The Forest Unseen_A Year's Watch in Nature

by David George Haskell

  The canopy’s renewal depends paradoxically on its being split open to let light reach the ground. Any change in the dynamics of gaps will therefore affect the viability of the forest. This makes me particularly concerned about the spindly tree growing at the back of the gap next to the mandala. This tree has grown several feet since the spring, thrusting its two-foot-wide, heart-shaped leaves into the opening. Fast-growing alien species such as this Paulownia tomentosa, or Princess Tree, are spreading through the eastern forests, taking over the forest by invading light gaps and outgrowing native species. Paulownia, and its invading partner, Ailanthus altissima, the Tree of Heaven, produce thousands of wind-dispersed seeds and thereby spread rapidly. They especially favor roadside edges and logged forests, but like most pioneers will readily invade openings after smaller-scale disturbances.

  Fast-growing invaders are particularly harmful to the regeneration of native trees that require full sunlight to grow: oaks, hickories, walnuts, tuliptrees. When Paulownia and Ailanthus sprout in a gap, they smother the slower-growing natives. In forests that are heavily disturbed by fire, logging, or housing development, nonnative trees can quickly erode native tree diversity.

  The study of twigs seems esoteric. But this impression is dangerously wrong. Counting back through bud scars, tallying yearly growth, I not only see the struggle among native and foreign trees, I read the ledger of the world’s atmosphere. Each twig yearly adds a few inches, and these inches, combined across the forest, create one of the world’s biggest stores of carbon.

  When we count all new growth—twigs, leaves, thickening trunks, extended roots—the mandala likely took ten or twenty kilograms of carbon from the air this year, a pile of sticks about as big as a small car. Summed over the world’s surface, forests contain about twice as much carbon as the atmosphere, over a thousand million million tons. This vast store is our buffer against calamity. Without forests, much of the carbon would be in the air as carbon dioxide gas, baking us in an awful greenhouse.

  As we’ve burned oil and coal, we’ve returned long-buried carbon stores to the atmosphere. But forests have saved us from the full brunt of the resulting climate change. Half our burned carbon has been absorbed by forests and the oceans. Lately, this buffering effect of forests has diminished—there is a limit to the rate at which trees can soak extra carbon from the atmosphere, especially as we accelerate our burning of fossil fuel. Nonetheless, forests continue to shield us from the more dire consequences of our profligacy. The study of twigs and bud scars is therefore the study of our future well-being.

  December 3rd—Litter

  I lie facedown at the edge of the mandala, readying myself for a dive under the surface of the leaf litter. The red oak leaf below my nose is crisp, protected from fungi and bacteria by the drying sun and wind. Like the other leaves on the litter’s surface, this oak leaf will remain intact for nearly a year, finally crumbling in next summer’s rains. These surface leaves form a crust that both hides and makes possible the drama below. Protected under the shield of superficial leaves, the rest of autumn’s castoffs are pulverized in the wet, dark world of the litter. Yearly, the ground heaves like a breathing belly, swelling in a rapid inhalation in October, then sinking as the life force is suffused into the forest’s body.

  Below the red oak leaf, other leaves are moist and matted. I tease away a wet sandwich of three maple and hickory leaves. Waves of odor roll out of the opening: first, the sharp, musty smell of decomposition, and then the rounded, pleasant odor of fresh mushrooms. The smells are edged with a richer, earthy background, the mark of healthy soil. These sensations are the closest I can come to “seeing” the microbial community in the soil. The light receptors and lenses in my eyes are too large to resolve the photons bouncing off bacteria, protozoa, and many fungi, but my nose can detect molecules that waft out of the microscopic world, giving me a peek through my blindness.

  A peek is about all that anyone is given. Of the billion microbes that live in the half handful of soil that I have exposed, only one percent can be cultivated and studied in the lab. The interdependencies among the other ninety-nine percent are so tight, and our ignorance about how to mimic or replicate these bonds is so deep, that the microbes die if isolated from the whole. The soil’s microbial community is therefore a grand mystery, with most of its inhabitants living unnamed and unknown to humanity.

  As we chisel away at the edges of this mystery, jewels fall out of the eroding block of ignorance. The earthy smell that embraces my nose comes from one of the brightest jewels, the actinomycetes, strange semicolonial bacteria from which soil biologists have extracted many of our most successful antibiotics. Like the healing chemicals in foxglove, willow, and spirea, the actinomycetes use these molecules in their struggle with other species, secreting antibiotics to subdue or kill their competitors or enemies. We turn this struggle to our advantage through medicinal mycology.

  Antibiotic production is a small part of the huge and varied role of actinomycetes in the soil’s ecology. There is as much diversity within the feeding habits of this group of bacteria as exists within all the animal kingdom. Some actinomycetes live as parasites in animals; others cling to plant roots, nibbling on them while fighting off more damaging bacteria and fungi. Some of these root dwellers may turn against their hosts and kill the plant by belowground assassination. Actinomycetes also coat the dead bodies of larger creatures, breaking them down into humus, the dark miracle ingredient of productive soils. Actinomycetes are everywhere but seldom enter our consciousness. Yet we seem to have an intuitive understanding of their importance. Our brains are wired to appreciate their distinctive “earthy” smell and to recognize the aroma as the sign of good health. Soil that has been sterilized, or that is too wet or dry for most actinomycetes, smells bitter and unfriendly. Perhaps our long evolutionary history as hunter-gatherers and agrarians has taught our nasal passages to recognize productive land, giving us a subconscious tie to the soil microbes that define the human ecological niche.

  The other members of the microbial community are harder to pin down in the complex smell that rises from the earth’s abdomen. Fungal spores contributed to the acrid mustiness; bacterial decomposers released sweet aromas from the remains of dead leaves. Tiny wafts of methane rise up from sodden patches where anaerobic microbes hide. Many other microbes live beyond the reach of my nose. Bacteria grab nitrogen from the air and pass it into the biological economy. Others take nitrogen from dead creatures and send it back into the air. Protists graze on fungi and bacteria that encrust decaying leaves. This secret microbial world has existed for a billion or more years. The bacteria, in particular, perform biochemical tricks that have fed them since the earliest years of life, three billion years ago. The smell in my nose therefore comes from a hidden world that is broad and deep, complex and ancient.

  Microbes may be invisible, but my window into the soil offers plenty else to see. Lightning-white fungal strands crackle over black leaves. Pink hemipteran bugs dance around orange spiders. A ghostly white springtail moves over the dark crumbs of last year’s decayed leaves. Everything lives in miniature. A buried maple seed towers over the animals like a mansion dwarfing its owner. The largest living creature is a rootlet, one tiny part of a plant, perhaps a sapling or tree. It is barely thicker than a pin, but it dominates the small hole that I have bored into the litter.

  The rootlet is a smooth, creamy cable sprouting a haze of hairs that radiates out into the soil’s matrix. Each of these hairs is a delicate extension of the root’s surface, a tentacle stretched out from a plant cell. The hairs creep around sand particles, sliding into the films of water that cling to the soil. By greatly increasing the surface area of the root, these hairs allow the plant to harvest water and nutrients that would otherwise be unavailable. So critical are these root hairs that if their intricate hold on the soil is broken by being uprooted or transplanted, the plant will wilt and die unless it receives extra watering from a gardener.

  Root hairs draw
water and dissolved nutrients from the soil, sending them upward to slake the leaves’ thirst and to supply the minerals that plants need for their building projects. The energy for this skyward motion comes mostly from the sun’s evaporative power, transmitted downward through columns of water in the xylem. But the root hairs are not just passive pipe ends sucking at the soil like a pump in a well. Their relationship with the physics and biology of the soil is reciprocal.

  The simplest of the roots’ gifts to the soil are hydrogen ions, pumped out by the root hairs to help loosen nutrients that are bound to clay particles. Each fleck of clay has a negative charge, and so minerals with positive charges such as calcium or magnesium stick to the clay’s surface. This attraction helps the soil hold on to its minerals, stopping them from washing away in the rain, but the bond also prevents plants from acquiring them in the flow of water into the root. The root hairs’ answer is to soak the clay particles in positively charged hydrogen ions. These dislodge some of the attached mineral ions from the surface of the clay. The released minerals float in water films that surround the clay and are swept into the root hairs by the water’s flow. The most useful of these minerals are easily dislodged, so the root hairs need release only a small amount of hydrogen ions to receive their reward. More vigorous applications of hydrogen ions, such as those that come with acid rain, release the more toxic elements such as aluminum.

  Roots also supply the soil with large amounts of organic matter. Unlike the deposition of leaves from above, most of the roots’ donations are actively secreted, not cast off as waste products. Dead roots certainly enrich the soil, but death’s contribution is dwarfed by the tangle of sugars, fats, and proteins that living roots infuse into the soil around them. This gelatinous sheath of food around the root creates a buzz of biological activity, particularly near the root hairs. As in a sandwich shop at lunchtime, much of the soil’s life is crowded into the narrow root zone or rhizosphere. Here, microbial densities are a hundred times higher than in the rest of the soil; protists crowd around, feeding on microbes; nematodes and microscopic insects push through the crowd; fungi spread their tendrils into the living soup.

  The ecology of the rhizosphere is mostly a mystery, made difficult to study by its paper-thin delicacy. Plants obviously stimulate life in the soil, but what do they receive in return? The explosion of biological diversity in the rhizosphere may protect roots from disease, just as a diverse forest is less likely than a bare field to be overrun with weeds. But this is speculation. We are explorers standing at the edge of a dark jungle, peering at the strange shapes in the soil’s interior, naming a handful of the most obvious novelties but understanding little.

  Despite the gloom, one relationship in the jungle of the rhizosphere is so important that even the most hasty explorers trip over its vines, then look up, astounded. The plants’ partners in this surprising relationship are visible in the window that I have created into the leaf litter. Fungal threads cover most of the soil like a subterranean spiderweb. Some are dusky gray and spread out seemingly at random, coating whatever lies in their path. Others grow their white strands in waving lines, diverging then reuniting like rivers in a delta. Each fungal thread, or hypha, is ten times finer than a root hair. Because hyphae are so thin, they can squeeze between microscopic soil particles and penetrate the ground much more effectively than can clumsy roots. A thimbleful of soil may contain a few inches of root hairs but a hundred feet of hyphae, spooled around every fleck of sand or silt. Many of these fungi work alone, digesting the decaying remains of leaves and other dead creatures. Some, however, work their way into the rhizosphere and begin a conversation with the root. This conversation is the start of an ancient and vital relationship.

  The fungus and the root greet each other with chemical signals and, if the salutation goes smoothly, the fungus extends its hyphae in readiness for an embrace. In some cases, the plant responds by growing tiny rootlets for the fungi to colonize. In others, the plant allows the fungus to penetrate the root’s cell walls and spread the hyphae into the interior of the cells. Once inside, the hyphae divide into fingers, forming a miniature rootlike network within the cells of the root. This arrangement looks pathological. I would be a sick man if my cells were infested with fungi in this way. But the ability of hyphae to penetrate plant cells is put to healthy use in this marriage with roots. The plant supplies the fungus with sugars and other complex molecules; the fungus reciprocates with a flow of minerals, particularly phosphates. This union builds on the strengths of the two kingdoms: plants can create sugars from air and sunlight; fungi can mine minerals from the soil’s tiny crevices.

  The fungus-root, or mycorrhizal, relationship was first discovered as a spin-off from the King of Prussia’s attempts to cultivate truffles. His biologist failed to domesticate the valuable fungus, but he discovered that the underground fungal network that produces truffles is connected to tree roots. He later showed that these fungi were not parasites, as he first suspected, but acted as “wet nurses” passing nutrients to trees and increasing their rate of growth.

  As botanists and mycologists worked their way across the plant kingdom, peering at root samples through microscopes, they found that nearly all plants have mycorrhizal fungi wrapped in or around their roots. Many plants cannot live without their fungal partners. Others can grow alone but are stunted and weak if they cannot meld their roots with a fungus. In most plants, fungi are the main absorbing surface in the soil; roots are just the connections to this network. A plant is therefore a paragon of cooperation: photosynthesis is made possible by ancient bacteria embedded in its leaves, respiration is likewise powered by internal helpers, and roots serve as connectors to an underground network of beneficial fungi.

  Recent experiments show that mycorrhizae take this relationship yet further. By feeding plants radioactive atoms, plant physiologists have traced the flow of matter in the forest ecosystem and found that fungi act as conduits among plants. Mycorrhizae are promiscuous in their embrace of plant roots. Seemingly independent plants are physically connected by their subterranean fungal lovers. The carbon taken out of the atmosphere and turned into sugar by the maple tree above the mandala may find itself transported to the tree’s roots and donated to a fungus. The fungus will then either use the sugar for itself or pass the sugar to the hickory, or to another maple, or to the spicebush. Individuality is therefore an illusion in most plant communities.

  Ecological science has yet to fully digest the discovery of the belowground network. We still think of the forest as being ruled by relentless competition for light and nutrients. How does the mycorrhizal sharing of resources change the aboveground struggle? Surely the race for light is no illusion? Could some plants be parasitizing others, using the fungi as friendly con men, or do the fungi mitigate and smooth out differences among plants?

  Whatever the answers to these questions, it is clear that the old “red in tooth and claw” view of the natural economy has to be updated. We need a new metaphor for the forest, one that helps us visualize plants both sharing and competing. Perhaps the world of human ideas is the closest parallel: thinkers are engaged in a personal struggle for wisdom, and sometimes fame, but they do so by feeding from a pool of shared resources that they enrich by their own work, thus propelling their intellectual “competitors” onward. Our minds are like trees—they are stunted if grown without the nourishing fungus of culture.

  The partnership between fungi and plants that undergirds the mandala is an old marriage, dating to the plants’ first hesitant steps onto land. The earliest terrestrial plants were sprawling strands that had no roots, nor any stems or true leaves. They did, however, have mycorrhizal fungi penetrating their cells, helping to ease the plants into their new world. Evidence of this partnership is etched into fine-grained fossils of the plant pioneers. These fossils have rewritten the history of plants. The roots that we thought were one of the earliest and most fundamental parts of the land plant body turn out to be an evolutionary afterthought
. Fungi were the plants’ first subterranean foragers; roots may have developed to seek out and embrace fungi, not to find and absorb nutrients directly from the soil.

  Thus cooperation gains another jewel in its evolutionary crown.

  Most of the major transitions in life’s history were accomplished through joint ventures such as the union between plant and fungus. Not only are the cells of all large creatures inhabited by symbiotic bacteria, but the habitats they live in are made by or modified by symbiotic relationships. Land plants, lichens, and coral reefs are all products of symbiosis. Strip the world of these three and you have stripped it virtually bare—the mandala would be transformed to a pile of rocks clothed in bacterial fuzz. Our own history mirrors this pattern: the agrarian revolution that unleashed humanity’s boom was created by entering into mutual dependence with wheat, corn, and rice, and by conjoining our fate with that of horses, goats, and cattle.

  Evolution’s engine is fired by genetic self-interest, but this manifests itself in cooperative action as well as solo selfishness. The natural economy has as many trade unions as robber barons, as much solidarity as individualistic entrepreneurship.

  My peephole into the soil gave me a glimpse into some new ways of thinking about evolution and ecology. Or are they so new? Perhaps soil scientists are rediscovering and extending what our culture already knows and has embedded into our language. The more we learn about the life of the soil, the more apt our language’s symbols become: “roots,” “groundedness.” These words reflect not only a physical connection to place but reciprocity with the environment, mutual dependence with other members of the community, and the positive effects of roots on the rest of their home. All these relationships are embedded in a history so deep that individuality has started to dissolve and uprootedness is impossible.