On a mid-November afternoon, the hardwood forests of Annandale-on-Hudson, New York are deep spaces. The leaf-covered forest floor wraps me in the comfort of warm browns and oranges. Somehow, though the leaves are dead, they still manage to glow gently. They amplify my footsteps with crunches and dampen my footfalls into a rhythmic sostenuto that has been playing for millions of years. How do you manage to be so still and so restless? Above me curves a hollow sky. No thick canopies insulate the ground on this autumn day. If I were to bend upward and take a picture, it may easily be confused as an image of death: bare branches, dark twigs, browning leaves. Yet standing in the middle of this vast sea of leaves and sky, the entire forest seems to hum. Those leaves still managing to hold on to their branches shake above me with each slight breeze. As if spontaneously, branches, and seeds drop all around. And at my feet these dead leaves, a symbol of darkening days and endings, are shivering with life.
To understand this feeling of impending life as I stand among you, I must first turn skyward. You now exist as various shades of dry browns and oranges on the forest floor, but once you shimmered green high above where I stand now. In those days you were packed with chlorophylls, molecules found in the cells of every leaf that give them their green pigment. It was from these chlorophylls that you, a tree’s representative offered to the sky, took in energy from the sun that in turn became the sugars that keep you alive. Yet, as everyone who lives among the temperate deciduous forests of the Hudson Valley knows, life passes seasonally. Since September, your greenery has been disappearing, a part of a seasonal process known as senescence. During senescence, individual cells and cellular structures within leaves die in a kind of programmed cell death. Some of the first structures to die are the chlorophylls, bringing out the reds, oranges, yellows, and browns so recognizable as autumnal. Eventually, the cells connecting you to your branch too must die. And so, you fall, transferring all your stored nutrients to the landscape of the forest floor and bridging the space between ground and ever-widening sky.
Following the path of a falling leaf back down to the ground, I can identify many types of leaves making up the litter at my feet. Oak leaves with thin pinnations, maples and tulip tree leaves with wide lobes, huge hickory leaves with leaflets in groups of three, wrinkled beech leaves sporting skeletal ribs where their vascular system used to run. Many of these leaves originated from nearby trees, but some have floated in from more distant locations. You move as far as the wind will carry you. Zooming in, I begin to understand the feeling of restless life that entrances me in the forest. Watching the litter closely, I see movement. Flies, ants, and beetles are crawling through the topography of the litter. In this miniature landscape they find shelter, nesting areas, and food. On wet days, close inspection reveals a world of tiny mushrooms, the fruiting bodies of fungi that are an essential part of the nutrient cycle of a forest. These dead and discarded leaves have become a world of micro-habitats, allowing life to flourish at even the smallest scales.
In exchange for the food and shelter, these insects and fungi provide a vital service for the adult trees who have sacrificed their leaves. Working together, insects and fungi break down leaf detritus into its basic elements. The insects take the lead. They consume the leaf litter, breaking it up into smaller pieces for the fungi. Then, along with a host of bacteria species also living within the litter and soil, the fungi reduce what is left of the leaves into water, carbon dioxide, and nitrogen, which can be consumed by nearby plants or released into the atmosphere. You form tight relationships with these tiny life-giving organisms. From the moment you unfurl out on the branches of your mother tree, an entire world of fungi is growing on you. As you eventually fall from the tree to meet the forest floor, this community comprised of thousands of species begins changing. In a process known as succession, old species of fungi give way over time to new ones, guiding the progression of your decay. Without you, they would have no food and no home. Without them, your nutrients stay locked up within you rather than nurturing the forest around you.
However, not all decomposition looks the same. Each species of leaf contains a unique combination of nutrients and structural molecules, causing no leaf to decay quite at the same rate as another. Mixed up together on the forest floor, each of you contributes to decomposition’s complex balance, shaping the rhythm of the forest’s regeneration. A prime example of the singularity of each leaf abounds at my feet: the American Beech. These delicate, papery leaves contain lower levels of nitrogen and higher levels of carbon than other species, a combination that is notoriously difficult for decomposition. They also have higher amounts of strong, starchy molecules guarding their cell walls known as lignins. These molecules are tough to break down, further impeding beech leaves’ decay.
Removing the magnifying glass and turning once again toward the sky, the tree canopy also shapes the progression of decomposition on the forest floor beneath it. While the tree canopy serves as a direct source of nutrients for the forest system by offering the fallen leaves that make up decomposing litter, a dense canopy also shades and protects the forest floor from sun and rain. The result is dryer, cooler ground less suited to fast decomposition. As for the beech, their canopies often are denser and more protective that those of other trees in this temperate forest. In areas of the forest where beech trees cluster together in stands, the high concentration of leaves with unfavorable chemistry and the density of the canopy can slow this vital regenerative decomposition process, keeping beeches from accessing the nutrients that they need.
The solution to the beech’s problem lies in its neighbors. As I look around me, I see not just beech, but hickory, oaks, maples, ashes. Each of these trees supplies its own leaves with their unique combination of nutrients to the litter landscape. This collaboration invites a trove of insects and fungi species to find a home in an increasingly complex terrain of microhabitats on the forest floor. Each of leaf species offers something different – a unique kind of shelter, a particular kind of food – making room for a great multitude of decomposers within the mix. In turn, each of these detritivores brings its own strengths to the tasks of litter decomposition and nutrient cycling. Many even serve as food sources to support a myriad of birds and mammals that make up the forest community.
But you, the individual leaves, even support each other. As leaf types differ in their chemical compositions, the combination of many different species can make up for slower decay among leaves of a particular species, like beech. This facilitation takes the form of a kind of nutrient sharing. When oak and hickory and maple leaves join beech leaves on the forest floor, these faster decomposers release nutrients to be shared by all. In turn, the beech leaves can then take up these nutrients, giving them a much needed boost in their decay process. A diversity of adult trees also transforms the canopy cover of the forest, creating patches of space within a previously dense beech-dominated canopy. These openings foster warmer, wetter litter, and in turn, faster decomposition. Surrounded by new neighbors, beeches and their leaves become part of a more productive team, fostering the cycle of growth and decay that forms the backbone of the forest ecosystem.
Turning around, I begin to tromp back out of the woods. As I attempt to retrace my path, winding in and out of the trees, my eyes are trained on the leaves at my feet. With each footstep, I notice a new leaf and regard it with joy and wonder. Though we think of you – when we remember to think of you at all – as a sign of death, you give the forest life. First reaching for the sky through spring and summer, then falling to the ground in fall and winter, you repeat an age-old dance of rejuvenation. In your complicated landscape across the forest floor, you make millions of homes for some of the smallest members of the forest community, who work together to nurture the trees around you and provide a foundation for the entire forest community. You support one another, sharing nutrients and fostering flexibility in the micro-climates of the forest floor, so that each of you may reach your potential. The comfort and exhilaration that I find in the forest, the life with which it brims, begins in your midst. Life begins in the litter.
Sources
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