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.

. Leaf water status and stem xylem flux in relation to soil drought in five temperate broad-leaved tree species with contrasting water use strategies. Annals of Forest Science, Springer Verlag/EDP Sciences, 2009, 66 (1), pp.1. Ann. For. Sci. 66 (2009) Abstract• Five temperate broad-leaved tree species were compared with respect to their water consumption strategies under ample and restricted water supply. We measured synchronously leaf conductance (g L ) in the sun canopy, xylem sap flux (J s ) and leaf water potential (predawn, Ψ pd and noon, Ψ noon ) in adult trees in a mixed stand and related them to the fluctuations in vapor pressure deficit (D) and soil moisture.• Maximum g L was particularly high in F. excelsior, C. betulus and T. cordata and revealed a higher D sensitivity. Ψ pd remained constantly high in A. pseudoplatanus, C. betulus and F. excelsior, but decreased in T. cordata and F. sylvatica with decreasing soil moisture.• J sd decreased linearly with decreasing soil matrix potential in all species except for F. excelsior. Apparent hydraulic conductance in the soil-to-leaf flow path (L c ) was higher in A. pseudoplatanus than in the other species.• F. sylvatica maintained a low maximum g L and reduced J sd markedly upon drought, but faced severe decreases in Ψ pd and Ψ noon . F. excelsior represents an opposite strategy with high maximum g L and stable Ψ pd .• The species drought sensitivity increases in the sequence F. excelsior < C. betulus < T. cordata < A. pseudoplatanus < F. sylvatica. • J sd a diminué linéairement avec le potentiel matriciel du sol pour toutes les espèces excepté F. excelsior. La conductivité hydraulique apparente du trajet sol-feuille (L c ) était plus élevée chez A. pseudoplatanus que dans les autres espèces. Mots-clés• F. sylvatica a maintenu une faible g L maximum et a réduit sensiblement J sd face à la sécheresse, mais a connu de graves diminutions de Ψ pd et Ψ noon . F. excelsior présentait une stratégie opposée avec une g L maximum élevée et un Ψ pd stable.• La sensibilité des espèces à la sécheresse augmente selon la séquence F. excelsior < C. betulus < T. cordata < A. pseudoplatanus < F. sylvatica.

No abstract

No abstract

Variations in crown forms promote canopy space-use and productivity in mixed-species forests. However, we have a limited understanding on how this response is mediated by changes in within-tree biomass allocation. Here, we explored the role of changes in tree allometry, biomass allocation and architecture in shaping diversity-productivity relationships (DPRs) in the oldest tropical tree diversity experiment. We conducted whole-tree destructive biomass measurements and terrestrial laser scanning. Spatially explicit models were built at the tree level to investigate the effects of tree size and local neighbourhood conditions. Results were then upscaled to the stand level, and mixture effects were explored using a bootstrapping procedure. Biomass allocation and architecture substantially changed in mixtures, which resulted from both tree-size effects and neighbourhood-mediated plasticity. Shifts in biomass allocation among branch orders explained substantial shares of the observed overyielding. By contrast, root-to-shoot ratios, as well as the allometric relationships between tree basal area and aboveground biomass, were little affected by the local neighbourhood. Our results suggest that generic allometric equations can be used to estimate forest aboveground biomass overyielding from diameter inventory data. Overall, we demonstrate that shifts in tree biomass allocation are mediated by the local neighbourhood and promote DPRs in tropical forests.