Determinants of photosynthetic capacity in six rainforest Piper species.

We examined the dependence of photosynthetic capacity on leaf nitrogen content and light environment in leaf microsites of six rainforest species growing naturally in understory and clearing habitats. All nine descriptors of the light environment in leaf microsites, encompassing canopy openness, potential exposure to sunflecks, and photosynthetically active photon flux density (PFD) integrated over one day, three months, and an entire year, were highly correlated with each other. Among species, A max was strongly dependent on leaf nitrogen and on all descriptors of the light environment in the leaf microsite. Leaf nitrogen was also dependent on all descriptors of the light environment. Simple regressions of A max on all measures of the light environment were significant for plants in the clearing, but not for plants growing in the understory. Regressions of leaf nitrogen on at least one descriptor of the light environment were significant for both clearing and understory plants. Over all species, and for the clearing plants alone, leaf nitrogen and a single descriptor of the light environment together explained significantly more variance in A max than either variable alone. Multiple regressions using leaf nitrogen and all nine descriptors of the light environment did not explain a significantly greater amount of variance in photosynthetic capacity than multiple regressions using leaf nitrogen and a single light descriptor. In most cases, A max was predicted better by the most general descriptors of the light environment, such as canopy openness and long-term photon flux density, and less accurately by shorter-term sensor measurements or descriptors based on the duration of potential sunfleck exposure. The gest predictors of A max were frequently different than the best predictors of leaf nitrogen. In every case examined, A max was less sensitive to variation in the light environment in understory than in clearing plants.