Beech bark disease: the temporal pattern of cankering in aftermath forests of Maine

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Abstract
In North America, beech bark disease occurs when bark of American beech (Fagusgrandifolia) is infested by beech scale (Cryptococcusfagisuga), then infected and killed by a fungus, Nectriacoccinea var. faginata, Nectriagalligena, or both. In long-affected stands, internal defect results as trees are cankered over time. The amount of defect, patterns of defect development, and the relationship of climate to these patterns were studied in 50 trees from two stands in eastern Maine. Two cross sections from each of five 1 m long bolts from each tree were selected at random by importance sampling. The total area of canker on the outside surface of each growth sheath of each bolt was estimated from the arc lengths of cankers on the annual rings of the sampled cross sections. Cankering began in stems 12–37 years old and 2–11 cm diameter. Rates of cankering increased over time; years of high or low cankering were synchronous between trees and stands. Cankering in year N + 1 was negatively correlated (R2 = 0.803) with October rainfall in year N and number of severely cold days from December in year N–1 through March in year N. Presumably, these factors adversely affect the survival and establishment of the beech scale and perhaps the development of and infection by Nectria spp. Mild winters and dry autumns since 1983 may have permitted the marked increases in beech scale and bark cankering observed in study plots throughout the range of beech bark disease.

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