Others did it for the simple joy of finding something new-none more admirably than Thomas Nuttall, a bright but unschooled journeyman printer from Liverpool who came to America in 1808 and discovered an unexpected passion for plants. He undertook two long expeditions, which he paid for out of his own pocket, made many important discoveries, and generously gave to the Liverpool Botanic Gardens plants that might have made him rich. In just nine years, from a base of zero, he became the leading authority on American plants. In 1817, he produced (literally, for he not only wrote the text but set most of the type himself) the seminal Genera of North American Plants, which stood for the better part of a century as the principal encyclopedia of American botany. Four years later he was named curator of the Botanic Garden at Harvard University, a position he held with distinction for a dozen years, and somehow also found time to become a leading authority on birds, producing a celebrated text on American ornithology in 1832. He was, by all accounts, a kindly man who gained the esteem of everyone who met him. Stories don’t get a great deal better than that.

Already in Nuttall’s day the woods were being transformed. The panthers, elk, and timberwolves were being driven to extinction, the beaver and bear nearly so. The great first-growth white pines of the north woods, some of them 220 feet high (that’s the height of a twenty-story building), had mostly been felled to make ships’ masts or simply cleared away for farmland, and nearly all the rest would go before the century was out. Everywhere, there was a kind of recklessness borne of a sense that the American woods was effectively inexhaustible. Two-hundred-year-old pecan trees were commonly chopped down just to make it easier to harvest the nuts on their topmost branches. With each passing year the character of the woods changed perceptibly. But until quite recent times-painfully recent times-one thing remained in abundance that preserved the primeval super-Eden feel of the original forest: the massively graceful American chestnut.

There has never been a tree like it. Rising a hundred feet from the forest floor, its soaring boughs spread out in a canopy of incomparable lushness, an acre of leaves per tree, a million or so in all. Though only half the height of the tallest eastern pines, the chestnut had a weight and mass and symmetry that put it in another league. At ground level, a full-sized tree would be ten feet through its bole, more than twenty feet around. I have seen a photograph, taken at the start of this century, of people picnicking in a grove of chestnuts not far from where Katz and I now hiked, in an area known as the Jefferson National Forest. It is a happy Sunday party, all the picnickers in heavy clothes, the ladies with clasped parasols, the men with bowler hats and walrus moustaches, all handsomely arrayed on a blanket in a clearing, against a backdrop of steeply slanting shafts of light and trees of unbelievable grandeur. The people are so tiny, so preposterously out of scale to the trees around them, as to make you wonder for a moment if the picture has been manipulated as a kind of joke, like those old postcards that show watermelons as big as barns or an ear of corn that entirely fills a wagon under the droll legend“A TYPICAL IOWA FARM SCENE.” But this is simply the way it was-the way it was over tens of thousands of square miles of hill and cove, from the Carolinas to New England. And it is all gone now.

In 1904, a keeper at the Bronx Zoo in New York noticed that the zoo’s handsome chestnuts had become covered in small orange cankers of an unfamiliar type. Within days they began to sicken and die. By the time scientists identified the source as an Asian fungus called Endothia parasitica, probably introduced with a shipment of trees or infected lumber from the Orient, the chestnuts were dead and the fungus had escaped into the great sprawl of the Appalachians, where one tree in every four was a chestnut.

For all its mass, a tree is a remarkably delicate thing. All of its internal life exists within three paper-thin layers of tissue-the phloem, xylem, and cambium-just beneath the bark, which together form a moist sleeve around the dead heartwood. However tall it grows, a tree is just a few pounds of living cells thinly spread between roots and leaves. These three diligent layers of cells perform all the intricate science and engineering needed to keep a tree alive, and the efficiency with which they do it is one of the wonders of life. Without noise or fuss, every tree in a forest lifts massive volumes of water-several hundred gallons in the case of a large tree on a hot day-from its roots to its leaves, where it is returned to the atmosphere. Imagine the din and commotion, the clutter of machinery, that would be needed for a fire department to raise a similar volume of water.

And lifting water is just one of the many jobs that the phloem, xylem, and cambium perform. They also manufacture lignin and cellulose; regulate the storage and production of tannin, sap, gum, oils, and resins; dole out minerals and nutrients; convert starches into sugars for future growth (which is where maple syrup comes into the picture); and goodness knows what else. But because all this is happening in such a thin layer, it also leaves the tree terribly vulnerable to invasive organisms. To combat this, trees have formed elaborate defense mechanisms. The reason a rubber tree seeps latex when cut is that this is its way of saying to insects and other organisms, “Not tasty. Nothing here for you. Go away.” Trees can also deter destructive creatures like caterpillars by flooding their leaves with tannin, which makes the leaves less tasty and so inclines the caterpillars to look elsewhere. When infestations are particularly severe, some trees can even communicate the fact. Some species of oak release a chemical that tells other oaks in the vicinity that an attack is under way. In response, the neighboring oaks step up their tannin production the better to withstand the coming onslaught.

By such means, of course, does nature tick along. The problem arises when a tree encounters an attacker for which evolution has left it unprepared, and seldom has a tree been more helpless against an invader than the American chestnut against Endothia parasitica. It enters a chestnut effortlessly, devours the cambium cells, and positions itself for attack on the next tree before the tree has the faintest idea, chemically speaking, what hit it. It spreads by means of spores, which are produced in the hundreds of millions in each canker. A single woodpecker can transfer a billion spores on one flight between trees. At the height of the American chestnut blight, every woodland breeze would lose spores in uncountable trillions to drift in a pretty, lethal haze on to neighboring hillsides. The mortality rate was 100 percent. In just over thirty-five years the American chestnut became a memory. The Appalachians alone lost four billion trees, a quarter of its cover, in a generation.

A great tragedy, of course. But how lucky, when you think about it, that these diseases are at least species specific. Instead of a chestnut blight or Dutch elm disease or dogwood anthracnose, what if there was just a tree blight-something indiscriminate and unstoppable that swept through whole forests? In fact, there is. It’s called acid rain.

But let’s stop there. I think we’ve both had enough science for one chapter. But hold that thought, please, and bear it in mind when I tell you that there wasn’t a day in the Appalachian woods when I didn’t give passing thanks for what there was.

So the forest through which Katz and I passed now was nothing like the forest that was known even to people of my father’s generation, but at least it was a forest. It was splendid in any case to be enveloped once more in our familiar surroundings. It was in every detectable respect the same forest that we had left in North Carolina-same violently slanted trees, same narrow brown path, same expansive silence, broken only by our tiny grunts and labored breaths as we struggled up hills that proved to be as steep, if not quite as lofty, as those we had left behind. But, curiously, though we had come a couple of hundred miles north, spring seemed further advanced here. The trees, predominantly oak, were more fully in bud, and there were occasional clumps of wildflowers-bloodroot and trillium and Dutchmen’s breeches-rising through the carpet of last year’s leaves. Sunlight filtered through the branches overhead, throwing spotlights on the path, and there was a certain distinctive, heady spring lightness in the air. We took off first our jackets and then our sweaters. The world seemed altogether a genial place.