On the Role of Photosynthetic Responses in Constraining the Habitat Distribution of Rainforest Plants

Abstract

Two rainforest species that differ in the range of light environments encountered by an individual provide a model system for illustrating implications of diurnal variation and spatial heterogeneity of the light environment within single canopies. The two species are similar in the acclimation of leaf-level photosynthetic characteristics to the leaf's past light environment, but are distinguished by the limited ability of the high-light species to decrease dark respiration in response to growth under shade. Over 24 h, total carbon gain from a leaf increases with increasing light, but may increase or decrease with increasing nitrogen. Intermediate nitrogen levels and photosynthetic capacities yield maximum daily carbon gain in only a narrow range of light environments. The ratio of daily photosynthesis to leaf nitrogen is a complex function of nitrogen, photosynthetic characteristics, and light availability in a microsite. Nitrogen levels in real leaves may be close to the optima, but leaf nitrogen levels are clumped relative to the optima. Daily carbon gain from a canopy of fixed total nitrogen tends to increase as the leaf area of the canopy decreases, a trend caused by two factors. First, photosynthetic capacity is a nearly linear function of leaf nitrogen up to the highest nitrogen levels observed in nature. Second, only leaves with nitrogen levels above a substantial threshold are competent to generate positive rates of net photosynthesis. Differences between the species in canopy architecture and leaf duration have counteracting effects on long-term nitrogen-use efficiency.