Leaf mass per area, nitrogen content and photosynthetic carbon gain in Acer saccharum seedlings in contrasting forest light environments

Total daily photosynthetic photon flux density (PPFD) varied by two orders of magnitude across forest understorey, gap and clearing habitats for sugar maple seedlings in SW Wisconsin. Area-based photosynthetic capacity (A_max/area) was greater in clearing or gap than in understorey seedlings, but little photosynthetic acclimation was observed between gap and clearing seedlings. There were significant correlations between A_max and leaf N content (N) among seedling populations from the three habitats when expressed on an area basis, but not on a mass basis because of scaling of leaf mass per unit area with N. A_max/N was higher and chlorophyll/N lower in clearing than in gap and understorey seedlings, possibly as a result of N partitioning to maximize capacity for either CO₂ fixation or light harvesting in the different light environments. Realized daily carbon gain under ambient field conditions appeared to be controlled largely by differences in total daily PPFD, both among habitats and across the growing season. Small gaps thus may present a favourable environment for photosynthesis and growth in sugar maple as a result of the limited ability of seedlings to acclimate to increases in PPFD over levels prevalent in the understorey and because of the moderate levels of other environmental stresses in small forest gaps. -Authors