Allozyme and mating-system variation in balsam fir (Abies balsamea) across a continuous elevational transect

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Abstract
Genetic structure and the mating system of a continuous balsam fir (Abies balsamea (L.) Mill.) population spanning a 610-m elevational gradient on the southeast slope of Mt. Moosilauke, New Hampshire, were investigated by assaying eight polymorphic isozyme loci. In contrast to steep elevational clines for seedling quantitative traits found in an earlier study on the same slope, no significant differences (P > 0.05) in allelic frequencies were found among four subpopulations sampled at 854, 1159, 1312, and 1464 m above sea level. Estimates of the proportion of progeny due to outcrossing (.cxa.tm) ranged from 0.78 to 0.99 (mean = 0.89) and were significantly different (P < 0.05) from .cxa.tm = 1.00 in three of the four subpopulations. At timberline (1464 m), .cxa.tm was lowest, yet the mean fixation index among adult trees was -0.149, suggesting that selection favors heterozygotes in this marginal environment. Despite the potential for extensive pollen dispersion along the slope, gene flow appears somewhat restricted because of substantial self-fertilization in some subpopulations, localized outcrossing within stands, and asynchrony of flower production among adjacent stands.