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 tall (250 to 600 cm tall) yellow birch, sugar maple, and beech individuals in low (< 10% of above-canopy PPFD (Photosynthetic Photon Flux Density)) and high (10 to 40% PPFD) Light environments in a mature sugar maple-birch-beech 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 often 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 general, differences found between light environments were smaller for the taller saplings, indicating that plasticity tends to decrease with increasing size in all three species. None of these crown structural differences found among species translated into differences in light attenuation within the sapling 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 since the photosynthetic to non-photosynthetic tissue ratio, as measured by the leaf area ratio (LAR), declines rapidly with seedling size up to 150 cm. We suggest that these three species co-dominate in this forest due to a combination of small but effective differences in physiological, morphological, and allocational traits and responses to increases in the understory light environment.