"If man, for any reason, drastically alters the natural order, setting in motion whole series of chain reactions, then he had better know what he is doing." David McCullough, The Johnstown Flood, 1968.

When I started teaching biology at Mount St. Mary’s, I always managed to take the class on a field trip to Indian Lookout on the mountain behind the college. It was a round trip of about two miles; it covered terrain that was steep and rocky in places, but it could be walked comfortably in a two-hour lab. Sometimes when classes were large I had to make the trip twice in the same afternoon; but I was younger then, and it was worth the effort because there was so much to see. Since I retired, the hill has grown steeper and the trail is longer, but I still manage to go up there at least once a year to watch the ongoing changes that began 300 years ago.

The mountain was covered with a mixed hardwood forest. The dominant tree species, chestnut oak, comprised as much as 90% of the cover on south-facing slopes where the soil is driest. On the east and north slopes there was more variety; red oak, black gum, red maple, and various birches were mixed with the chestnut oaks. On the thin, rocky soil at the crest of ridges were stands of Virginia pine. In ravines where the soil stays damp, tulip poplar and beech trees could be found. A few places that had been burned by forest fires were covered with dense stands of aspen. Under the trees were a variety of wild flowers, including pink lady’s slippers and Indian pipes, as well as ferns, mosses and lichens. To the students, most of whom were city-bred, it seemed like the primeval forest that had been there since the Ice Age, and they were ready to believe that bears and wildcats were waiting around the next turn of the path.

It was a healthy forest, but by no means primeval; it was in an ongoing series of changes which began when European settlers arrived. Some of these changes were severe enough to be called ecological disasters; they were caused either directly by human activity or by alien species of pests that were introduced by humans. Other changes resulted from ecological succession, the process by which forests recover from such disasters.

"Disaster" is admittedly a pejorative term; lumbering was a sign of progress to the people of the growing nation, but to the forest it was devastating. Between 1700 and 1900, most of the original forest was cut for lumber and firewood. Nature was resilient; the forest grew back, and was cut again, as trainloads of firewood were shipped to Baltimore and Washington. As the 20th century opened, coal became more economical for heating, so large-scale tree-cutting stopped and the forest began growing back into what looked like its original form.

Originally, the dominant tree was the American chestnut, but around 1930 the second disaster arrived in the form of chestnut blight. It was caused by a fungus introduced from Asia, and it killed all of the adult chestnut trees within a few years; they were replaced by the chestnut oak. So although the forest looked undisturbed to my students in the late 1950s, it didn’t take an expert to notice that trees with trunks over 18 inches in diameter were rare, and that scattered among them were the still un-rotted stumps and logs of chestnut trees that had died 30 years earlier.

In 1957 all northbound traffic went comfortably through the stoplight at the square in Emmitsburg; but things were changing. The post-war baby boom was followed by the automobile boom of the ‘50s and ‘60s, and cars multiplied like rabbits. Within 10 years it was necessary to build bypasses around small towns like ours, and air pollution was becoming a problem. By the 1970s the air pollution inversion from Washington, D. C. spread as far north as Emmitsburg every summer, and on some days the haze was too thick to see the top of the mountain from town. The lichen called reindeer moss, which was well known as an indicator species, disappeared from the forest floor. My ecology classes also noticed that the trees were growing more slowly than they had two decades earlier. These sorts of things were happening all over the country, and they provided evidence that led to the passage of environmental regulations in the mid-70s. The problems were not cured instantly, but within ten years air quality began to improve. And it was about then that the next disaster arrived.

Gypsy moths are native to Europe and Asia; they were brought to America by a misguided French astronomer in 1869*. After escaping near Boston, they spread southward and appeared in Maryland around 1980. That year they defoliated just three acres of forest, but within a decade over 100,000 acres had been destroyed, and they continued to spread. Ecologists call them an irruptive species because they undergo population explosions at irregular intervals. Their favorite food is oak, and since this is the most common tree in our forests, they are especially devastating here. They hatch as tiny caterpillars in early spring when tender new leaves are just coming out. They eat leaves and grow for the next several months; their presence was evident in late May and early June this year, when motorists on Route 15 could see patches of pale green in the forest along the top of College Mountain; as summer wore on, these patches turned brown as the trees were completely defoliated. They are nearing the end of their life cycle now; they will pupate and emerge as adults in mid-July, and each adult female will produce from 400 to 800 eggs.

The last bad outbreak in our local area was about six years ago. Last summer I walked to Indian Lookout with my granddaughter and showed her a stand of oak trees killed in 2001. We found an egg mass on the bark of a surviving oak tree, and crushed it with a stick, but as we looked around we began to see hundreds more of them, far too many to destroy by hand. It was obvious that a bad outbreak would come this year; what we could not foresee at the time was that the effect of the moths on the trees would be compounded by drought. It is likely that next year will see more destruction; then, if history repeats as it has, the moth population will die out for a few years. But they will return again.

Trees will grow back in the areas where the oaks have been killed, but it will be a different kind of forest. Without the oaks, faster growing but less desirable trees will come in; they will create a forest, but it will not be like the one that we have now. Without the annual crop of acorns, many native animals, birds and insects will decline or become extinct locally; without the annual layer of slow-to-decay oak leaves, soil will erode faster and its chemical composition will change. Without the shade from mature trees, ferns and wildflowers will be crowded out by grasses, weeds and briars. Biodiversity will decrease, and the forest will be less stable and resilient when the next disaster comes. I cannot tell my grandchildren what that will be, but I know it will happen; new pests like the emerald ash borer have reached Maryland in the past year, and unknown others are waiting in the wings.