Sapflow measurements in forest stands: methods and uncertainties

This paper discusses the respective advantages and disadvantages of three sapflow techniques used for measuring tree transpiration in forests: heat pulse velocity, tissue heat balance (Cermak-Type), and radial flowmeter (Granier-Type). In the EUROFLUX programme, aiming at analysing and modelling water and CO2 fluxes above European forests, the two latter techniques are used at several sites. These two techniques were compared on the same trees, and resulted in similar flux estimates. Principal problems of the methods are linked with the influence of natural thermal gradients in the trunks and with effects of heat storage and conduction within the tissue. Sapflow probes can be typically left in place during one vegetation period, without any apparent modification of water transfer properties of the xylem. Different sources of sap flux variability related to temporal and spatial scale are discussed. Accuracy of sapflow estimates at the stand level can only be achieved by appropriate sample size of flux measurements and structural scalars. In a homogeneous, untreated stand, the appropriate sample size is usually about ten but increases depending on species, conducting type of the xylem and spatial heterogeneity of the site. It is recommended to combine sapflow measurements with eddy covariance techniques in order to separate tree transpiration from total forest water vapor flux and to examine spatial heterogeneity of fluxes within forest stands. ((C) Inra/Elsevier, Paris.).