The Forest Unseen: A Year's Watch in Nature (30 page)

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.

O
ur everyday experience of the animal kingdom is dominated by two groups of animals: the vertebrates and the insects. These two branches on the tree of life occupy most of our culture’s zoological field of vision, yet they represent just a fraction of the structural diversity of animals. Biologists divide the animal kingdom into thirty-five groups or phyla, each one defined by a distinctive body plan. The vertebrates and insects represent two subphyla among the thirty-five.

Why have the birds and the bees captured our imagination, leaving the nematodes, flatworms, and the rest of the world’s bestiary in a dusty back room of our consciousness? The simple answer is that we don’t run into nematodes very often. Or, we think we don’t. A deeper answer seeks to explain why the larger part of animal diversity is hidden from us. We get out and about enough, so why do we not run into our neighbors?

Unfortunately for the richness of our experience, we live in a strange and extreme corner of the world’s available habitat. The animals that we encounter are those few that also inhabit this unusual niche.

The first cause of our estrangement is our size. We are tens of thousands of times larger than most living creatures, therefore our senses are too dull to detect the citizens of Lilliput that crawl around and over us. Bacteria, protists, mites, and nematodes make their homes on the mountains of our bodies, hidden from us by the dislocation of scale.
We live in the empiricist’s nightmare: there is a reality far beyond our perception. Our senses have failed us for millennia. Only when we mastered glass and were able to produce clear, polished lenses were we able to gaze through a microscope and finally realize the enormity of our former ignorance.

Our living on land further distances us from the rest of the animal kingdom, augmenting the handicap of gigantism. Nine-tenths of the animal kingdom’s main branches are found in water—in the sea, in freshwater streams and lakes, in watery crevices within the soil, or in the moist interiors of other animals. The desiccated exceptions include the terrestrial arthropods (mostly insects) and the minority of vertebrates that have hauled themselves onto land (most vertebrate species are fish, so terrestrial life is unusual even for a vertebrate). Evolution has plucked us out of our wet burrows, leaving our kin behind. Our world is therefore populated by extremists, giving us a distorted view of life’s true diversity.

My first dive into the soil helped me escape my strange ecological hermitage and gave me a taste of the treasury that lives below the surface. My thirst is whetted, so I dip down again. At three spots around the edge of the mandala I peel away a small clump of leaves, create a small hole in the litter, peer down with my hand lens, then replace the leaves. The contrast with the aboveground world is striking. Aboveground, apart from a passing titmouse, I seem to be alone in the forest. Yet animals abound an inch below the surface of the litter.

The largest animal in my foray is a salamander curled in the cup of a fallen oak leaf. The salamander would fit on my thumbnail but is hundreds of times larger than all the other animals that I encounter. This salamander is a crocodile among minnows, watched by a nearsighted whale.