Annual carbon balance and nitrogen-use efficiency in tropical C₃ and CAM epiphytes

Over the course of 12 months, diel (24 h) measurements of gas exchange were performed on leaves of three epiphytic species growing in the crown of a kapok tree on Barro Colorado Island, Panama: a drought-deciduous orchid with the C₂ , pathway (Catasetum viridiflavum Hook.), an evergreen C₂ fern (Polypodium crassifolium L.), and an evergreen epiphyte with an intermediate C₃ -C₃ -CAM pathway of photosynthesis (Clusia uvitana Pitt.), The gas exchange characteristics of all three species; were strongly affected during the four-month dry season. Compared with the rainy season, mean daily carbon gain of Clusia uvitana was reduced by almost 40° paralleled bY a strong decrease in daytime CO₂ uptake and an increase in CO₂ uptake at night. The orchid, growing new leaves in the second half of the dry season, showed markedly decreased stomatal conductances and greatly reduced carbon gain. In the fern, daily carbon balance became negative during the dry season and chronic photoinhibition was indicated by reduced F_V /F_M ratios and a decreased photon-use efficiency of photosynthetic O₂ evolution. Annual carbon gain was similar far the three species (about 1000₂ CO₂ m^-2 yr_-1 ) as was long-term nitrogen-use efficiency (annual carbon gain/mean leaf nitrogen content, about 1·1 g CO₂ mg N^-1 yr_-1 ). In the C₃ CAM epiphyte, the long-term water use efficiency of net CO₂ uptake was more than twice as high as in the two C₃ epiphytes.