Effects of light availability and sapling size on the growth, biomass allocation, and crown morphology of understory sugar maple, yellow birch, and beech

The patterns of above-ground growth, biomass allocation, crown morphology, and light attenuation were compared between small (50 to 250 cm tall) and t all (250 to 600 cm tall) yellow birch, sugar maple, and beech individuals i n low (< 10% of above-canopy PPFD (Photosynthetic Photon Flux Density)) and high (10 to 40% PPFD) Light environments in a mature sugar maple-birch-bee ch stand near Quebec city, Canada. Significant differences in above-ground growth, crown morphology, and allocation patterns were found among (i) the three co-dominating tree species, (ii) short and tall individuals, and (iii ) low and high light environments. The direction of the differences in most baits investigated between low and high light environments were strikingly similar among the three species, but the magnitude of the differences ofte n varied. Overall, yellow birch differed more in several traits in terms of its responses to light and size compared to beech and sugar maple. In gene ral, differences found between light environments were smaller for the tall er saplings, indicating that plasticity tends to decrease with increasing s ize in all three species. None of these crown structural differences found among species translated into differences in light attenuation within the s apling crowns. The maximum height observed in individual trees of all three species tended to decrease sharply below approximately 4% PPFD. We suggest that maximum tree height is restricted in such low light environments sinc e the photosynthetic to non-photosynthetic tissue ratio, as measured by the leaf area ratio (LAR), declines rapidly with seedling size up to 150 cm. W e suggest that these three species co-dominate in this forest due to a comb ination of small but effective differences in physiological, morphological, and allocational traits and responses to increases in the understory light environment.