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

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Abstract
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 FV /FM 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.