Leaf- and plant-level carbon gain in yellow birch, sugar maple, and beech seedlings from contrasting forest light environments

Leaf-level photosynthetic-light response and plant-level daily carbon gain were estimated for seedlings of moderately shade-tolerant yellow birch (Bet ula alleghaniensis Britton) and shade-tolerant sugar maple (Acer saccharum Marsh.) and beech (Fagus grandifolia Ehrh.) growing in gaps and under a clo sed canopy in a sugar maple stand at Duchesnay, Que. All three species had a higher photosynthetic capacity (A(max)) in the gaps than in shade, but ye llow birch and beech responded more markedly than sugar maple to the increa se in light availability. The high degree of plasticity observed in beech s uggests that the prediction that photosynthetic plasticity should decrease with increasing shade tolerance may not hold when comparisons are made amon g a few late-successional species. Unit-area daily carbon gain (C-A) was si gnificantly higher in the gaps than in shade for all three species, but no significant difference was observed between light environments for plant-le vel carbon gain (C-W). In shade, we found no difference of C-A and C-W amon g species. In gaps, beech had a significantly higher C-A than sugar maple b ut similar to that of birch, and birch had a significantly higher C-W than maple but similar to that of beech. Sugar maple consistently had lower carb on gains than yellow birch and beech but is nevertheless the dominant speci es at our study site. These results indicate that although plant-level carb on gain is presumably more closely related to growth and survival of a spec ies than leaf-level photosynthesis, it is still many steps removed from the ecological success of a species.