title: Variability in radial sap flux density patterns and sapwood area among seven co-occurring temperate broad-leaved tree species.
abstract: Forest transpiration estimates are frequently based on xylem sap flux measurements in the outer sections of the hydro-active stem sapwood. We used Granier's constant-heating technique with heating probes at various xylem depths to analyze radial patterns of sap flux density in the sapwood of seven broad-leaved tree species differing in wood density and xylem structure. Study aims were to (1) compare radial sap flux density profiles between diffuse- and ring-porous trees and (2) analyze the relationship between hydro-active sapwood area and stem diameter. In all investigated species except the diffuse-porous beech (Fagus sylvatica L.) and ring-porous ash (Fraxinus excelsior L.), sap flux density peaked at a depth of 1 to 4 cm beneath the cambium, revealing a hump-shaped curve with species-specific slopes. Beech and ash reached maximum sap flux densities immediately beneath the cambium in the youngest annual growth rings. Experiments with dyes showed that the hydro-active sapwood occupied 70 to 90% of the stem cross-sectional area in mature trees of diffuse-porous species, whereas it occupied only about 21% in ring-porous ash. Dendrochronological analyses indicated that vessels in the older sapwood may remain functional for 100 years or more in diffuse-porous species and for up to 27 years in ring-porous ash. We conclude that radial sap flux density patterns are largely dependent on tree species, which may introduce serious bias in sap-flux-derived forest transpiration estimates, if non-specific sap flux profiles are assumed.
authors.affiliation: Plant Ecology, Albrecht-von-Haller-Institute for Plant Sciences, Georg-August-University of Göttingen, Untere Karspuele 2, 37073 Göttingen, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Gebauer
authors.forename: Viviana
authors.initials: V
authors.lastname: Horna
authors.forename: Christoph
authors.initials: C
authors.lastname: Leuschner
mesh_terms: D001692:Biological Transport / Q000502:physiology; D018526:Plant Transpiration; D013045:Species Specificity; D014197:Trees / Q000033:anatomy & histology* / Q000254:growth & development / Q000378:metabolism; D052584:Xylem / Q000033:anatomy & histology / Q000502:physiology