Water relations and hydraulic architecture of woody hemiepiphytes

Several parameters related to the water relations of eight woodyhemiepiphytes with different photosynthetic pathways were studiedin situ and in the laboratory on Barro Colorado Island, Panama.As a group, woody hemiepiphytes tended to have less conductivestems per unit leaf area (lower k_L) and invested less wood cross-sectionper unit leaf area compared to free-standing trees, while theirspecific conductivity (K_s) was significantly higher. Among hemiepiphytes,there were significant differences between C₃ and CAM (CrassulaceanAcid Metabolism) species in respect to leaf characteristics,transpiration rates, diel patterns of water flow through aerialroots, and in hydraulic architecture parameters. Average transpirationrates of the two Clusia species (C₃-CAM) were lower by aboutan order of magnitude compared to the C₃ species. In all C₃species, sap flow through aerial roots (F) closely followedtranspiration (E), whereas E and F were decoupled in time inthe C₃-CAM species Clusia uvitana: considerable long-distancewater flow occurred at night. The hydraulic efficiency of theother C₃-CAM species, C. minor was the lowest of the five speciesinvestigated. Key words: Hemiepiphytes, water relations, hydraulic architecture, aerial roots, CAM