A FOREST'S CYCLE OF LIFE
The southwest wind of an early-April storm in 1486 blows inland from the Pacific Ocean. High in a Douglas-fir tree growing along a stream on the western flank of Misty Mountain, a female cone awaits the pollen borne aloft on currents of air. As millions of pollen grains ride the wind, swirling about the receptive cone, some fertilize the waiting, female gametes.
Over the days and weeks, the seeds mature. With the approach of autumn, the seeds are blown from their resting places in the cone by a gust of wind. Landing in an opening on the forest floor, most are eaten by small animals. Five, however, remained to germinate. Of those, only two seedlings survive the next two and a half centuries (1486 to 1736), passing through the stages of adolescents and maturity, to enter the portal of old age.
In 1836, when the trees are 350 years old, a severe windstorm knocks the top out of one of the trees and breaks some of its branches, which allows a fungus to enter the wounds and begin to rot its body. Simultaneously, a falling 800-year-old monarch hits the tree's sibling hard enough to remove some of its protective bark near the ground, where it causes a small, elongated wound. Although tiny compared to the size of the tree, the wound attracts wood-boring beetles, which chew their way into the wounded area. In so doing, the beetles expose the wood to fungal spores, which germinate and begin attacking the tree's tissues. The fungus spreads, killing the tissues and weakening the tree.
By 1911, when the tree is 425 years old, it has a large weakened area ten feet above the ground. That year, a newly matured, queen carpenter ant sets up housekeeping in the area of the old wound. As the ant colony grows, new tunnels are continuously chewed in the sapwood by worker ants, each of which is intent on expanding the colony. A pileated woodpecker finds the ant colony in 1970 and pecks a large, squarish hole into the mainstream of ant life, where it returns periodically over the years to dine on the ants.
In 1986, a windstorm, with gusts reaching seventy miles per hour, blows over a small group of Douglas-fir trees that had been weakened by a root-rot fungus. One falls against the tree inhabited by the ants, which is now 500 years old. It breaks off in the area of the old wound and falls diagonally upslope into the water of a small stream. There it stays, creating habitat for fish and other organisms, until 1996, when a spring freshet dislodges the old tree and moves it a hundred yards downstream, where it is once again anchored to the stream's bank by the vegetation growing there.
In the year 2000, there is a flash of fading silver, a swirl of bright water, and a female salmon flexes her tail against the swift current as she propels herself to a small gravely bar just under the surface near the headwaters of the stream on Misty Mountain, into which the old tree fell. Again a flash of silver, then another, and another as other salmon press against the rush of clear, cold water, each seeking the exact spot to which its inner drive to spawn impels it.
Suddenly, from somewhere in the shadow of trees overhanging the stream, there comes a large, magnificent male of metallic luster now blotched with a whitish, life-sucking fungus; he swims alongside the female with powerful undulations of his body. They touch, and the female immediately turns on her side and fans the gravel with strong beats of her tail.
She continues spraying gravel into the current until a shallow depression comes into definition, after which she begins depositing reddish-orange eggs, as the male squirts milky-white sperm into the water. The cloud of sperm, enveloping the eggs as the current carries it downstream, fertilizes them as they settle into the shallow "nest."
Having spent themselves to ensure the essence of their existence through their offspring, the female covers the nest with powerful strokes of her tail against the gravely bottom. Now she and her mate, having fulfilled their life's purpose, swim into deeper water, where they rest and die. Their death, however, is the gift of life for last year's generation of salmon because the youngsters glut themselves on the decaying carcasses of the adults, just as this year's generation will do next year.
But for now, there is an orange, opaque egg in the gravely bottom of the stream inside of which a female salmon is developing. In time, the baby salmon hatches and struggles out of the gravel into the open water of protected, hidden places in the stream. Here, she will grow for a year, until it's time to leave the stream of her origin and venture into life.
It's now 2002, and the young salmon can go only one way—downstream to larger and larger streams and rivers, until at last she reaches the ocean, all the way beset by increasing numbers of distracting nooks and crannies to explore and dangers to overcome.
On her way to the sea, the young salmon depends on the driftwood that accumulates in the streams and rivers as instream habitat—including an old Douglas-fir tree that fell into the stream in the 1986 and has been in its current location since 2000, where it protects her from the swiftness of the current and from opportunistic predators. In addition, large, well-anchored pieces of wood also help to stabilize the stream's channel, increasing the predictability of its configuration from year to year, as well as helping to form the under-water bars of clean gravel in which salmon spawn.
As important as the driftwood is, salmon from all of the various streams, rivers, and estuaries leave it behind as they mingle in the open ocean, where external influences, such as ocean currents and the up-welling of cold water, affect them in common in what might be called a pool of commonality. It is therefore impossible to view salmon in the open ocean as discrete populations because they behave as an aggregate individual with no visible affinity to a particular river and stream.
Only after some years at sea will the inner urge of individual salmon dictate their approaching time to spawn. This inner urge will drive the adult salmon along the Pacific Coast to find the precise river they descended years earlier, and in so doing, the aggregate individual will differentiate into identifiable freshwater populations that are reproductively isolated from one another, each with its own affinity to a particular river.
Once in the river, they will again differentiate as discrete subpopulations, each with its own affinity to a particular stream within the river system. A salmon's ability to find its required spawning area depends on its guidance system secreted in the genetic code within each member of the population, which leads it back to its home waters, when the time to spawn finally arrives.
Thus, in youth the many traveled seaward to become in aggregate the one. Although most died either on that journey or at sea, the rest confronted the external commonalties that helped to shape their lives. Then comes the time of maturity, when the compelling inner drive to spawn, to achieve their life's purpose, causes them to separate into smaller groups. Many more died on the upstream journey, which reaches its climax with the act of spawning, after which they all die, returning to the Great Mystery from whence they came.
But the female that descended the stream in 2002 and rested under the old Douglas-fir, finds it again in this year of 2007, the year of her sexual maturity. Here, she rests awhile before compelling her battered, fungus-blocked body to its final destination, the place where she will spawn and die.
As the dead salmon wash into the shallow water along the edge of the stream's banks, they enter the atomic interchange, where they represent a biophysical mechanism through which the elements of their bodies become concentrations of nutrients and energy that subsidize the forest ecosystem that helped nourish them in preparation for their journey to the sea. This massive infusion of decomposing salmon in the forest stream promotes the growth of algae and bacteria, which help sustain aquatic insects.
Juvenile salmon, steelhead, and cutthroat trout also poke around the expired, rotting bodies, eating the eggs left in the females and, eventually, picking off pieces of flesh. This huge addition of nutriments is critical for the young salmon because the rich banquet of dead fish enables youngsters to double their weight in about six weeks. The added body weight greatly increases the chances that a particular fish will survive to swim the gauntlet from the stream of its origin far out into the North Pacific Ocean and return again years later to spawn in the place it hatched.
As a carcass decomposes underwater, its dissolved nitrogen and carbon are soaked up by algae and diatoms, the one-celled plants that form a scum on the gravel and rocks, which in turn is grazed by aquatic insects, which in turn is food for the salmon that will hatch the next spring. In addition, birds and mammals that feast on the carcasses, such as eagles, jays, ravens, wrens, skunks, otters, raccoons, bears, foxes, mice, and shrews, deposit their droppings on the forest floor.
The culmination of this great infusion of nutrients is that the scum on the gravel and rocks and the plants along the stream's banks, including trees, suck up the nitrogen from the rotting salmon because nitrogen is an element in short supply in the soils of the Pacific Northwest. Sitka Spruce in southeast Alaska, for example, take only 86 years to reach a trunk thickness of twenty inches when fed by the decomposing carcasses of spawned-out salmon, as opposed to the normal 300 years to reach that trunk thickness without the benefit of the salmon carcasses. The forest plants then drop their leaves, needles, and twigs into the stream, providing more food for the aquatic insects and, ultimately, the young salmon, as well as shade for young fish in which to hide, eat, and grow.
Nature's feat of nourishing the plants and animals requires about one salmon carcass per every three square feet of stream edge. This can be roughly translated into approximately one dead salmon for the amount of water that would today fill a standard bathtub.
One turn of the cycle is thus completed—the reciprocal gifts of driftwood and young salmon from the forest to the sea and of adult salmon from the sea to the forest. As driftwood travels down the streams and rivers, it carries the carbon and nitrogen of its body to the food chain of the ocean and creates stabilizing instream structures and habitat for young salmon and other fish as the driftwood rests here and there along the way. As salmon travel seaward, they also bring elements, such as nitrogen and carbon, from the forest to the sea. Those that die at sea leave their forest-derived elements in the ocean, whereas those salmon that survive to swim the gauntlet back to their stream of origin leave their ocean-derived elements in the streams and vegetation of the forest.
The winter of 2016 is heralded by a series of violent rainstorms, causing severe flooding that sweeps the once-anchored Douglas-fir down river and out to sea. Heavy seas and strong winds move the tree northward several miles and cast it onto a beach. Barkless and battered, the tree lies bleaching in the sun; still visible, however, are the holes made by the pileated woodpecker 45 years earlier.
The tree remaining in the forest, on the other hand, will stand firm for another two and a half centuries (until 2266), despite the storm that snapped out its top and broke some of its branches, despite the fungus that entered the wounds and began to rot its body. But in 2266, the now-ancient tree will not be able to withstand an ice storm, which in this year weights it so heavily on its downhill side that it will succumb to the call of gravity. The toppling of the 780-year-old tree will forever alter the forest and start an irreversible chain of events that will last for more than four centuries.
The initial response of the forest denizens to the falling of the ancient tree will vary. By night, a deer mouse, which felt the vibrations sent through the Earth by the impact of the old tree, will come to investigate. A passing black-tailed deer will stop to eat lichens off the branches, which are suddenly and unexpectedly accessible. By day, a chickaree (a small, tree-dwelling squirrel) will climb onto the old tree and establish a lookout from which to survey its territory. Birds will land on the branches to warm themselves in the sun coming through the new opening in the forest canopy or to search for insects. Flies will land on the tree to sun themselves, and ants will include the newly fallen tree in their foraging territories.
The fallen tree, warmed in the sun, will attract thousands of bark beetles that will chew through the bark and thus connect the outside world with the inside of the tree. As they enter and begin to use the tree, they will introduce fungal spores and also initiate the inevitable nutrient cycle with the first deposit of their bodily wastes.
The character of the available wood, like that of all trees, will vary greatly in different parts of the tree, where proteins are concentrated in the living tissues and carbohydrates are concentrated in the dead, woody tissue. The living tissue will be more easily digested than the nonliving sapwood, but moist sapwood will be more digestible than the drier heartwood.
The living tissue, which is located just under the bark, furnishes the most nutritious food, so this microhabitat will be promptly occupied. The area of next importance is the sapwood, then the heartwood, and finally the bark. Each portion of the fallen tree will thus support a characteristic group of insects adapted to that specific microhabitat. The numbers of any one species will be regulated by the quantity and quality of their food supply, as well as predator-prey relationships.
A year will pass. Needles and bits of lichen from neighboring trees will join the snow and rain as it comes through the forest canopy to collect in the fissures in the bark of the fallen tree. Another year will pass. Seeds of western hemlock will land on the fallen tree and germinate. Those that land in a crevice filled with organic debris from the canopy will grow for a time. But as summer arrives and the forest becomes hot and dry, the seedlings will shrivel because their tiny roots cannot yet penetrate the thick, outer bark of the fallen Douglas-fir.
A Townsend chipmunk, collecting salal berries in its cheek pouches, will scamper along the top of the fallen tree. Suddenly, out of the shadows bounds a long-tailed weasel, which will bite the chipmunk through the base of the skull. Some of the salal seeds will spill onto the tree. Again, it is too soon, and they will germinate only to perish.
An ever-changing variety of animals will use the tree for shelter, food, foraging, and as perches during its first two years on the ground. And throughout each year, the surrounding vegetation will continue to grow and change, gradually adding another dimension of ever-increasing diversity to the fallen tree.
When the old tree falls, it will create a notable hole in the forest canopy. Without further disturbance, the increased light striking the ground will "release" the shade-tolerant understory trees to grow and, with time, to fill in the hole. (Shade-tolerant means that a plant can survive in the shade of another plant; when the shade is removed, however, the plant responds with increased growth). There are many scenarios in the response of ground vegetation to the falling of a tree because each tree falls differently, further compounded by the tree's size, species, and health, as well as characteristics of the surrounding forest.
It is likely that understory vegetation, both existing and potential, will be released when the old Douglas-fir falls because of the large opening created in the canopy that admits light to the floor of the forest. Suddenly, space and resources for plants to germinate and grow will become available: first in the mineral soil of the newly exposed rootwad; second on the fallen tree itself as the thick, furrowed bark accumulates litter that forms a seed bed; and third, in the decaying trunk, where plants will become established in the wood under the bark.
The newly fallen tree will interact only passively with the surrounding forest because its interior will not yet be accessible to plants and most animals. But once fungi and bacteria, which are smaller than the wood fibers, gain entrance, they will slowly dissolve the tree's tissue and enter the cells; meanwhile, wood-boring beetles, carpenter ants, and termites will chew their way through the wood fibers. But many other organisms, such as green plants, mites, springtails (also called collembolans), amphibians, and small mammals, will have to await the creation of internal spaces before they can enter.
When the old tree has been on the ground for a century, plant roots will have penetrate the decaying wood, which they will split and compress as the roots elongate and thicken in diameter. Because of all this internal activity, the longer the fallen tree rests on the forest floor, the greater the development of its internal surface area. Most internal surface area will result from biological activity, the cumulative effects of which not only increase through time but also act synergistically—insect activity promotes decomposition through microbial activity that, in turn, encourages the establishment of rooting plants, and so on.
Thus, the old tree will increasingly offer myriad organisms multitudes of external and internal habitats that will change and yet persist through the decades. Of the multitude of organisms, a casual observer might notice only a few, such as mushrooms or bracket fungi. These structures, however, are merely the fruiting bodies produced by the colonies of mold that will run for miles within the tree. Many such fungi can be seen only when a decaying tree is torn apart because they fruit within the body of fallen tree. Even then, only a fraction of the fungi present might be noticed because the fruiting bodies of most appear for only a short time.
As vegetation becomes established on and helps to stabilize the "new soil," created by the decomposing tree, and as invertebrates and small vertebrates begin to burrow into the new soil, they will both enrich it nutritionally with their feces and urine and mix it through their burrowing activities.
As for the Douglas-fir that washed out to sea in 2016, it remains beached for fourteen years before a winter storm along the coast in 2030 again washes it out to sea, where it floats for a year before being seen by a 25-year-old tuna fisherman. Setting his nets, he makes a bumper catch under the floating tree, which is the only shade-producing structure for hundreds of miles in the open ocean.
In the winter of 2033, the tree is again carried toward land and this time is deposited in a mudflat along an estuary. A storm in 2040 moves the now-waterlogged tree toward the mouth of the estuary, where it finally sinks. Marine, wood-boring invertebrates, such as gribbles, are attracted to the tree and penetrate its wood. The sunken tree is fragmented over the years as the marine invertebrates tunnel their way throughout it. In addition, much of the wood fiber has been excreted in the feces of gribbles, tubeworms, and other organisms that live in the sunken tree. During this time it has served as cover and habitat for gribbles, shrimp, crabs, and fish, to name but a few.
Meanwhile, in the forest, the flow of plants and animals, air, water, and nutrients between a fallen tree and its surroundings increases, as the tree's decomposition process continues. The water-holding capacity of the fallen tree varies by day, season, year, decade, and century, adding yet another dimension of diversity to the forest.
Surface area develops within the fallen tree through physical and biological processes because the tree suffered some cracks and splits when it fell, which gradually began to dry. In addition, microbial decomposition has broken down the cell walls and further weakened the wood. Larval, wood-boring beetles and termites tunneled through the bark and wood, not only inoculating the fallen monarch with microbes but also opening it to colonization by other microbes and small invertebrates. Wood-rotting fungi produced zones of weakness, especially between the tree's annual growth rings, by causing the woody tissue laid down in spring to decay faster than that laid down in summer.
A series of violent rainstorms in 2211 cause severe flooding, and the sunken tree in the estuary begins breaking into chunks that wash back and forth with the tides. By 2250, all that remains of the tree that went to sea is the substance of its cells, called lignin, which are now part of the organic material that enriches the floor of the continental shelf, as part of the ocean's never-ending story off the coast of the Pacific Northwest of the United States.
Time passes, as it inexorably does, and the last vestige of the old tree's body in the forest disappears in 2675, after having influenced that site on which it germinated, grew, matured, died, and decomposed over nearly a thousand years. Thus is the soil enriched by the life of a single tree as an integral and ongoing participant in the forest's never-ending story.