Adaptation and Dimorphism in Eastern Hemlock, Tsuga canadensis (L.) Carr

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Abstract
Eastern hemlock''s (T. canadensis (L.) Carr.) habitat distribution, and variations in its growth rate response to macroclimatic variation, sensitivity to climatic and topographic variation, and gross needle and branch morphology, were quantified in 8 major geographical areas throughout the species'' range in the eastern USA. Two apparent sympatric ecotypes or morphs which occur throughout most of the species'' range were discussed. These ecotypes may be differentiated by habitat preference, growth rates, sensitivity to moisture and temperature, and certain gross morphological characters. The high-response hemlock prefers mesophytic habitats, and exhibits a high growth rate, high sensitivity to moisture and low sensitivity to temperature. The low-response type also occurs in mesic sites, but is more common on subxeric slopes. It exhibits a low growth rate, low sensitivity to moisture, and high sensitivity to temperature. Throughout southern Canada and New England, the 2 ecotypes are distinct in terms of growth rate, morphology and response to macroclimatic variation, despite their sympatric occurrence in mesophytic habitats. Only limited intergradation is observed. In the Alleghenies and southern Appalachians, the 2 types broadly intergrade, providing a spectrum of phenotypes, the extremes of which correspond to the 2 New England ecotypes. Only the high-response type was found in the Cumberland Mountains and Plateau, while the disjunct populations studied in Indiana were composed entirely of intermediate individuals. Differentiation into ecotypes and subsequent introgression between the 2 ecotypes appear to confirm a competitive advantage to T. canadensis, giving it a phenotypic elasticity which allows it to occupy a broad range of habitats. There also appears to be sufficient divergent selection against the intermediates to prevent the 2 ecotypes'' convergence or fusion into a single phenotype.

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