Fresh from the Woods
The Underground Forest
By Joe Rankin
Walking in the forest you are bathed in sounds, sights, smells – chirping of birds, sighing of the wind in the branches, flashes of light off the leaves, warmth of sun on your skin, the skittering of tiny creatures in the underbrush. There’s peace there, and beauty.
But there’s more to the forest than the trees.
Look down. Scuff your boots through the leaves and humus. Turn over a rotting log. Gently, gently, there’s an entire universe down there. There lies the Underground Forest. The unseen forest. One at least as interesting as the forest of tree trunks and leaves, deer and bears and wild turkeys. And as complex. And much less understood.
Suppose that we shrink our forest visitor down . . . and down . . . and down. At some point this person will enter a brave new world, teeming with life. A voracious, predatory world of slimy monsters and alien beings. Jurassic Park in miniature.
Leonardo Da Vinci said, “we know more about the movement of celestial bodies than about the soil underfoot.”
In these days of GPS systems and satellite telescopes old the Italian master’s statement is still true.
“I think a lot of it has to do with out-of-sight, out-of-mind,” says Mike Cline, the director of the Tin Mountain Conservation Center in New Hampshire and a plant physiologist. “It’s not the easiest thing to see. Most of the organisms are microscopic, flora and fauna. But if you get involved with them you find them very fascinating. There’s a lot of stuff going on down there.”
While tree trunks, branches and leaves are impressive, there’s a lot of the tree underground, 20 to 25 percent of their biomass is below the surface of the soil, in their root systems. And those root systems tend to be one of the main characters in this play.
“In forest soils a lot of the action below ground has to do with the trees,” says Ivan Fernandez, who teaches about forest soils at the University of Maine.
“A pinch of soil may contain millions of bacteria. It’s a phenomenal number of organisms that are present,” marvels Bryan Dail, a research scientist at the University of Maine.
And it’s a world where everything preys on everything else with voracious enthusiasm.
Salamanders prey on invertebrates. Nematodes chomp on bacteria, fungi and even roots.
Earthworms suck in decaying matter at one end and excrete nutrient-rich castings at the other. Voles eat plants. Springtails eat fungi. Bacteria eat each other, and just about anything else that is present in soil, living or dead. This messy chaos is niftily designed to break down organic matter, to recycle carbon. Good thing too, for without all these creatures busily doing their jobs we’d be neck deep or better in non-decaying stuff.
Of course we can’t speak accurately about “forest soil.” Just as the aboveground biomes on earth are many and varied, there are many “forest soils.” They differ because of climate, type of bedrock, terrain, the trees that like to grow there, and what humans have wrought on the landscape, among many other factors.
The soil under the beeches on this south facing slope might be different from the soil under those firs on that ridgeline. Not to mention different from that of a Scottish pine forest or a New Guinea cloud forest.
Northern forests, like ours, tend to have acid soils, which limit the how much some creatures, such as earthworms and bacteria, can participate in the work of nutrient cycling.
Although bacteria outnumber all other creatures in the forest soils – a cupful of forest soil could hold more bacteria than there are people on the planet – they’re not the heavy lifters in your typical northern New England forest.
That title is held by the fungi: the mushrooms, toadstools, shelf fungi and their ilk.
Some species of fungi break down rotting wood. But there’s another type of fungi – mycorrhizal fungi – that are a critical component of the underground forest. These fungi have developed a symbiotic relationship with essentially all plants, including trees. The fungi feed the tree nutrients they gather from the soil, the tree shares some of the nutrients it creates through photosynthesis. It’s an artful relationship. Kind of like a good marriage, where each party complements the other, strengthening the whole.
“What fascinates me is just how evolution and natural selection got these two organisms to be living in combination and dependent on the other,” said Cline, who studied mycorrhizal fungi and trees in grad school and researched their relationship during seven years with International Paper Co.
In many cases a species of mycorrhizal fungi tends to prefer a particular partner. Birches, say, or oaks, lending kind of a monogamous overtone to the whole partnership.
And the relationship is close. Cline notes that the fine feeder roots of trees are virtually enveloped by the fungi.
“They essentially put on a fungal glove. It’s called a mantle. The fungus grows in such proximity to the root itself that there’s just a little bit of free space. It actually grows between cells. It’s not harming the plant. It allows the fungus and the root to exchange material,” he said.
You can see the mycorrhizal fungi on a rainy fall day in the mushrooms popping up through the leaf litter and duff like, well, mushrooms.
“The majority of fungi you see in the fall . . . do not decompose wood. They don’t have the enzymes to allow them to do use dead wood. Thoseare the mycorrhizal fungi,” said Cline.
The mushrooms are the ephemeral fruiting bodies of a vast fungal net called the hyphae that can extend under entire square miles of forest, essentially one huge organism. The mushrooms exist to reproduce, to spread their spores before they get eaten by voles or, like many do, turn to slime between nightfall and the sun’s rising. Their real life, who they are, is underground in the fibrous net called the hyphae.
“If you dig down and peel back the soil at the point where the topsoil meets mineral soil you can actually see the hyphae,” said UM’s Fernandez. “There is often an abrupt boundary between the two, and right at that point you’ll see the hyphae, usually white or yellow.”
Those filamentous strands cross the mineral soils, helping bind and stabilize them, he notes.
But the mycorrhizal fungi offer other benefits as well helping feed the tree and stabilize mineral soil. They help extend its reach.
“That glove around the roots protects a little bit from drought,” said Cline. “But most importantly, think about a little short root. With root hairs sticking out a millimeter or so around it. That’s the absorbing area of a ‘non-infected’ root. The fungal glove increases the absorbing area of that little rootlet over a thousand times. That mycorrhizal fungus also needs phosphorous and nitrogen. It takes up nutrients from this vast area in the soil that would not be available to the tree from its root hairs alone.”
We have learned a lot about forest soils in the centuries that have passed since Da Vinci’s day.
For instance, we’ve learned that, though nitrogen is the most commonly limiting nutrient in forests, human-caused pollution often adds more than the forest can use. That forest soils are a huge sink for carbon. We’ve learned about the role of the lowly and underappreciated fungi in breaking down organic matter. That thawing permafrost under tundra and high subarctic forests is contributing to global warming.
Another example: in a study published last year, French researchers found that some trees growing in nutrient-poor forest soils may get what they need by cultivating specific root microbes to create compounds they require.
Still, the forest soils are a fertile field, pardon the pun, for research. Too many questions remain about the underground forest and how it works, and how it connects to the aboveground forest.
“What we know now is that we have such a poor understanding of the complex underground world of interactions in forest soils between the physical, chemical and biological,” said Fernandez.
“We know forests are often limited by nitrogen, sometimes phosphorus, maybe even calcium, and yet we can not determine with precision which forests are vulnerable to harvesting pressures. We are uncertain why some lakes recover and others do not even with declining sulfur deposition, and the answer is often in the soils. We have a load of questions about what are the sources and sinks of the various greenhouse gases in forest soils. We do not adequately understand the processes taking place within forested wetlands in a forest landscape, nor the role of those wetlands in landscape processes. We also are struggling to understand and predict the reaction of forests to climate change.”
Considering the stress that scientists are placing on trees’ ability to pull carbon from the air in times of rising CO2 levels and sequester it in their trunks and in the soil, it might behoove us to find out some more answers.
“When it comes to carbon, there’s an awful lot we don’t know, and don’t know enough in a practical way to apply it to current problems and questions,” said Fernandez.
But Cline said he’s encouraged with the progress made so far.
Scientists today know that old forests continue “to sequester carbon even after they have reached biological maturity,” said Cline. “This is somewhat counter to past ideas that the carbon uptake in old forests is at equilibrium at best, carbon uptake matched by carbon dioxide emissions. Although young trees initially fix carbon through photosynthesis at a faster rate, older forests play a more substantial role in storing carbon than younger forests. As a result, old growth forest soils are now recognized as major terrestrial carbon sinks. Although there is much carbon stored in old growth trees, the soil does the heavy lifting; it stores about twice as much carbon as the trees.”
Still, researching the Underground Forest is not easy. It is a complicated world down there, and some efforts to tease out answers have failed simply because isolating one piece of the soil’s complex and interrelated community often doesn’t work very well. Everything down there seems to work hand-in-glove. To paraphrase John Muir, when you pick out one strand of the web of soil life you find it’s connected to everything else.
Cline, Dail and Fernandez agree that you can’t understand forest productivity without understanding forest soils.
Dail sees the forest soil as “crucial to making available nutrients that underlie forest productivity. It’s spatially complex because we don’t have a grasp on this complexity; this is often made evident when we seek to balance the sources and fates of nutrients in the forest ecosystem. There are a lot of relationships in the soil that are difficult for us to fully appreciate.”
And while many debate the merits or effects of clearcutting, shelterwood harvests and selective cutting; managed forests versus wilderness; high grading and low grading, few loggers, foresters or landowners ask how chopping down the trees will affect the ecosystem underneath their boots.
“All those things we’re doing have an effect on the soil, whether logging or development or agriculture. Everything has an influence,” said Cline. “They upset the soil equilibrium in some way. That happens in nature as well. But we disturb much more of the landscape. It’s important to develop a better understanding so it’s not out-of-mind, out-of-mind.”
Forest soils, and the mycorrhizal fungi, seem to be resilient. If the trees are cut and replaced by different species, the forest soil and the characters that inhabit it will change. It will recover, given time. Fungi are omnipresent. They produce zillions of spores and, though some are killed by sunlight, others are infinitely patient in the environment, just waiting for the day when causes and conditions are right for them to link up, hand-in-glove, with their favored tree partner.
Dail said he believes that the most important forest movement in the past decade or so is the one that seeks to more holistically regard the forest. Not just seeing it as a source of timber, but “tying it to the things we know are important: clean water, clean air. The ecosystem services approach.”
He remembers a 1990 United National Food and Agriculture Organization report that estimated
the cost of duplicating an acre of farmland in a hydroponic system at $250,000. Just the infrastructure, the setup. Not operating costs. And that, of course, in dollars that are now but a dim memory.
For crops like tomatoes or cucumbers or eggplants, whose life is measured in weeks and months, you can make the numbers work, of course. It’s done all the time.
Trees. Well, it’s a different story.
“You’d never do that with a tree, with a plantation or a grove of trees,” said Dail. “It’s just not feasible. So I guess there’s the value of the soil to plant growth and timber. It’s important to us, and we’re going to need it.”