of a leaf are easier eating than others. Often you’ll find one that is just a skeleton of veins, like a picked-over turkey carcass. But sooner or later even the tough parts will become lunch for snails or fungi.

Fungi are everywhere in the leaf litter and the top layers of soil. We notice them when they form a mushroom, a temporary device for distributing their spores. But fungi are there all the time, as spreading networks of nearly invisible filaments called hyphae that penetrate dead wood, leaves, roots, and animals, extracting nutrients and piping them away, often to share with living plants.

The activity in the soil borderland—the “organic horizon,” as Scharenbroch puts it—sustains life because it’s not a one-way street; it’s a bazaar where nutrients are swapped among different living things.

As millions of bacteria, insects, earthworms and other organisms die, fungi and bacteria break down their bodies into nutrients such as nitrogen that are essential for the lives of trees and other plants. Most tree roots are in the top layers of soil where these nutrients are handy. Trees often live in symbiotic relationships with fungi; the roots swap some energy-rich sugars to the fungi in return for help collecting nutrients and water.

The leaf litter food chain has its carnivores—spiders, salamanders, crayfish—and carnivores of carnivores—robins, moles, skunks. “The litter also is important for camouflage,” says Education Guide Beth Bengtson, who leads school groups and scouts on field trips in the woods. Some animals, like snakes and toads, use its concealment to hunt. Others, such as shrews and mice, are hunted, hiding from owls and hawks.

These animals, too, are part of a community that cycles the chemicals essential to life from the dead to the living. The forest floor at the side of the trail is where last year’s leaves become tomorrow’s.

An abbreviated form of this story appeared in the Spring 2014 issue of Seasons, the member magazine of The Morton Arboretum.