The life that springs from dead leaves in streams

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Photo: Raul Angel / Unsplash

On a cool afternoon, I step down a shaded path that leads into a small nature reserve near my home in southern Maine. I’m slowly stepping under the pine trees, eager to explore the boundaries of my pantry and home office after weeks of COVID-19 homework assignments.

As I crunch over dead leaves in the shade, I am drawn further in by the rushing water, and soon a wide stream rises to the left. I’ve never visited this seven acre wooded area before so I’m soaking up its novelty. The damp branches lit by dappled sunlight, the large clumps of dead leaves that build up eddies: they take some of the burden off the pandemic.

All of this debris may look like a mess, but it is actually a sign of health. A muddy brook brings new life; a sterile stream produces nothing. I stare at a brown leaf that hangs and waves in the current. Where are we going? What new life will it brew?

What will become of this dead leaf?

For me it’s an empty thought. For Jane Marks this is part of everyday life. As a biologist at Northern Arizona University in Flagstaff who is studying leaves, she’s been trudging through streams for decades thinking about messy rubble, and when we call from our home office she tells me it never gets old. “There are always little surprises,” she says.

Dead leaves, Marks explains, are a primary food source for life along the entire food chain in and around streams, from the fungi and bacteria that originally colonized the leaves, to the insects that chew them, to birds and fish who have favourited eat these insects and so on. Prefer different organisms different types of leavesThe greater the diversity of trees along a stream bank, the greater the diversity of life they support.

These “brown” or “dead” food webs can be far wider than the “green” food webs that a leaf feeds when it’s still alive, says Marks. A fresh leaf could feed caterpillars or beetles, which in turn feed insectivorous carnivores, but the pool of nutrients released by dead leaves by rotting in the water adds another dimension to their contribution. “Understanding what happens when it’s dead,” she says, “is actually just as important or more important than understanding what happens when it’s alive.”

Illustration: Victor Leshyk and Abigail Downard. Source: JC Marks / AR Ecology, Evolution and Systematics, 2019 /Well-known magazine

There are many nuances, writes Marks in an article entitled writes in an article entitled ‘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 nutrient-rich or digestible leaves and which groups 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 land mass on a given continent, but often contain an oversized dose of biodiversity that drives ecosystems beyond.

So Marks’ results have practical applications that refute the simple wobbling of the brown leaf I see on my walk, as ecologist Amy Marcarelli explains when I call her a few days later: Knowing how leaf litter breaks down helps in efforts ranging from fisheries management to water quality, improvements in 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 safely trap carbon dioxide underground if they are spilled and not consumed. So understand this “fate of dead leaves that fall into streams” – a poetic formulation borrowed from a newspaper published in the 1970s – “is really important,” says Marcorelli.

Illustration: Victor Leshyk and Abigail Downard. Source: JC Marks / AR Ecology, Evolution and Systematics, 2019 /Well-known magazine

Marks addresses the problem by packing different types of leaves in small mesh bags, leaving them in streams, and observing their possible disappearance over several months.

First, she says, freshly fallen leaves act like a tea bag in water – some carbohydrates and sugars seep out almost instantly and float downstream. But there is much left, and that provides nourishment for the mushrooms that are now thriving. The fungi make it easier for the bacteria to break down the leaf and after about four to six days insects begin to participate in the feeding frenzy.

The first insects to arrive are usually tiny mosquitos, says Marks. Then bigger insects so-called shredders descend with mouthparts intended to tear apart large pieces of leaves. Shredders include the larvae of insects like caddis flies and stone flies and tend to be quite messy. They leave a trail of smaller particles that feed on another group of insect larvae known as collectors. These in turn leave behind particles that are passed on to animals such as freshwater clams or black fly larvae, which filter food directly from the water. The sheet, says Marks, “is simply processed and processed until it is gone.”

This whole collapse can last up to three to four months in relatively warm rivers such as those surveyed in the American Southwest, and up to a year in colder waters. But depending on the tree species, events can unfold much faster. Preserving a variety of trees along the banks of streams helps ensure that the food sources for insects – and the animals that eat them – persist throughout the year and do not disappear all at once.

Illustration: Victor Leshyk and Abigail Downard. Source: JC Marks / AR Ecology, Evolution and Systematics, 2019 /Well-known magazine

This is the type of information land managers can use in choosing which trees to plant in restoring degraded creek and river banks. But focusing on biodiversity alone may not be enough: Individual trees within a particular species can also differ greatly in terms of nutrient content and digestibility, explains biologist Thomas Whitham from Northern Arizona University, who works with Marks.

For the past decade, Whitham and colleagues have studied the molecular and genetic diversity of trees and found that the variability within a species can be greater than that between species. For example, leaves from two different narrow-leaved poplars can contain up to ten times the difference in difficult-to-digest compounds called tannins, with one leaf containing 3% tannins and another leaf containing 33%. “People thought you saw a poplar, you pretty much saw them all,” he says. “But that’s not true.”

Whitham has partnered with the Nature Conservancy, Grand Canyon Trust, Bureau of Land Management, and other groups to plant hundreds of thousands of trees on restoration projects in the Southwest. With a view to the poplar leaf lessons, his team is working to understand which trees are particularly digestible or nutritious, and helping land managers plant a range of species that can together support the needs of the ecosystems they seek to restore. “The diversity in the plants will affect everything around them,” he says.

Now the warming climate has added another thread to this rippling dynamic – one that may begin to unravel these systems. Heat increases microbial activity, making fungi and bacteria the more voracious consumers of leaves in a warmer world. As a result, there may be 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 current increases by 2 degrees Celsius, you have lost some of the carbon that would otherwise end up in the food web,” she says.

Illustration: Victor Leshyk and Abigail Downard. Source: JC Marks / AR Ecology, Evolution and Systematics, 2019 /Well-known magazine

She directs me to a paper that refers to aquatic food chains as “Confused networks”, and she speaks of rivers and streams as “water pipes” of our continent. I sense an preponderance of poetry within science, which studies the fate of dead leaves in streams – more than you normally find in academic papers. I ask Rosemond about this, and she agrees that poetics has found its way into the field. “I think scientists are artists,” she says. “Ecologists see holism and are attracted to nature because of its beauty and connections.”

Now, on my daily forest walks during this pandemic, connections bubble when I pause to record a dirty scene. The old tree that fell over the creek near my house isn’t just an obstacle – it’s a dam for leaves that feed all winter for the fungi and bacteria, the choppers and foragers that pop up in the spring , saves. and the adult insects and the birds that eat them. Without these dams, this stream would be like a lifeless pipe.

For a while, says Marks, pipes were what a lot of people thought they wanted out of rivers and streams. As the world industrialized, they wanted streams to come to water easily or boats to be easy to pass through. They cleared away the fallen branches and let those leaves wash off. Nowadays, river restoration projects often purposely dump dead trees into streams to damm up the leaves and keep them in place. “Since then, we’ve realized that all of this natural complexity is really very important to everything that lives in it,” says Marks. Dead leaves and the food stored in them remind us that beauty and life can be found in disorder and decay.

This article originally appeared in Well-known magazine, an independent journalistic endeavor by Annual reviews.


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