Partitioning of soil water among canopy trees in a seasonally dry tropicalforest

Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical fc,rests. We assessed spatial and temporal pattern s of soil water utilization in several canopy tree species on Barro Colorad o Island, Panama, during the 1997 dry season. Stable hydrogen isotope compo sition (delta D) of xylem and soil water, soil volumetric water content (th eta(v)), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water delta D to decrease from about -15 parts per thousand at 0.1 m to -50 to -55 parts per thousand at 1.2 m dept h. Groundwater sampled at the sources of nearby springs during this period yielded an average FD value of -60 parts per thousand. theta(v) increased s harply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem delta D values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where theta(v) dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths gre ater than 1 m where theta(v) remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem delta D value s were less certain because variation in soil water delta D between 20 and 70 cm was relatively small. Xylem water delta D was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This rel ationship appeared to be species independent. Trees able to exploit progres sively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem delta D values, were also able to maintain cons tant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increa singly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater delta D values thus pointed to spatial and temporal partitioning of water resources among several tropical forest cano py tree species during the dry season.