The Forest Unseen_A Year's Watch in Nature

by David George Haskell

  The storms came with strong winds, some of which swirled into tornadoes. None of the columns of vengeful air touched the mandala, but the forest floor is strewn with the evidence of agitation in the canopy. Freshly torn leaves festoon the litter. Cracked twigs and fallen branches are tangled in the understory. The wind’s force has yet to die down. It surges across the forest in pulses, setting the trees into violent motion. The canopy protests with a loud hiss, the sound of millions of pummeled leaves. The forest groans and cracks as tired wood fibers are pushed beyond their endurance.

  The air is quieter at ground level. Strong breezes rush past me, but it is calm enough for mosquitoes to circle my arms and head, weaving in and away as they plot their attack. The mosquitoes and I sit in the middle of a dramatic gradient of physical energy. The surface of the canopy is the shore against which the air beats itself, crashing wave after wave onto the treetops. The shrub layer of the forest, where I sit, is heavily buffered by the trees above and receives only feeble eddies from the breakers pounding the canopy. The mandala’s surface is calmer yet. Snails feel hardly a breeze as they graze on the leaf litter. No insect or snail is active in the canopy today; only a few brave the gusts below, but life continues as usual in the leaf litter.

  Trees are ill suited to absorb the force of the wind. Leaves are designed to intercept as much sun as possible. Unfortunately this also makes them excellent wind catchers. The saillike surfaces of leaves are pulled leeward by the flow of air. Leaves and twigs do not stretch much, so the pull is transmitted to the rest of the tree. As the wind gets stronger, leaves start to flutter. A fluttering leaf creates more drag than a stiff leaf, so the pull on the tree increases sharply. The force of tens of thousands of leaves dragging in the wind is accentuated by the height of the tree’s crown. The trunk acts as a lever, turning the tree into a huge crowbar. The wind pulls at one end, the trunk multiplies the force, and snap! the tree is shattered or uprooted.

  Natural selection does not allow trees to take the obvious way out, namely to abandon the lever arm and hug the ground. Competition for light among plants in the forest forestalls this possibility. Any tree that fails to grow a tall trunk will be unable to gather much sunlight and will leave few, if any, offspring. Trees therefore grow as tall as their supporting architecture will allow, each individual tree reaching up to secure an unshaded spot in the canopy. A second solution to the problem of wind would be to stiffen the trunk, toughen up the twigs, and turn the leaves into solid plates. This is the human approach: our solar panels and satellite dishes are firmly anchored and flap in the wind only when things go wrong. But this approach is costly. Solid trunks and leaves would require a hefty investment in wood. Platelike leaves would also be less effective at photosynthesizing, having lost their gauzy openness to light and air. Such leaves would also take longer to make, delaying the tree’s springtime growth. Bulking up is therefore a poor solution.

  The tree’s answer to the wind’s force echoes the Taoism of the lichens: don’t fight back, don’t resist; bend and roll, let your adversary exhaust herself against your yielding. The analogy is reversed, for the Taoists drew their inspiration from nature, so “the Tao is Tree-ist” is more accurate.

  In moderate winds, leaves bend back and flutter. As the wind’s force increases, leaves change their behavior and absorb a portion of the wind’s strength, using it to furl into a defensive posture. The leaves fold onto themselves, rolling their margins to the center. They take on shapes of strange fish, shedding air from their aerodynamic surfaces. The compound leaves of hickories fold each leaflet to the central stalk, forming a loosely rolled cigar. Air rushes past, its death grip loosened. As the wind abates, the leaves spring back, unrolling into sails again. Lao Tzu reminds us: “Grass and trees are pliant and fragile when living, but dried and shriveled when dead. Thus the hard and strong are the comrades of death; the supple and the weak are the comrades of life. A weapon when strong is destroyed; a tree when strong is felled.”

  The trunk also yields to the wind’s push rather than resisting like a rock. It is designed to stretch and flex, absorbing energy in the microscopic cellulose fibers from which wood is woven. The fibers are arranged in coils, so each one acts as a spring. The coils are layered over one another, forming the water-carrying tubes that run up and down the trunk. Each tube has many coils, and each coil is wound at a slightly different angle. The result is a trunk filled with springs, each spring designed to exert its maximal pull at a different degree of stretch. The tightly wound springs resist strongly as the wood is first stretched. Looser springs take over as the tension increases and the tight springs fail.

  I look out across the forest and see nothing but trunks in motion. They scissor past one another, bending alarmingly as their crowns surge back and forth. Despite their elegant accommodation and avoidance of the wind’s power, there is a good chance that some of them will fall. Within five paces of the mandala there are two large fallen trees. Judging from their freshness, they likely fell within the last year or two. One, a hickory to the east, was uprooted. The other, a maple to the north, snapped its trunk four feet from the ground. Both trees were smaller than those that surrounded them. Perhaps their vigor was sapped by shading from larger competitors? If so, they would have grown little new wood, and fungi may have invaded the weakened trunks and roots, chewing up the cellulose coils. Bad luck may also have been involved. Either tree could have been hit by a particularly strong gust, and the hickory was growing amid root-blocking boulders. Whatever the particularities of their history, these fallen trees have now started the next part of their journey through the ecology of this old-growth forest. Fungi, salamanders, and thousands of species of invertebrates will thrive in and under the rotting trunks. At least half a tree’s contribution to the fabric of life comes after its death, so one measure of the vitality of a forest ecosystem is the density of tree carcasses. You’re in a great forest if you cannot pick out a straight-line path through fallen limbs and trunks. A bare forest floor is the sign of ill health.

  Today the forest floor is strewn not only with fallen trees and limbs but with green maple helicopters, the discarded immature fruits whose seeds were defective or whose stems were too weak. The seed embedded in each fruit was fertilized by sperm from a wind-borne pollen grain. The fruit is an airfoil, so as it spins it creates an upward push and its descent is slowed, increasing the distance it can travel. The wind is therefore the maple’s goddess of both sexual union and childhood wanderlust.

  The diversity of shapes of the maple helicopters scattered on the mandala suggests that maples are not passive recipients of the wind goddess’s whims; trees have the potential to mold themselves to her character through natural selection. Variation in fruit design may lead to evolutionary adaptation: those helicopter shapes best suited to the nature of wind in their corner of the world will survive and prosper. Even without such evolutionary change, the diversity of helicopter shapes allows each tree to buy hundreds of tickets in the aerodynamic lottery. Whether the sky howls, squalls, or puffs, the maples will have a helicopter design to suit the mood. The Taoist embrace of the wind is therefore a philosophy that applies throughout the tree’s life. Leaves furl, trunks bend, and fruits are variable enough to conform to, then use, the wind’s forceful nature.

  May 18th—Herbivory

  Springtime’s perfect leaves have turned ragged. Their smoothness is broken by irregular gashes or tidily incised bite marks. The interminable storms of the past several weeks are partly responsible. A sassafras sapling hangs low, its leaves shredded by hail. Maple leaves are similarly slashed. This physical violence is dramatic, but it accounts for only a small portion of the damage borne by the mandala’s leaves. The main culprits are the mouths of insects. They gnaw, suck, nibble, and rasp day by day, tearing down what the plants build up.

  Half all insect species are plant eaters, and insects account for half to three-quarters of the species on earth. Plants are therefore plagued by six-legged robbers. Small p
lant species such as clover have to contend with one to two hundred species of herbivorous insects, while trees and other larger species have a thousand or more. These estimates are from northern areas, so the number of insect species that might browse or suck on each plant species in the mandala is likely much higher. Species richness is higher yet in the tropics. The world is full of marauding vegetarians; no plant escapes their attention.

  The most obvious signs of herbivory in the mandala are holes in leaves. The bloodroot’s leaves are naturally deeply indented, but insects have disrupted the flow of these lines with gouges and nips. Toadshade trillium likewise is etched with irregular gaps. Spicebush leaves are dotted with oval excisions, and perfect semicircles are carved out of its leaf edges. The perpetrators—or artists, depending on your perspective—have left the scene. They are likely caterpillars, the larval stage of moths and butterflies. Caterpillars are champions of herbivory, designed to focus exclusively on turning leaves into insect flesh. But no caterpillars are visible, apart from one chewing on a maple leaf, the animal’s pulsing gut visible through its thin green skin. I search leaf margins, stems, and growing tips, finding nothing. The insects are either hidden in the leaf litter or have moved up the food web, perhaps in the belly of a nestling bird.

  Leaf miners have also left their mark, mostly in the leaves of seedling maples. Miners are like those humans who tear open a sandwich or cookie, eating the filling and leaving the crust. Miners do so not by opening the cookie but by diving inside, squirming their tiny flattened bodies between the upper and lower skins of the leaf. They tunnel into the cookie’s center, munching at the cells inside, inching forward and leaving behind a feeding scar. Over one thousand species of miner work North America’s leaves, and each species marks the leaf with its own style of scar. Some species move in circles, creating brown spots on leaves; others wriggle in seemingly random lines, scrawling thin paths across the leaf. More fastidious species move back and forth, systematically eating out the whole leaf, leaving a pattern like that in a freshly mown lawn. Leaf miners are the larvae of a taxonomically diverse selection of flying insects, including the young of flies, moths, and beetles. When the larvae have completed their work, they turn into winged adults that lay eggs on leaves, producing the next generation of miners.

  The viburnum shrub in front of me has an entirely different kind of herbivore on its stem. The insect sits on the tender new growth at the tip of the shrub, perfectly color-matched in rich green. Its head is down, facing away from the stem’s tip; its wings and body are slightly raised, shaped like an oriental slipper or a fancy Dutch clog. The overall effect is almost perfect mimicry of a bud. But this is no innocent bud. The green slipper is a leafhopper, an insect that attaches ticklike to its hosts.

  Leafhopper jaws are stretched into a thin flexible needle that can wriggle between plant fibers, reaching down into the plant’s blood vessels, the xylem and phloem. These are the same kinds of vessels that run up tree trunks, but in the thin-skinned new stems of viburnum the vessels are close to the surface and easily tapped by leafhoppers. Xylem carries mostly water, whereas phloem runs rich with sugars and other food molecules. Leafhoppers therefore prefer to feed on phloem, sliding their sharp mouthparts into the vessels. Because phloem is pressurized by the flow of sugary water from the leaves to the roots, leafhoppers simply tap into the vessels and let the plant squirt its food into their mouths. Leafhoppers, and their relatives the aphids, are so adept at tapping the phloem that they are put to use by scientists studying the plants. No human needle can match the exquisite delicacy of the insect’s mouth, so researchers parasitize the parasite by snipping off the needle, killing the insect but leaving a probe lodged inside the phloem cells.

  Insects feeding on plant sap face a bigger problem than the occasional sorry end in a lab. Phloem is a wonderful source of sugar but has few of the building blocks of proteins, amino acids. Xylem has little food of any kind. Phloem sap is ten to one hundred times poorer in nitrogen than are leaves, and leaves themselves are ten times poorer than animal flesh. Living on sap is therefore like trying to get a balanced meal from a case of soda. Leafhoppers solve the problem by drinking two hundred times their dry body weight in sap per day, equivalent to a human’s drinking nearly one hundred cans of soda each day. This enormous volume compensates for sap’s low concentration of nitrogen.

  The leafhoppers’ mega-drinking strategy creates another problem: how to flush out the excess water and sugar without also eliminating the nitrogen? Evolution has solved this problem by creating two paths for the phloem liquid that the leafhoppers drink. Their gut has a filter that sends unwanted water and sugar down a bypass, admitting only precious food molecules. The bypassed water and sugar is voided in drops from the anus, creating the sticky “honeydew” that coats plants infested with leafhoppers, aphids, or scales. Some entomologists claim that this honeydew is the manna that the Israelites ate during the Exodus. This is possible, of course, but it is hard to imagine anyone subsisting for forty years on the nutrient-poor excretions of leafhoppers, although honeydew supplemented by flocks of roasted quail might be feasible.

  Even with a sophisticated filtering system in their guts, the diet of leafhoppers is inadequate, or it would be if they did not receive help from bacteria. Not only is plant sap watery but it contains an unbalanced mixture of amino acids; some of the amino acids necessary for insect growth are present, but some are not. Insects cannot make the missing amino acids from scratch. Instead, leafhopper guts have cells specially designed to hold bacteria that make amino acids. This is a mutually beneficial arrangement: the bacteria get a place to live and a continual supply of food, and the insects get their missing nutrients. Unlike the microbes that swim freely in the deer’s rumen, these bacteria are embedded inside the cells of their host. Like the algae in lichens, the bacteria cannot live outside their host, nor can the host live without its internal helpers. The leafhopper on the branch in front of me is therefore a fusion of lives, another Russian doll in the mandala.

  The dependence of leafhoppers on their bacterial helpers is of particular interest to entomologists in the pest control business. Leafhoppers and aphids exact a heavy toll on crops and often transmit disease to the plants that they puncture. If the relationship between the insect and its bacteria could be poisoned or otherwise disrupted, the entomologists might be able to wash fields clean of these troublemakers. This idea has yet to be put into practice, but I hope that if it ever is, we will not let the bright light of our ingenuity blind us to the possible costs of our actions. Chemicals that sever the tie between beneficial bacteria and their hosts may have effects well beyond ridding crops of leafhoppers. The soil’s vitality depends on the action of such bacteria, as does the health of our own gut. At a deeper level, all animals, plants, fungi, and protists have ancient bacteria living inside their cells. Leafhoppers are the tip of the iceberg. Hammering at this tip risks sending fractures throughout.

  The mandala contains insects designed to steal every part of a plant. Flowers, pollen, leaves, roots, sap are all preyed upon by a diverse toolbox of insect mouthparts. Yet the mandala is green. Leaves are a little tattered, but they still dominate the forest. Above, leaves are stacked in layers, blocking my view of the sky; around me, shrubs stretch out across the hillside, again hemming in my sight; below, my feet rest in a carpet of saplings and forest herbs. The forest seems to be an herbivore’s heavenly banquet. Why is the mandala not stripped bare? This is a simple question, but it is much fought over, and it stirs up controversy among ecologists for good reason. The relationship between herbivores and plants sets the stage for the rest of the forest ecosystem. If we don’t get the answer right, or if we cannot produce an answer, our understanding of forest ecology is shipwrecked and we are swimming in ignorance.

  Birds, spiders, and other predators may give part of the answer. Their hunger might hack back the ravenous hordes of insects, protecting plants by preventing herbivore populations from getting large enough to fulfi
ll their destructive potential. A corollary of this idea is that herbivorous insects seldom compete among themselves; they are suppressed by their predators, not by their peers. This is important because competition is the force that drives evolution. If herbivore populations were limited solely by predation, we would expect natural selection to have lavished more effort on helping herbivores to avoid predators than on giving them an edge in competition for food.

  The idea that insect populations are suppressed by their predators has been tested by building cages around plants. If predation rules the insects’ world, insect numbers should explode inside the cages, and caged plants should be eaten down to nubs and stumps. The results of caging experiments are mixed. Insect populations do sometimes increase when their predators are kept away, but the jump in numbers is seldom dramatic, and in some seasons and locations the cage has no effect at all. Even when cages do boost insect populations, the caged plants remain leafy green, albeit more chewed than their uncaged kin. Predation cannot, therefore, be the only explanation for the seeming paucity of herbivores.

  We too are plant eaters, and our feeding behavior suggests another approach to the puzzle of the greenness of the woods. I live surrounded by maples, hickories, and oaks but have never sat down to a tree-leaf salad. Forest herbs grow in profusion at my feet but, again, I have not dined on them. My medicinal botany books tell me that small doses of the herbs in the mandala may alleviate ailments, but anything more than a nibble will cause (depending on the species of herb) heart stoppage, glaucoma, stomach upset, tunnel vision, or irritation of the mucous membranes. Our domesticated crops have had the toxins bred out of them, giving us a distorted view of the realities of herbivory. Granted, we have not evolved to be leaf eaters and lack the detoxifying biochemistry of most true herbivores, but our inability to eat most of the plants that surround us reveals an important point: the world is not as green as it seems. This point is reinforced by the very fact that other herbivores have specialized biochemical methods to neutralize the toxins in their food. The mandala is not a banquet waiting for guests to arrive but a devil’s buffet of poisoned plates from which herbivores snatch the least deadly morsels.