GROWTH AND MORPHOLOGICAL RESPONSES OF YELLOW BIRCH, SUGAR MAPLE, AND BEECH SEEDLINGS GROWING UNDER A NATURAL LIGHT GRADIENT

Height and lateral growth, biomass distribution, leaf morphology, and crown architecture were studied in yellow birch (Betula alleghaniensis Britton), sugar maple (Acer saccharum Marsh.), and beech (Fagus grand ifolia Ehrh.) seedlings growing under 1-50% of above-canopy light in a sugar maple stand, in Quebec. All three species showed increasing gro wth with increasing light, but growth of yellow birch was higher and m ore responsive than that of sugar maple and beech. All three species s howed typical sun-shade morphological responses, such as decreasing sp ecific leaf area and leaf area ratio, and increasing leaf area index, with increasing light availability. Sugar maple was morphologically mo re plastic than the other species. It showed variations in biomass all ocation to leaves and branches, a decrease in branch length to seedlin g height ratio, and a marked increase in the ratio of leaf area to ste m length. Although our results clearly demonstrate the ability of thes e three species to modify several of their morphological features in r esponse to variations in light, they do not show a clear relationship between species shade tolerance and morphological response to light va riations. We suggest that species-specific developmental patterns may act as important constraints to morphological acclimation to light var iation.