5 min read

As Leaves Fall, Celebrate All of the Life that Unfolds on Their Stage

On a green leaf, there is a small brown patch with an oval-shaped hole cut in it—an empty leaf mine.
This spot on a black tupelo leaf (Nyssa sylvatica) was created by a tiny caterpillar (Antispila nysaefoliella) living in between the layers of leaf tissue. Photo: David Jeffrey Ringer

Ten minutes after sunset, I stood below a black tupelo in Central Park, agape. From its broad, upright crown to the sloping branches that nearly touch the water several stories below, the tree was so completely orange, so brilliant in the dim evening, that I thought I could feel its radiant heat on my cheeks, and for a moment, I believed that the faint pink glow on the clouds overhead was cast there by the tree itself.

“Your face says it all,” offered a passerby.

Every leaf on a tupelo tree is a theater for life’s great unfolding.

Some of the performers are organic molecules that interact with photons from the sun. The tupelo leaves were dark, glossy green all summer long, crammed full of chlorophyll molecules that absorb the energy of red and blue light (letting the green wavelengths bounce away, perhaps into a human retina) in the course of their work to create sugars that feed the plant and most other life on Earth as well—and producing atmospheric oxygen as a byproduct, another fundamental requirement for many of us living beings.

But winter is coming, and the tupelo must shed its leaves to survive that harsh season. So it begins to starve them, pulling some valuable chemicals and nutrients out of the leaves and into its woody tissues for safekeeping during the dark, cold, dry times and ultimately cutting off the leaves from water and nutrients with a physical barrier that grows at the base of each leaf stalk.

As the chemical environment inside them begins to change, tupelo leaves produce red-light reflecting pigments called anthocyanins. Chlorophyll molecules break down and are not rebuilt as the leaf begins to die.

Then, for a few precious days, the colors we see are determined by the anthocyanin newcomers and the yellow-light reflecting carotenoid pigments that have been present all summer long, working hard alongside the chlorophyll molecules but effectively outshone. No longer, though: This is their time. The ascendence of these molecules drives billions of dollars in human economic activity every year. We like to watch.

Other actors on the leafy stage are more familiar to us in principle if in fact rather remote both in scale and lifestyle. The moth species Antispila nysaefoliella, the tupelo leafminer, is smaller than a short grain of rice as a winged adult. Despite bold silver and ebony stripes on its tiny wings, its minuscule size and finicky habits ensure that billions of human beings live and die without ever seeing one or even suspecting its existence.

To us, leaves can seem basically two dimensional, but of course, that’s not correct. Leaves are made of cellular layers stacked upon each other like pages of a magazine, with various specialized structures like veins to move liquids around, hairs and waxy coatings for protection, and flexible pores or stomata to exchange gasses with the outside air.

The larva (or caterpillar if you prefer) of the tupelo leafminer moth lives its life entirely in between the layers of a single tupelo leaf, munching away on the nutrient-rich cells inside the leaf and leaving the tough epidermis and cuticle layers intact to shield itself from most external threats and influences. Extending the magazine metaphor for the leaf, the larva is eating the interior pages and leaving the covers intact.

What a life: surrounded by more food than you could ever eat and protected above and below from all of your enemies—well, almost all. The sole aim of certain small female wasps is to find a tupelo leafminer larva, pierce its leaf mine fortress, and lay her egg inside so that her offspring can consume the moth alive.

She will locate her quarry by tapping, tapping, tapping on the leaves and sensing the change in vibrations between normal, intact leaf tissue and hollow leaf mines containing moth larvae.

The leafminer can sense that tapping, though, and it has tools to fight back. Candace Low discovered that tupelo leafminer larvae use special hardened structures on their body segments to generate vibrations of their own, making clicks and rattles strong enough when amplified by the leaf tissue that a even a human being can hear them. Dr. Low’s observations suggest that the larvae use some of these vibrations to deter parasitoid wasps from completing their singular mission—perhaps by confusing or frightening them.

These life-or-death rhythmic, acoustic performances play out on the leaves of every tupelo tree in Central Park and across much of eastern North America, whether any of us pay attention or not.

If a tupelo leafminer larva is lucky and successful, it will grow to its maximum size, approximately 3 millimeters long. It will have created a hollow mine in the leaf approximately the size of a human thumbnail. It will then use its jaws to cut its way out of the leaf. It will chew out an oval-shaped piece of leaf epidermis from the empty mine, creating a pocket-like shelter for itself by binding the upper and lower bits of leaf epidermis together with silk. Picture a kid in a potato sack race.

It will depart the leaf that has been its entire world up to this point, rappelling down on a silk fiber and wriggling away—it does not have legs—until it finds a suitable place, like a small bark crevice, to affix its tiny shelter and wait. Hopefully, its camouflage job is good enough to evade the probing bills of Brown Creepers or Ruby-crowned Kinglets and the hungry jaws of ants through autumn and the winter months.

Imagine it there, secure in its shelter, which is smaller than the end of a pencil eraser, as the first frost glazes the world and the first snowfall buries it. The larva waits and waits, snug inside its sleeping bag made from a tupelo leaf, as Earth tips back and forth, and finally, when the time is right at the height of the new year, it will complete its metamorphosis and emerge, winged.

That is just one species on just one plant. Tupelo leaves also host two microscopic mite species: One creates pimple-like bumps on the leaves, and the other creates fancy-looking scalloped edges on the leaves it inhabits. There is a second leaf-mining caterpillar, Ectoedemia nyssaefoliella, and several much larger caterpillars that feed externally on tupelo leaves, such as the larva of the striking black-and-white Hebrew moth. Those caterpillars might become food for warblers or titmice as the solar energy captured by a leaf moves through Earth’s food web.

Nearby, the oak leaves have their own countless dramas in progress, as do the willows, the maples, the apples, the asters, and even tripartite poison ivy—so much life emanating from every single leaf.

Across the northern latitudes, trillions of leaves are shutting down, flushing outrageous colors in celebration of everything they have accomplished on behalf of creatures great and small. Even now, their work is not done: They will shelter cocoons, nurseries, and fungal spores through the winter, and their nutrients and carbon will ultimately return to the earth.

So in this season of dying and suspended animation, let your spirit rise in ovation for the leaves and all of the life they have fostered, including your own!