Variation in leaf structure, nitrogen, and photosynthesis across light gradients in a temperate and tropical forest

I investigated plasticity in leaf structure and nitrogen content across three light/successional gradients in a temperate forest and a tropical rain forest to examine the implications of such plasticity to variation in field photosynthetic carbon assimilation capacity in temperate and tropical forest species. The goal was to determine the ability of species to modify leaf characteristics across gradients in light availability and among contrasting forest environments. Leaf mass per unit area (LMA) and area-based photosynthetic capacity were greater in seedlings growing in sun than shade habitats in both sugar maple (Acer saccharum Marsh.) and in four other mid- to late-successional species in a temperate and tropical forest across a light availability gradient from a clearing to a light-gap to the forest understory. Relatively small differences in photosynthetic capacity and maximum stomatal conductance were observed for sugar maple seedlings in a clearing and gap habitat compared to those between seedlings in a gap and understory habitat, but leaf water relations characteristics showed large differences between clearing seedlings versus gap or understory seedlings. Light limitations to photosynthesis appear to be strongest for understory and gap sugar maple seedlings, whereas stress conditions associated with high light may impose limitations to photosynthesis by seedlings in the gap and more so for clearing seedlings.; Photosynthetic capacity per unit area was highly linearly correlated with leaf nitrogen (N) content per area in sugar maple across a horizontal gradient in habitat light availability from clearing to understory and across a vertical gradient from the upper to lower canopy positions in a mature forest; there were no correlations between mass-based photosynthetic capacity and leaf N concentration across either of these light gradients. LMA was highly correlated with area-based photosynthetic capacity and N content per area among seedlings in different light environments and among leaves in different canopy positions.; Early successional tropical species growing in clearings after slash-and-burn agriculture showed strong correlations between mass-based photosynthesis-N parameters rather than area-based relationships. Differences in mass versus area-based photosynthesis-N relationships among species may reflect differences between prevailing environmental limitations of nutrient and light availability through succession. (Abstract shortened with permission of author.)...