On a cool afternoon, I enter a shaded path leading into a small nature preserve near my home in southern Maine. I tread slowly beneath the pines, eager to explore beyond the confines of my pantry and home office after weeks of Covid-19 stay-at-home orders.
As I crunch over dead leaves into the shadows, I’m pulled farther in by the sound of rushing water, and soon a wide stream emerges to the left. I have never visited this seven-acre patch of woods before, so I soak up its newness. The moist twigs lit by dappled sunlight, the large clumps of dead leaves damming up eddies: They relieve some of the weight of the pandemic.
All this debris may look like a mess, but it’s actually a sign of health. A mucky stream breeds new life; a sterile stream breeds nothing. I stare at one brown leaf, hanging and wagging in the current. Where’s it headed? What new life will it brew?
What will come of this dead leaf?
For me, it’s an idle thought. For Jane Marks, it’s all in a day’s work. As a biologist at Northern Arizona University in Flagstaff who studies leaf litter, she’s been tromping through streams and contemplating messy debris for decades and, when we speak over the phone from our home offices, she tells me that it never gets old. “There are always little surprises,” she says.
Dead leaves, Marks explains, provide a primary food base for life all the way up the food chain in and around streams, from the fungi and bacteria that initially colonize the leaves, and the insects that chew them, down to the birds and fish that eat those insects, and so on. Different organisms prefer different types of leaves, so the greater the variety of trees along a stream bank, the greater variety of life they support.
These “brown” or “dead” food webs can be far more expansive than the “green” food webs a leaf nourishes when it’s still alive, Marks says. A fresh leaf might feed caterpillars or beetles that in turn feed insect-gobbling carnivores, but the pool of nutrients that dead leaves release by decaying in water adds another dimension to their contribution. “Understanding what happens once it’s dead,” she says, “is actually as important or more important than understanding what’s happening when it’s alive.”
The makeup of tree communities alongside streams influences how carbon moves through the ecosystem, as shown by these three examples. Leaves from a mix of deciduous tree species (left) will break down at different rates, offering a staggered food supply that supports diverse life including fish, insects and birds. In this scenario, much of a leaf’s carbon is assimilated into other organisms rather than being released into the atmosphere. If there is a single species of tree but those trees are genetically diverse (center), much of the leaves’ carbon is still integrated into the food chain. In the third scenario (right) — a single tree species with leaves that decay quickly — microbes use most of the leaf material, which robs the food web of carbon and results in release of higher levels of carbon dioxide into the atmosphere. Invasive tamarisks in the Southwest US are an example.
The nuances are many, Marks writes in an article titled “Revisiting the Fates of Dead Leaves That Fall into Streams” in the Annual Review of Ecology, Evolution, and Systematics. No two trees produce the same two leaves, so she is working to understand which species produce more nutritious or digestible leaves, and which groupings of trees are best suited to feed different types of aquatic ecosystems. Riparian zones — areas on land that border rivers and streams — make up only a fraction of the landmass on any given continent, but often contain an outsized dose of biodiversity that fuels the ecosystems beyond.
So Marks’s findings have practical applications that belie the simple wag of that brown leaf I observe on my stroll, as ecologist Amy Marcarelli explains when I call her some days later: Knowing how leaf litter breaks down helps inform efforts ranging from fisheries management to water quality improvements to large-scale river restoration projects.
The research also has implications for climate science, adds Marcarelli, who studies aquatic ecosystems at Michigan Technological University. Dead leaves can release carbon dioxide into the atmosphere when they decay. Or they can trap carbon dioxide safely underground if they become buried and remain unconsumed. So understanding these “fates of dead leaves that fall into streams” — poetical phrasing borrowed from a paper published in the 1970s — “is really important,” Marcarelli says.
A fallen leaf is broken down by fungi, bacteria and invertebrates such as insects that shred and consume leaf material. These insects and their wastes are, in turn, eaten by other insects as well as birds and fish (not shown). The leaf’s carbon and nitrogen are assimilated and released at various stages of the process.
Marks attacks the problem by packing different types of leaves into small mesh bags, leaving them in streams and observing their eventual disappearance over the course of several months.
First, she says, newly fallen leaves behave like a tea bag in water — some carbohydrates and sugars seep out almost immediately and float downstream. But plenty remains behind, and that provides fodder for the fungi that now flourish. The fungi make the leaf easier for bacteria to break down and then, after about four to six days, insects start to join in on the feeding frenzy.
The first insects to arrive are usually small gnats, Marks says. Then bigger insects called shredders descend, with mouthparts designed to tear apart big chunks of leaves. Shredders include the larvae of insects like caddisflies and stoneflies, and tend to be quite messy. They leave behind a trail of smaller particles — which feed another group of insect larvae called collector/gatherers. Those, in turn, leave behind particles that pass on to animals like freshwater mussels or black fly larvae that filter food directly out of the water. The leaf, Marks says, “just keeps getting processed and processed until it’s gone.”
This whole breakdown can take as much as three to four months in relatively warm rivers like the ones she studies in the southwestern United States, or up to a year in colder waters. But depending on the tree species, events can unfold far faster. So maintaining a diversity of trees along streambanks helps to ensure that food sources for insects — and the animals that eat them — persist through the year and don’t disappear all at once.
To measure how energy (in the form of carbon), and nutrients (such as nitrogen), flow from a tree’s leaves into the surrounding food web, researchers infuse trees with isotopes, versions of elements that can be tracked. Scientists then put the leaves in mesh bags, place them in a stream and observe their fates. In this example, a caddisfly was able to consume more energy in the form of carbon from Gambel oak (Quercus gambelli) leaves than from faster-decaying Fremont cottonwood (Populus fremontii) leaves. This suggests that slower-decaying leaves may provide more nutrition for some stream life than faster decaying ones.
This is the type of information that land managers might use when choosing what trees to plant when restoring degraded stream- and riverbanks. But focusing just on species diversity may not be enough: Individual trees within a given species can also differ widely in their nutritiousness and digestibility, explains biologist Thomas Whitham of Northern Arizona University, who collaborates with Marks.
Over the past decade or so, Whitham and colleagues have been studying the molecular and genetic diversity of trees and have found that variability within a species can be greater than between species. Leaves from two different narrowleaf cottonwood trees, for example, can contain up to a tenfold difference in difficult-to-digest compounds called tannins, with one leaf containing 3 percent tannins and another leaf 33 percent. “People thought, you see one cottonwood, you’ve pretty much seen them all,” he says. “But that’s not true.”
Whitham has partnered with the Nature Conservancy, the Grand Canyon Trust, the Bureau of Land Management and other groups to help plant hundreds of thousands of trees in restoration projects across the Southwest. With cottonwood-leaf lessons in mind, his team works to understand which trees are especially digestible or nutritious and to help land managers plant an array of types that, together, can support the needs of the ecosystems they’re trying to restore. “Diversity in the plants will have ripple effects on everything around them,” he says.
Now the warming climate has added another thread to these rippling dynamics — one that may begin to unravel these systems. Warmth increases the rate of microbial activity, so fungi and bacteria may become more voracious consumers of leaves in a warmer world. This may leave less food available for insects and the creatures that eat the insects, says Amy Rosemond, an ecologist at the University of Georgia who studies the effects of global change on rivers and streams. “As soon as a stream increases 2 degrees C, you’ve lost some of that carbon that would otherwise be going into the food web,” she says.
As temperatures rise, microbial activity picks up speed. This means that fewer nutrients from the leaves are available for the insect community. That community may also change should some insect species — such as ones known as shredders that tear apart leaves — fail to tolerate higher temperatures. Shifts like these could therefore result in increased release of carbon dioxide into the atmosphere, making less food available to non-microbial aquatic life.
She points me to a paper that refers to aquatic food chains as “tangled webs,” and she talks about rivers and streams as the “plumbing” of our continent. I sense a preponderance of lyricism within the science that explores the fates of dead leaves in streams — more than you typically find in academic papers. I ask Rosemond about this, and she agrees that poeticism has found its way into the field. “I think scientists are artists,” she says. “Ecologists see holism, and are drawn to nature because of its beauty and its connections.”
Now, on my daily strolls in the woods during this pandemic, the connections bubble up when I pause to take in a mucky scene. The old tree that has fallen across the stream near my home isn’t just a thing in the way — it is a dam for leaves that stores food all winter for the fungi and bacteria, the shredders and the gatherers that emerge in the spring, and the adult insects and the birds that eat them. Without those dams, this stream would be like a lifeless pipe.
For a while, Marks says, pipes are what many people thought they wanted out of rivers and streams. As the world industrialized, they wanted streams to be easy to get water from, or easy to get boats through. They cleared away the fallen branches and let those leaves wash away. Today, river restoration projects often deliberately drop dead trees into streams to dam up the leaves and keep them in place. “We have since realized all this natural complexity is really, really important to everything that lives in it,” Marks says. Dead leaves, and the nourishment they store, remind us that there’s beauty and life to be found in disorder and decay.
