HOLLOWS, PEEPERS, AND HIGHLANDERS: AN APPALACHIAN MOUNTAIN ECOLOGY

5. Catastrophe and the Appalachian Quilt

33 5 Catastrophe and the Appalachian Quilt I n ecology, as in other areas of inquiry, answers to even the most elementary questions hinge on the breadth of the observer’s vision. Take, for example, the question of how the males of a particular bird species, like the wood thrush, distribute themselves across a landscape. They could appear uniformly distributed when viewed on the small scale, being spaced evenly by their mutual territorial repulsion . Since the birds’ territories only occupy certain habitats, the individuals could also appear clumped in discrete groups if viewed at an intermediate scale covering several hills. Finally, the birds could appear randomly dispersed across a large geographic region where the habitat blocks are scattered haphazardly. To understand why the Appalachian Mountains have continued to host so many species, we need to apply this practice of observing the forest at several scales. Although viewing through a continuum of scales provides the most realistic impression, it also proves unmanageable, so we’ll simply examine the extremes: surveying some of the broad causes of biological diversity, such as environmental gradients and ecological disturbances; then narrowing our scope to explore small-scale factors such as canopy gaps. As you climb a mountain, the temperature gradually cools. This thermal gradient helps determine which plants grow where on the slope. Moisture gradients similarly influence plant communities. In the oak forests of Virginia, several trends become obvious as you move from moderately moist to dry sites: the canopy coverage, stand 34 height, and total area of the trees’ bases decrease, while in the shrub level these features increase. At the continental scale, environmental gradients contribute to the maintenance of plant diversity. The boreal forest covers a huge area across southern Canada and southward along the high ridges of Appalachia. Dominant boreal trees include balsam fir; black and white spruces; red, white, and jack pines; quaking and bigtooth aspens ; paper birch; and balsam poplar. Farther south, the broad-leaved trees of the mixed deciduous forest take over. The mixed deciduous forest may assume several forms, including northern hardwood, oakchestnut , oak-pine, mixed mesophytic, and cove hardwood forests. Mesophytic refers to a medium level of moisture. A cove is a bowlshaped valley with rich, damp soil. One way of appreciating the great diversity of communities within the mixed deciduous forest is to compare three of its subtypes: the northern hardwood forests of New England, the mixed mesophytic forest of central Appalachia, and the cove hardwood forests of southern Appalachia. The northern hardwood, or transition, forest combines features of the boreal woodland and the deciduous forest of southern Appalachia. Common in New England and at high elevations in the southern Appalachians, the northern hardwood forest contains only a few dominant species: red maple in wet spots, beech in drier conditions , and yellow birch on rock slides. The mixed mesophytic forest of central Appalachia supports a marvelous diversity of tree species. Commonly, three dozen species are prominent in a mature community. Canopy trees include white basswood, beech, sugar maple, tulip poplar, red and white oak, hemlock , magnolia, white ash, and black cherry. Tulip poplars inhabit lowlands, hemlocks can monopolize ravines, and oaks dominate drier ridge tops. The cove hardwood forests of southern Appalachia exhibit the greatest diversity of tree species in North America. Within a single stand, eight to ten tree species may share dominance in a total assemblage of forty species. Trees common in southern cove forests include tulip poplar, sugar maple, yellow buckeye, basswood, beech, yellow birch, red oak, black cherry, and Carolina silverbell. Hollows, Peepers, and Highlanders 35 In addition to the contributions of altitude and latitude, ecological disturbance and the introduction of exotic species contribute to high biotic diversity. At some sites, episodes of fire and windstorm may actually be more important in determining tree species composition than long stretches of tranquility. Fire, so important in conifer-dominated systems such as boreal forests and southern Appalachian pine forests, also plays a role in the drier parts of deciduous forests. Fire alters light intensity, water and nutrient contents of the soil, microclimate at the soil’s surface, and reseeding sources. Repeated fires favor species with fire-adaptive traits, such as serotinous cones that release seeds when stimulated by the heat of a fire, resprouting by roots, and even increased flammability of the plant community as a whole. At first glance, fire appears to be a force imposed from without. However, the...