HABITAT, CO2 UPTAKE AND GROWTH FOR THE CAM EPIPHYTIC CACTUS EPIPHYLLUM-PHYLLANTHUS IN A PANAMANIAN TROPICAL FOREST

In the tropical forest of Barro Colorado Island, habitat characteristi cs, diel acidity changes, CO2 uptake and growth were investigated for the epiphytic cactus Epiphyllum phyllanthus (L.) Haw. It occurred most frequently in tree cavities with its roots in canopy soil and was esp ecially abundant on two tree species: Platypodium elegans J. Vogel and Tabebuia guayacan (Seem.) Hemsl. Its maximum net CO2 uptake rates wer e low under natural conditions (1.4 mu mol m(-2) s(-1)) but were compa rable to those of other CAM and C-3 epiphytes under wet conditions in a screenhouse. Under both natural conditions and in the screenhouse, p artial shade enhanced growth and CAM activity. When plants grew under a photosynthetic photon flux of c. 4 mol m(-2) d(-1), their nocturnal acidity increase and total net CO2 uptake were twice as much as for pl ants growing at lower (an average of 2.4 mol m(-2) d(-1)) and higher ( 7.7 mol m(-2) d(-1)) photosynthetic photon fluxes. Stem elongation was 27% greater at the intermediate photosynthetic photon flux. Seedlings of E. phyllanthus survived three months of drought and responded rapi dly to rewetting, recovering fully within three days. Transpiration ra tes and nocturnal acidity increases also recovered to the values of we ll-watered plants a few days after rewetting, indicating that this spe cies can take advantage of episodic rainfall during the dry season.