Natural variation in morphology among diverse populations of yellow birch (Betula alleghaniensis) and sweet birch (B. lenta)

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Abstract
Natural variation was studied in 47 morphological attributes of yellow birch and sweet birch collected from adult trees along 5 elevational transects in the Appalachian Mountains. Populations of one or both species were analyzed from 30 locales extending from southern North Carolina to northern Vermont and New Hampshire [USA]. The major objectives of the study were as follows: to test for differences in foliage, fruits, bark and pollen among species and among populations; to determine if patterns of variation associated with latitude could be strengthened by elevational adjustments; and to determine the degree of similarity in patterns of variation for the 2 species. The results indicated considerable within-population variation and significant differences among populations and species. However, few morphological characters showed consistent trends in size, shape or pubescence with changes in elevation and latitude. Where relationships were consistent, they were not particularly strong as indicated by linear correlation coefficients (few exceed 0.70). These findings are in contrast to earlier studies demonstrating a strong relationship of growth periodicity to elevation and latitude of seedling populations grown in a uniform garden. Preliminary evidence suggests that expression of morphological variation may be greater in the less extreme portions of the species latitudinal and elevational distributions.

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