Sap flow measurements of trees are today the most common method to determine transpiration at the tree and the forest canopy level. They provide independent measurements for flux comparisons and model validation. Sap flow measurements of Norway spruce (Picea abies) which were conducted in the mountain range of the Fichtelgebirge (Germany) are reviewed with respect to analyses of structure-function relationships, atmospheric control and horizontal heterogeneity of stand transpiration in the Lehstenbach catchment, and local vertical gradients from the forest floor over various heights in the tree up to the atmosphere above. It is concluded that sap flow measurements have importance for the analysis of physiologically controlled plant process at the organism and community level and could first demonstrate age dependence of managed forest stands. The measurements also supplement long-term micrometeorological measurements and have the potential for cross validation of independent measurements of water vapor fluxes and the water equivalent of latent heat fluxes from eddy covariance technique. For short-term comparisons, the time lag between sap flow and evaporation and effects of changes in tree water storage need to be considered.
