MICROHABITATS AND WATER RELATIONS OF EPIPHYTIC CACTI AND FERNS IN A LOWLAND NEOTROPICAL FOREST

The pattern of canopy distribution and some water relations characteri stics of the epiphytic cacti Epiphyllum phyllanthus and Rhipsalis bacc ifera and the epiphytic ferns Polypodium crassifolium and Polypodium p hyllitidis were examined in the tropical forest of Barro Colorado Isla nd, Panama. The epiphytic cacti were 6-fold more succulent than the ep iphytic ferns, which had a 9-fold higher root:shoot ratio. The four sp ecies, especially the cacti, were more abundant on the deciduous trees Ceiba pentandra, Platypodium elegans, and Tabebuia guayacan than on t he evergreen Anacardium excelsum. The water-holding capacity of the ba rk was 2-fold higher for A. excelsum and T. guayacan than for C. penta ndra and P. elegans; the main crotches of the latter species intercept ed 2-fold more rainfall than chose of the former. Well-watered planes of all epiphytic species showed similar total daily transpiration. Aft er 2 weeks of drought, daily transpiration decreased an average of 48 percent for the cacti and 73 percent for the ferns. After 4 weeks of d rought, daily transpiration decreased 80 percent for the cacti and was eliminated for the ferns; succulence then decreased an average of 29 percent for the cacti and 98 percent for the ferns, but all species re covered fully within 2 d after rewetting. The epiphytic ferns had a lo w relative capacitance (0.16 M/Pa), causing these presumably C-3 plant s often to be under water stress. A high relative capacitance (0.50 M/ Pa) and crassulacean acid metabolism allowed the epiphytic cacti to oc cur most frequently on the driest sites of this tropical forest.