PARTITIONING OF WATER-RESOURCES AMONG PLANTS OF A LOWLAND TROPICAL FOREST

Source water used by plants of several species in a semi-evergreen low land tropical forest on Barro Colorado Island, Panama, was assessed by comparing the relative abundance of deuterium, D, versus hydrogen, H (stable hydrogen isotope composition, delta D) in xylem sap and in soi l water at different depths, during the dry season of 1992. Ecological correlates of source water were examined by comparing xylem water del ta D values with leaf phenology, leaf water status determined with a p ressure chamber, and rates of water use determined as mass flow of sap using the stem heat balance method. Soil water delta D values decreas ed sharply to 30 cm, then remained relatively constant with increasing depth. Average delta D values were -13 parts per thousand, for 0-30 c m depth and -36.7 parts per thousand for 30-100 cm depth. Soil water 6 D values were negatively associated with soil water content and soil w ater potential. Concurrent analyses of xylem water revealed a high deg ree of partitioning of water resources among species of this tropical forest. Xylem water delta D of deciduous trees (average = - 25.3 +/- 1 .4 parts per thousand) was higher than that of evergreen trees (averag e = - 36.3 +/- 3.5 parts per thousand), indicating that evergreen spec ies had access to the more abundant soil water at greater depth than d eciduous species. In evergreen shade-tolerant and high-light requiring shrubs and small trees, delta D of xylem water was negatively correla ted with transpiration rate and leaf water potential indicating that s pecies using deeper, more abundant water resources had both higher rat es of water use and more favorable leaf water status.