Journal
TREE PHYSIOLOGY
Volume 28, Issue 12, Pages 1821-1830Publisher
OXFORD UNIV PRESS
DOI: 10.1093/treephys/28.12.1821
Keywords
annual growth rings; diffuse-porous; dyes; Fagus sylvatica; Fraxinus excelsior; Granier method; ring-porous; sap flow; Tilia; Weibull function
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Funding
- German Research Foundation (DFG)
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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 I 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 stein 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.
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