-Tree transpiration was observed with sapflow methods in six Norway spruce (Picea abies) stands located in the Lehstenbach catchment, Fichtelgebirge, Germany, differing in age (40 years up to 140 years), structure, exposition and soil characteristics. The seasonal pattern in tree canopy transpiration, with the highest transpiration rates in July, was very similar among the stands. However, young dense stands had higher transpiration compared to older less dense stands. Because of forest management practices, stand density decreases with increasing stand age and provides the best predictor of canopy water use. Measured xylem sapflux density did not differ significantly among stands, e.g. vary in correlation with stand density. Thus, differences in canopy transpiration were related to differences in cumulative sapwood area, which decreases with age and at lower tree density. While both total sapwood area and individual tree sapwood area decrease in older less dense stands, leaf area index of the stands remains high. Thus

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-Tree xylem sapflow rates of 140-year-old Norway spruce (Picea abies) were scaled to the stand level and compared to canopy transpiration predicted by the stand gas exchange model STANDFLUX. Variation in sapflux densities between individual sensors was high (coefficient of variance = 0.4) and included both variation within and between trees, but it was not different between two applied sapflow methodologies (radial flowmeter

-Differences in canopy exchange of water and carbon dioxide that occur due to changes in tree structure and density in montane Norway spruce stands of Central Germany were analyzed with a three dimensional microclimate and gas exchange model STANDFLUX. The model was used to calculate forest radiation absorption, the net photosynthesis and transpiration of single trees, and gas exchange of tree canopies. Model parameterizations were derived for six stands of Picea abies (L.) Karst. differing in age from 40 to 140 years and in density from 1680 to 320 trees per hectare. Parameterization included information on leaf area distribution from tree harvests, tree positions and tree sizes. Gas exchange was modelled using a single species-specific set of physiological parameters and assuming no influence of soil water availability. For our humid montane stands, these simplifying assumptions appeared to be acceptable. Comparisons of modelled daily tree transpiration with water use estimates from xylem sapflow measurements provided a test of the model. Estimates for canopy transpiration rate derived from the model and via xylem sapflow measurements agreed within ± 20%, especially at moderate to high air vapor pressure deficits. The results suggest that age and density dependent changes in canopy structure (changes in clumping of needles) and their effect on light exposure of the average needle lead to shifts in canopy conductance and determine tree canopy transpiration in these managed montane forests. Modelled canopy net photosynthesis rates are presented, but have not yet been verified at the canopy level.norway spruce / xylem sapflow / canopy transpiration / canopy light use efficiency / biosphere-atmosphere interactions Résumé -Effet de l'âge et de la densité sur la modélisation des échanges gazeux dans la canopée de peuplements de Picea abies (L) Karst. dans le Fichtelgebirge (Allemagne). Les différences d'échange d'eau et de dioxyde de carbone au niveau de la canopée qui se produisent à la suite de changements dans la structure et la densité des arbres dans des peuplements d'Epicéa commun en zone de montagne de l'Allemagne centrale, ont été analysées selon le modèle tridimensionnel de microclimat et d'échange gazeux STANDFLUX. Le modèle a été utilisé pour calculer l'absorption du rayonnement par la forêt, le bilan photosynthétique net et la transpiration d'arbres individuels, et les échanges gazeux au niveau de la canopée. La paramétrisation du modèle a été effectuée à partir des mesures dans six peuplements de Picea abies (L.) Karst. dont l'âge variait de 40 à 140 ans et la densité des peuplements de 1680 à 320 arbres par hectare. La paramétrisation incluait l'information sur la distribution de la surface foliaire à partir de l'exploitation d'arbres, la position des arbres et leurs dimensions. Les échanges gazeux ont été modélisés en utilisant une série de paramètres physiologiques spécifiques de l'espèce, et en supposant qu'il n'y avait aucune influence de la disponibilité en eau. Pour les peuplements consi...

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