title: Partitioning of soil water among canopy trees in a seasonally dry tropical forest.
abstract: Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θv), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about -15‰ at 0.1 m to -50 to -55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of -60‰. θvincreased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θvdropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θvremained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD 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 relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant 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 increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season.
authors.affiliation: Hawaii Agriculture Research Center, 99-193 Aiea Heights Drive, Suite 300 Aiea, HI 96701 USA e-mail: meinzer@hawaii.edu Tel.: +1-808-4865357 Fax: +1-808-4865020, , , , , , US.
authors.forename: F C
authors.initials: FC
authors.lastname: Meinzer
authors.affiliation: Hawaii Agriculture Research Center, 99-193 Aiea Heights Drive, Suite 300 Aiea, HI 96701 USA e-mail: meinzer@hawaii.edu Tel.: +1-808-4865357 Fax: +1-808-4865020, , , , , , US.
authors.forename: José Luis
authors.initials: JL
authors.lastname: Andrade
authors.affiliation: Department of Botany, University of Hawaii, 3190 Maile Way, Honolulu, HI 96822, USA, , , , , , US.
authors.forename: Guillermo
authors.initials: G
authors.lastname: Goldstein
authors.affiliation: Biological Laboratories, Harvard University, Cambridge, MA 02138, USA, , , , , , US.
authors.forename: N Michele
authors.initials: NM
authors.lastname: Holbrook
authors.affiliation: Departamento de Ciencias Biológicas, Universidad de los Andes, Bogotá, Colombia, , , , , , CO.
authors.forename: Jaime
authors.initials: J
authors.lastname: Cavelier
authors.affiliation: Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Ancón, Republic of Panamá, , , ,.
authors.forename: S Joseph
authors.initials: SJ
authors.lastname: Wright
keywords: Key words Roots; Leaf phenology; Panama; Sap flow; Stable hydrogen isotope ratio