The Forest Unseen_A Year's Watch in Nature

by David George Haskell

  Can plantations ever be restored to foresthood? The lesson of the ice age is that such annihilation can be reversed, but at a pace measured in millennia, not decades. But the question is premature. The ice is not pulling back. Every major type of native forest in the southeastern United States is in decline. Only plantations are on the rise.

  The scale, the novelty, and the intensity of this change are unquestionable threats to the diversity of life in the forests. Whether or how we should respond to this erosion is a moral question. Nature seemingly provides no moral guidance; mass extinction is another of her many flavors. Neither can moral questions be answered by our culture’s obsession with policy think tanks, scientific reports, or legal contests. I believe that the answers, or their beginnings, are found in our quiet windows on the whole. Only by examining the fabric that holds and sustains us can we see our place and, therefore, our responsibilities. A direct experience of the forest gives us the humility to put our life and desires into that bigger context that inspires all the great moral traditions.

  Can the flowers and bees answer my questions? Not directly, but two intuitions come to mind by contemplation of a multifarious forest whose existence transcends my own. First, to unravel life’s cloth is to scorn a gift. Worse, it is to destroy a gift that even hardheaded science tells us is immeasurably valuable. We discard the gift in favor of a self-created world that we know is incoherent and cannot be sustained. Second, the attempt to turn a forest into an industrial process is improvident, profoundly so. Even the apologists for the chemical ice age will admit that we are running down nature’s capital, mining the soil, then discarding the spent land. This rash ingratitude, justified by the economic “necessity” created by our ballooning consumption of inexpensive wood, seems to be an outward mark of inner arrogance and confusion.

  Wood and wood products such as paper are not the problem. Wood provides us with shelter, paper with nourishment for the mind and spirit—unarguably wholesome outcomes. Wood products can also be much more sustainable than the alternatives such as steel, computers, and plastic, all of which use large quantities of energy and nonrenewable natural products. The problem with our modern forest economy lies in the unbalanced way that we extract wood from the land. Our laws and economic rules place short-term extractive gain over all other values. It does not have to be this way. We can find our way back to thoughtful management for the long-term well-being of both humans and forests. But finding this way will require some quiet and humility. Oases of contemplation can call us out of disorder, restoring a semblance of clarity to our moral vision.

  April 2nd—Flowers

  An impossible number of flowers blaze out of the mandala. Confusion sets in when I try to count them: two hundred and eighty, three hundred and twenty, too many crowded into one square meter. The flowers’ valets are in attendance, buzzing and humming in smart furry dress, fussing over the floral royalty. I join them in their observance and genuflect, then prostrate myself, hand lens pressed to my eye.

  A fountain of anthers arches from the chickweed’s open bloom. A central dome, the ovary, is ringed by gracile, creamy filaments holding up tawny knots of pollen grains. These filaments soar away from the dome, holding the pollen away from the flower’s own pollen landing pads, the stigmas. The chickweed has three stigmas, planted at the peak of the ovary’s onion dome, each one waiting for a pollen-dusted bee to brush past.

  The surface of the stigma is a forest of microscopic fingers, reaching out to embrace pollen grains. If the petals do their job and attract a bee, the stickiness of the stigma traps the rough-coated grains. Once pollen is caught, the stigma assesses it, rejecting any from different species. The plant also shuns its own pollen and that from close relatives, preventing self-fertilization and inbreeding. In a few species, this rule against self-fertilization is broken if no other suitable pollen arrives. Such self-fertilization is a strategy of last resort used by Hepatica and other species that bloom in the early spring. For these species, desperate self-love is better than no love at all when inclement weather grounds their pollinating insects.

  If the biochemical matchmaking goes well, the stigma’s cells release water and nutrients to melt the pollen’s tough armor. The pollen grain’s shell cracks open, ruptured by the swelling pair of cells within. The larger of these two cells grows, amoeba-like, out of the ruptured pollen coat and starts to burrow down between the enveloping cells of the stigma, forming a tube. Each stigma is at the tip of a stalk known as the style, and the pollen tube works its way down the style, either pushing between the cells or, if the style is hollow, flowing down the style’s inner wall like a drop of oil. The second, smaller pollen cell divides and forms two sperm cells. These float down the pollen tube, carried along like rafters in a flowing river. Unlike the sperm cells of animals, mosses, and ferns, these rafters have no oars; their movement is entirely passive.

  The style’s length is caused by the need to hold the stigmas up where bees will bump against them. This creates a challenging odyssey for the pollen tube and, therefore, a convenient testing ground on which the plant can assess her suitors. Bees dump many pollen grains onto each stigma, so the style may have several tubes growing at once. If so, the style becomes the Kentucky Derby of plant love. The sperm cells jockey their tubes toward the ovule, which contains the plant’s egg; the price of failure is the annihilation of the rider’s genes. There is some evidence that vigorous plants produce fast pollen tubes, so the style’s length allows the flower to select mates with a history of success. Perhaps the styles are a little longer than strictly necessary for intercepting bees, just to give the pollen stallions a good hard run.

  When the pollen tube reaches the base of the style, it burrows into the fleshy ovule. Here the pollen tube releases its two sperm cells. One sperm cell joins with the egg to make an embryo; the other joins with the DNA from two other tiny plant cells to make a larger cell with a triple complement of DNA. This triply endowed cell divides, fattens up, and becomes a food storage area for the developing seed, a store that humans have put to use as wheat flour and cornmeal. Such double fertilization is unique to flowering plants; sexual union in all other creatures requires only one sperm cell and one egg cell.

  The chickweed in front of my hand lens is a hermaphrodite, producing both pollen and eggs, male and female, in each blossom. Every flower contains all the necessary reproductive apparatus: pollen grains; anthers to make and store the grains; filaments to hold up the anthers, stigmas, styles; and an ovary to contain the eggs. These parts are all crowded within the cup of the flower, ringed by colored petals designed to appeal to animal eyes. Such tiny, tidily arranged complexity makes for a compelling display.

  All the mandala’s spring ephemeral flowers are hermaphrodites, a strategy well suited to these tiny plants that produce just a few flowers during a short, unpredictable season. By combining male and female into one flower, the plants leave open the door to self-breeding. They also spread their investment between male and female functions, increasing the chance that at least some of their genes will pass to the next generation. Other species, such as many wind-pollinated trees—oaks, walnuts, elms—use a different strategy, producing great numbers of unisexual flowers. In this case, each flower has a specialized task, either to shed pollen or to harvest pollen from the wind.

  Although the mandala’s plants share a hermaphroditic design, their geometry differs markedly from species to species. The Hepatica’s anthers grow in a thick bush around a cluster of pillarlike styles. Blue cohosh’s pallid ivory flowers have globular anthers squatting around a bulbous ovary with minute stigmas. Toothwort’s petals enclose a sheath around the hidden anthers. Only spring beauty has a flower somewhat like the chickweed. Its three stigmas sit atop a drooping trident, circled by five pink-tipped anthers.

  This variety reflects the tastes of the plants’ pollinators but is also caused by less obvious forces. Nectar robbers, for example, exert a stealthy but powerful influence over floral design. An ant h
as buried its head in a spring beauty flower in front of me. I use the hand lens to watch it bypass the pollen and stigma, then upend and steal the flower’s sugary nectar. This robbery is the cost borne by open-cupped flowers for welcoming a diverse set of pollinators: freeloaders move in and exploit your openness. Spring beauty flowers choose the most welcoming, and therefore the most vulnerable, strategy by freely offering nectar inside an open cup that is accessible to any insect. Hepatica and rue anemone also produce open cups, but neither offers nectar. These nectarless flowers lose little energy to thieves, but they are also less attractive to bees. Toothwort offers nectar enclosed in a tube that excludes robbing ants but restricts the number of bees that can reach into its recesses for nectar.

  The diversity of floral design is also affected by the longevity of plants and their flowers. Blooms that last for just a few days, such as those of spring beauty, are desperate for pollinators. This favors a bohemian style, risking all for the kiss of a bee. If the bee’s embrace is accompanied by some ne’er-do-wells, so be it. Longer-lived flowers can be more restrained, holding back the nectar or enclosing their bloom in the knowledge that sooner or later a decent suitor will come along. The longevity of the plant that produces the bloom also factors into the economy of flowering. All the spring ephemerals are perennials that sprout from underground roots or stems. If a creeping stem lives for three decades, it can afford to be restrained in its search for pollinators. A shorter-lived root might be more willing to tolerate a few freeloaders. Both factors, the duration of the bloom and the longevity of the plant, are variants of the same theme: shorter lives must burn brighter.

  Flowers therefore perform economic gymnastics as they balance losses to robbers with the need to lure pollinators. How this performance unfolds depends not just on the insects flying around the mandala but also on the ancestry of the plants. Natural selection tinkers with the raw materials provided by previous generations, so each flower’s design is shaped by the particularities of its genealogy. Different plant families have different sets of equipment, constraining their acrobatics.

  Hepatica and rue anemone belong to the same family, the buttercups, all of which produce nectarless flowers with open cups. Great chickweed belongs to the pink family. This family’s name comes from the common name of Dianthus, a sweet-smelling garden flower. The flower named the color pink, and the jagged edges of its petals also gave their name to pinking shears, scissors that dressmakers use to cut zigzag edges. The “pink” plant family is named for the shears, not for the color—great chickweed inherits a tendency for serrated petals. At first glance, its ten slender white petals seem to have broken away from familial tradition. A closer look shows that the flower has just five petals, each one so deeply cleft that it seems to be double. Chickweed therefore pushed to the limit its family’s preference for ornamented displays and created an illusion of extra petals.

  Like all living creatures, our own lives included, the flowers layer adaptation over history, creating the tension between diversity and unity, individuality and tradition, that makes the mandala’s immoderate blaze so compelling.

  April 8th—Xylem

  The weather has been unsettled lately, dropping sleet on one day, then blazing with hot sunshine the next. The pace of life in the mandala follows these variations. On slushy days, leaves droop and the forest is silent except for the drumming of woodpeckers. Today, the sun is out and life has quickened, with revived greenery, a dozen species of singing birds, several small swarms of flying insects, and an early tree frog chirping from a low branch.

  Last week, the forest’s green lay across the ground, a carpet of photosynthesis that ended at ankle height. Now the maples are unfurling leaves and dangling green flowers from branches. Like a tide rising, the green glow is reclaiming the forest from the ground up. The upward surge floods the mountainside with a sense of renewal.

  Sugar maple branches hang over the mandala, and their new leaves block the sun’s rays, shading the understory. Of the hundreds of spring wildflowers, only a dozen remain; the maple has snuffed their spark. But not all the trees around the mandala are in leaf. The maple’s exuberance contrasts with the dour, lifeless pignut hickory that stands on the other side of the mandala. The hickory’s massive gray trunk rises straight to the canopy where it holds out dark, bare branches.

  The contrast between the maple and the hickory expresses an inner struggle. Growing trees must throw open the breathing pores on their leaves, allowing air to wash the wet surfaces of their cells. Carbon dioxide dissolves into the dampness before it is turned to sugar inside the plants’ cells. This transformation of gas into food is the trees’ source of life, but it comes at a cost. Water vapor streams out of the leaves’ open breathing pores. Every minute, several pints of water are exhaled into the air by the maple above the mandala. On a hot day, the seven or eight trees whose roots penetrate the mandala send several hundred gallons of water out of their leaves as vapor. This reverse waterfall quickly dries the soil. When the supply of water is exhausted, the plant must close its breathing pores and cease growing.

  All plants face this trade-off between growth and water use. But trees have another devilish layer of difficulty. By thrusting their leaves skyward they have become slaves to the physics of their plumbing systems. Inside each trunk lies the vital connection between earth and sky, soil’s water and sun’s fire. The rules that govern this connection are stringent.

  Inside the trees’ leaves, sunlight causes water to evaporate from cell surfaces and drift out of breathing pores. As vapor wafts away from wet cell walls, the surface tension of the remaining water tightens, particularly in the narrow gaps between the cells. This tension yanks more water from deep in the leaf. The pull moves to the leaves’ veins, then down the water-conducting cells in the tree’s trunk, finally all the way to the roots. The pull from each evaporating water molecule is minuscule, like a breath of wind tugging at a silk thread. But the combined force of millions of evaporating molecules is strong enough to haul a thick rope of water up from the ground.

  The trees’ system for moving water is remarkably efficient. They exert no energy, instead letting the sun’s power draw water through their trunks. If humans were to design mechanical devices to lift hundreds of gallons of water from roots to canopy, the forest would be a cacophony of pumps, choked with diesel fumes or run through with electrical wires. Evolution’s economy is too tight and thrifty to allow such profligacy, and so water moves through trees with silence and ease.

  Yet this efficient water-lifting system has an Achilles’ heel. Sometimes the rising columns of water are broken by air bubbles. These embolisms plug the flow of water. Winter weather makes these blockages more likely because air bubbles form when water freezes inside water-conducting cells. These are the same bubbles that haze ice cubes in kitchen freezers. Thus icy weather peppers the trunk with air gaps that wreck the trees’ plumbing. Maple and hickory have found two different solutions to this challenge.

  With its bare branches, the hickory looks wintry and inactive, but this is an illusion. Inside, the tree is building a whole new plumbing system, readying itself for the flowers and leaves that will emerge in a couple of weeks. Last year’s plumbing system is useless, blocked by embolisms. So, hickory trees spend the first part of April growing new pipes. Just below the bark, a thin sheet of living cells wraps the trunk. These cells divide and create the season’s new vessels. The outer layer of cells, those that lie between the bark and the sheet of dividing cells, will become phloem, a living tissue that transports sugars and other food molecules up and down the tree. The new cells formed on the inner side will die and leave their cell walls to become the xylem, or wood, that conducts water up the trunk.

  Hickory xylem tubes are long and wide. These pipelines offer little resistance, so the flow of water is prodigious when the tree finally leafs out. But the width of these tubes makes them particularly vulnerable to blockages by embolisms. Once blocked, they become useless and, because the tr
ee has relatively few of these wide conduits, the flow of water drops significantly with just a few embolisms. This design means that hickories must delay the growth of their leaves until the danger of frost is past. The trees miss out on the warm sunny days of spring, but they recoup these losses when their pipelines are thrown open later in the season. The hickory is therefore like a sports car—kept off the road by ice until late in the spring, but outstripping all rivals on warm summer days.

  Hickory trunks have one more problem. Their wide, long xylem tubes are weak, like thin-walled straws. These tubes cannot hold up heavy branches or cope with the force of wind pulling on leaves. Therefore, later in the year, after the springtime xylem has grown, the tree grows thick-walled, smaller-bored xylem vessels. This summer-grown xylem provides the structural support that the water-carrying vessels lack. The yearly alternation is visible in cut hickory wood as a “ring porous” pattern of wide porous cells separated by denser wood.

  If hickories are sports cars, then maples are all-wheel-drive passenger cars. Their xylem is frost-resistant and lets them leaf out weeks before the hickories. But, come summertime, maples will lag behind hickories in their ability to carry water and thereby feed on sunlight. The maples’ xylem cells are more numerous, shorter and narrower than the hickories’, and they are separated by comblike plates. Unlike the broad, open tubes of hickory, the maple’s design confines embolisms to the small cells in which they form. Because maples have so many small tubes, each embolism blocks just a tiny fraction of the trunk. Unlike the ringed patterns in hickory wood, maple wood is more uniform, showing a “diffuse porous” pattern. These differences are visible in furniture and other wood products—maple is smooth-grained, whereas hickory has regular rows of pinholes.