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Predicting Invasions of Nonindigenous Plants and Plant Pests
matured, they have been subject to oak decline, a condition that appears to develop as physiologically mature trees experience insect defoliation or drought (Houston 1987). Although the native insect defoliators, such as elm spanworm (Ennomos subsignarius), have been implicated in this decline, the introduced gypsy moth (Lymantria dispar) has been the most important agent of damage. Gypsy moth defoliation disturbs the carbohydrate physiology of oak root systems and makes them highly susceptible to native root-invading fungi (Armillaria spp.) and insects, particularly the two-lined chestnut borer (Agrilus bilineatus), a root collar insect (Houston 1987, Oak 1998). As the gypsy moth has continued its spread southward and westward from New England through the Appalachians, the oak decline-gypsy moth situation has resulted in significant oak mortality. Species that have replaced oaks include more-shade-tolerant trees, such as black gum, red maple, white ash, and yellow poplar. Oak mortality has created dens for wildlife and increased the amount of coarse woody debris. The forests that eventually emerge from the impact of chestnut blight and gypsy moth defoliation may be more tolerant to oak decline and defoliation but will be structurally and compositionally very different plant and animal habitats.
Two other nonindigenous organisms, a scale insect (Cryptococcus fagisuga) and a fungus (Nectria coccinea var. faginata), are influencing the eastern forests of North America as they operate in concert to cause beech bark disease (Houston 1994). The small scale insect creates tiny feeding wounds in the thin bark of beech, which are colonized by the fungus. Eventually, mature beech trees die as the many fungal infections coalesce and girdle the tree. The killing front of this insect–pathogen complex has spread from the introduction point of the scale insect in Nova Scotia to central Pennsylvania; small outbreaks of the disease now occur as far south as the Great Smoky Mountains National Park. The dynamics of beech bark disease begin as infestations by the scale insect and fungal infections of the bark occur along the killing front. As native beeches dies, the remaining beech trees are riddled with nonlethal infections that grotesquely deform them. Further deaths lead to the development of beech thickets that arise from root sprouts. General structural changes in the forest include loss of beech in the canopy, increased snags and downed woody debris, and overabundance of infected beech stems. Long-term effects of the elimination of beech are uncertain, but some stands are already being replaced by sugar maple and yellow birch or have been transformed into grasslands or shrub lands (Oak 1998). Although American beech is not highly prized economically, it had a valuable wildlife role in producing beechnuts, an important food for some birds, squirrels and chipmunks (Martin et al. 1961).
Evaluating Impacts on Communities
There is a fundamental need to identify common standard measures of impacts that would create a more reliable basis for comparison, interpolation, and