Variations on a Forest Theme

Hardwood forest, with its animal inhabitants, is obviously the dominant natural theme in Shenandoah. But within this theme are variations wrought by both nature and man. A walk to South River Falls, in the central section near Swift Run Gap, takes us through a forest sequence that well illustrates some of these variations, and climaxes at one of the park's most beautiful waterfalls. So let's go there!

It is June, and as we leave the South River picnic grounds we pass through a tangle of locust, black cherry, persimmon, dogwood and grapevine. The leaves are fully out, and animal life is at a peak of activity. Down the trail, a thick stand of Virginia and pitch pine marks a former pasture. The songs of towhees and chestnut-sided warblers, as well as the tree species, proclaim this an early stage of second growth. A rotting rail fence and occasional rock piles tell us that man was responsible.

(Photo by Ross Chapple)

Now we step into the pines. In little openings among the trees, grassy patches from pasture days remain. But scattered here and there we find a few seedlings of yellow-poplar, hickory, ash, oak, and sugar maple—the beginnings of a future forest.

Returning to the trail and following it downhill, we come to a sudden change in the forest. At an old rail fence, where the slope pitches downward more steeply, the young growth on the former pasture ends, and tall oaks begin. We guess by their foot-thick trunks and 80-foot height that they are 50 to 60 years old. We see no pines or locusts among them. Perhaps, then, this steep mountainside was logged shortly after the turn of the century, and seedlings of the cut trees began to reproduce the original forest. A few old sawed-off stumps bolster our theory. We continue down the switchbacking trail into these taller woods.

Now a boulder beside the trail invites us to sit. Looking up, we see a maze of leaves against the blue sky, and we pick out the different trees by the shapes of their leaves—the pointed lobes of red oak leaves, the wavy-edged leaves of chestnut oak, the four-pointed mitt of the yellow-poplar. We search harder and find a few pointed leaflets of hickory and ash. Round about us, on our own level, we see an understory of small hickories, black birch, dogwood, redbud, and sugar maple. The ground cover is rather open, but there are seedlings of some of the larger trees as well as of maple-leaved viburnum. Beside the trail, Christmas, hay-scented and maidenhair ferns spread their lacy fronds. Now that the spring burst of wildflowers has passed, and the leafy forest roof has closed, little is blooming on the forest floor except a few Gray's penstemons, wild hydrangeas and bluets. But some plants without chlorophyll—ghostly-white Indian pipes and brown-stained squaw-root—are pushing up. Obtaining nutrients from decayed material or roots of living plants, these fleshy saprophytes are freed from direct dependence on sunlight.

We shut our eyes and let our ears investigate the forest. They pick up the voices of the red-eyed vireo, scarlet tanager, wood pewee, wood thrush, downy woodpecker, ovenbird, and redstart. A chipmunk, suddenly aware of our presence, squeaks nearby. The rustle overhead is a gray squirrel, springing from tree to tree.

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All this seen and heard life, along with a great deal more unseen, unheard life, is part of a complex forest community, each of whose members exploits the forest's resources in a different way, all living together in a shifting equilibrium of competition and cooperation. One reason so many forms of life can live here is that the mature forest provides a diversity of micro-habitats. The gray squirrel spends much time in the canopy, eating buds, nuts, and seeds of trees. Up there with him is that glowing red ember with black wings, the scarlet tanager. This bird, along with others, feasts on a great assortment of insects—such as moth caterpillars, leaf miners, and aphids—that live on tree foliage just under the surface of the canopy. Red-eyed vireos nest and do much of their feeding in the forest understory, hanging their neat little basket nests from forks of branches. The wood thrush generally nests in shrubs, such as spicebush, and hunts food on the ground, while chipmunks make their homes in burrows or under rocks or logs, and feed from the ground up into shrubs and small trees. Restricting most of its operations to the ground layer, the ovenbird builds its domed nest and feeds among the leaves of the forest floor, flying to higher levels only to sing.

At ground level, most animal life is secretive and tends to escape the notice of the casual observer. But turn over a rotting log and you may find a little ring-necked snake, eater of small prey such as insects and worms. An even more likely find under that log would be a red-backed salamander. Herpetologist Jaeger considers this the most abundant vertebrate in Shenandoah National Park and possibly in the eastern United States. He estimates that, where the ground surface does not dry out, there is one redback for each 2.9 square yards in the park, and five redbacks if the subsurface area is included. This salamander coexists with such larger ground-dwelling predators as the ring-necked snake by taking smaller prey. On wet nights it extends its hunting territory by climbing shrubs and trees.

If we searched a long time on this slope we're descending, we might also find a copperhead. well-camouflaged in the leaves by the hour-glass patterns on its reddish-brown back. A white-footed or deer mouse, venturing out of its nest to hunt for seeds, would have to be sharp-eyed indeed to notice the copperhead, lying motionless in wait.

As we near the last switchback going down to the stream, we notice some sugar maples and basswoods among the oaks and hickories—a good sign that the soil holds more moisture here. And below the last switchback, another trailside rock offers a place to sit and enjoy the coolness, shadowy mystery, and music along the stream. Shafts of sunlight pierce the canopy to light the silver-gold trunks of yellow birch, the furrowed, red-brown trunks of aged hemlocks, the cracked, platy trunks of sweet ("black") birch. Spots of sunlight fall too upon mossy boulders bearing gardens of ferns and small flowering plants. Through the boulders burble many litile channels of South River, here in its infancy. The stream's music is accompanied by songs of the Louisiana waterthrush and is punctuated by the calls of a phoebe, an Acadian flycatcher, and numerous chipmunks. A red squirrel chatters at us from a hemlock branch.

The richness of life in this stream valley can hardly escape us. If we look carefully about, we see an even greater variety of trees than on the slope above. Besides the hemlock and yellow and sweet birches, there are sugar and red maple; red, white, and chestnut oak; yellow-poplar; hickory; ash; basswood; butternut; black gum; and American and hop hornbeam. Insects, some of which dance above the stream, are more abundant here, too. We have descended along a gradient of increasing soil moisture, depth, and fertility to a sort of vegetational climax in this hollow.

Now as our eyes swing upslope behind us, they catch an anomaly in the forest pattern. Here, instead of scattered big trees, we see a thick stand of young sweet birches, some draped with grapevines. We walk up to investigate. Aha! Here is a depression in the hillside, outlined by leaf-covered rocks; near it is a smaller one. Someone a few generations ago, liking this spot as much as we do, built his home here, and beside it dug a root cellar. Sometime after he left, sweet birch seedlings claimed the opening, and leaf litter and the infinitesimal creep of soil began to return the slope to its original contour. Perhaps in another couple of generations only an expert eye will be able to discern that man once disturbed the forest here.

The barred owl lives in the deep woods of the coves and stream valleys (top, left). (Photo by Paul Favor) The red-backed salamander feeds on small invertebrates (middle, left). Though the ringneck snake is common, its small size and secretive habits make it an inconspicuous part of the forest community (bottom, left). (Photo by Frank Deckert) The white-footed mouse feeds on seeds, nuts, and insects (top, right). The gray squirrel prefers forests of oak, beech, and hickory (middle, right). (Photo by Hugh Crandall)The copperhead is well camouflaged when in its normal habitat, the leaf-strewn forest floor (bottom, right).

Pleased with our sleuthing ability, we watch more carefully for clues to the nature of this community as we walk down the hollow. Suddenly, among the roots of a big yellow birch, which burrow into the soil like huge banded worms, we see white droppings and fragments of crayfish shells. Who was here? Probably a barred owl or broad-winged hawk sat in this small dead tree beside the birch and regurgitated these indigestible parts of crayfish taken from the stream. For both of these woodland birds eat aquatic animals as well as mice and other land animals.

Thinking about these winged predators, which hunt over many acres of slope and valley, we wonder about some of the other large animals that surely wander this South River country. The forests in this valley are too grown up to provide good browse for deer. Hoofprints along the stream tell us, however, that deer have come here to drink. Perhaps on our way we will find the fur-filled droppings of a bobcat, which might very well have a den in the rocks on these steep slopes. Bobcats, seldom seen because of their shyness and nocturnal habits, are nevertheless considered fairly common in the park, especially at the higher elevations. Rabbits, squirrels, mice, and birds form most of their diet, though like many predators they will eat almost anything they can catch, even occasionally a deer. No doubt, too, a flock of turkeys roams these slopes, scratching among the leaves for acorns and insects. It takes a bit of luck to spot turkeys, and usually the glimpse is brief as the wary birds trot rapidly off.

But the animal that most captures our thoughts is the black bear. Along the trail we have seen characteristic droppings and decaying logs that have been torn apart—evidence that one has been here. But as usual the maker of these signs does not appear. Nevertheless, his felt presence lends excitement to our walk.

From thoughts of bears we return our focus to the forest around us. Having admired the diversity of life along the path, we begin to be aware that death, too, is part of the forest scene. Uprooted giants lie moldering on the slopes; trees felled long ago by man leave their tell-tale stumps; and others, snapped by the wind, hang in their neighbors' branches.

Fire, too, has taken a toll. Blackened stumps and tree bases show that fire has long been a fact of life in these forests; it can come again any time.

Among the principal agents of tree destruction are insects and disease, often working together. The most devastating tree disease ever to hit North America was the chestnut blight, caused by a tiny fungus. Introduced around 1900, it wiped out virtually every mature chestnut tree in America by the late 1930's. Once abundant in Shenandoah, the chestnut exists now only as gray, twisted stumps and logs, and as sprouts that grow from the surviving roots only to die after a few years. In these South River forests and elsewhere, many of its bleached skeletons stand in dignified death among their living associates, preserved this long by the tannin in their wood.

The Shenandoah Food Pyramid. Because there is about a 90% loss of energy in each stage of the food chain, it requires about 1,000 pounds of green plants to produce 100 pounds of plant eaters. When taken as food, these 100 pounds of plant eaters will produce only 10 pounds of animal eaters. Thus, any plant-and-animal community contains a much greater mass of green plant material than the total mass of plant eaters. And there will be a much greater mass of herbivores than carnivores.

Many other diseases and insects attack Shenandoah's trees, but most are taken in stride as part of an environment to which these tree species have become adapted over the centuries. Usually it is a foreign invader, such as the chestnut blight, that is not part of the native system of checks and balances, that proliferates and causes real devastation. But trees, like people, must die somehow. As a tree ages, its ability to heal wounds and grow new leaves decreases. Fungi, bacteria, viruses, and insects eventually block its food- and water-conducting vessels or strip its foliage, thus ending its life.

The death of animals, though continual, is a less conspicuous part of the forest drama. Disease takes many. Starvation and severe weather take some. Accidents take a few. (Once, on Big Run, I found a water snake that had crawled into a hollow limb and then tried to crawl out before its back half was in; the snake got stuck in the hole and died.) Predation accounts for a great many animals. Eating and being eaten is a very important part of natural regulation, usually helping to prevent any species from getting out of hand to the detriment of the rest. However death comes to an animal, it usually comes early, returning that organism to the great revolving fund of the forest.

Death in the forest, whether it be of trees, worms, or chipmunks, means life for the next generation. All the nutrients taken up through the soil or obtained by eating plants or animals now return to the soil. An army of visible and microscopic organisms, such as bacteria and fungi, keeps all this substance in circulation by ingesting dead plant and animal matter and converting it into chemical forms which can again be used by living things. Without their help, the supply of nutrients, such as nitrogen and phosphorous, in dead organisms would remain locked up, and life would cease. Thus the billions of minute soil organisms, though least noticeable, are perhaps the forest's most important inhabitants.

Thus far, the forest has captured our attention, but now the stream takes center stage. After a half mile of many-channeled meandering through its flat-bottomed valley, it now narrows into a rocky slot and quickens its pace. It cascades over boulders and finally, emerging from the forest shade, leaps into the sunlit opening of South River Falls. We exult in its moment of freedom. On our return, past other discoveries of human and animal occupancy, we are fueled by the memory of this scene, and are continually repaid for our exertion by the variety and beauty of forest life en route.

A hike to South River Falls will take you through some of the park's finest forest habitats. (Photo by Hugh Crandall)

Our walk, starting at 3,000 feet elevation and dropping to 2,000 feet at the falls, has taken us through some of the richest forest in Shenandoah. Here coolness and fairly deep soil formed from easily-fractured, water-holding greenstone preserve moisture for plants. We have seen the more favorable end of the spectrum of Shenandoah forests. Slopes and ridges that are less sheltered, or face southwest or west, or lie upon sedimentary rocks, support less plant and animal life and may look much different. As conditions become drier, such trees as yellow birch, hemlock, yellow-poplar, sugar maple, and basswood drop a out, and the forest becomes mainly one of red, chestnut, and scarlet oaks and several hickories. In the driest places, such as many of the quartzite ridges in the south section, pines and thickets of scrub oak take over. On the highest mountains and ridgetops, where moisture and soil may be favorable but exposure is severe, wind and the weight of ice create forests of stunted, twisted red and white oaks. Branches are continually being broken, so that they have no chance to grow long in one direction. Branches that venture above the general treetop level are sheared off by winter storms, so that from a distance the forest has a pruned, hedge-like appearance. Within, these forests have a mysterious, gnomish quality, especially on misty days, when forms are indistinct and the moisture turns the trunk-encrusting lichens green. If we follow environmental gradients to the coolest, wettest places in the park, such as Limberlost or the top of Hawksbill, we find such northern conifers as spruce and fir among the more temperate trees. All these variations in Shenandoah forests tend to occur gradually over space, individual species dropping out or coming in over several hundred feet of elevation change. From a distance, the effect of a seamless green robe prevails.

Because of its elevational range of 600 to 4,000 feet and its variety of topography and rocks, Shenandoah National Park displays much of the forest variation to be found in Eastern United States. Lucy Braun, who spent a lifetime studying the deciduous forests of eastern North America, classified these forests into nine regions: mixed mesophytic, western mesophytic, oak-hickory, oak-chestnut, oak-pine, southeastern evergreen, beech-maple, maple-basswood, and hemlock-white pine-northern hardwoods. Shenandoah falls within her oak-chestnut region, but the park contains most of the trees that typify the other regions except for those of the southeastern evergreen forest. The type with the greatest diversity—the mixed mesophytic—is approached in some of Shenandoah's hollows, such as the one we have just visited along South River.

Some places in the central section that rival South River in variety and size of trees are White Oak Canyon, Cedar Run, and the Rapidan River from Camp Hoover down. In the south section, the Falls Trail along Jones Falls Run and Doyle's River passes some giants. In the north section, the forest around the Elkwallow picnic area has a particularly pleasant diversity. But walk from any high ridge down into a hollow, and you will see much of the variation on Shenandoah's forest theme.