Sapwood as the scaling parameter defining according to xylem water content or radial pattern of sap flow?

Sapwood cross-sectional area is a simple biometric parameter widely used for scaling up the transpiration data between trees and forest stands. However, it is not always clear how the sapwood can be estimated and considered, which may cause scaling errors. We examined the sapwood depth according to xylem water content and more precisely according to radial patterns of sap flow rate in five coniferous and four broad-leaved species of different diameter, age and site conditions. Sapwood estimated by the two methods was almost equal in some species (e.g. Cupressus arizonica), but differed significantly in other species (e.g. Olea europaea, Pinus pinea). Radial pattern of sap flow rate is a more reliable indicator of sapwood then xylem water content for sap flow scaling purposes. Percentage of sapwood along radius changed with tree diameter and age. Sapwood also changes substantially under severe drought (e.g. in spruce, Picea abies, up to 1:3 in the course of several months). Sapwood should be used for upscaling sap flow data from measuring points to the whole trees and from trees to stands only for the period when it was actually measured, or the radial profile of sap flow should be measured continuously to avoid possible scaling errors. ((C) Inra/Elsevier, Paris).