SAPFLOW MEASUREMENTS IN FOREST STANDS - METHODS AND UNCERTAINTIES

This paper discusses the respective advantages and disadvantages of th ree sapflow techniques used for measuring tree transpiration in forest s: heat pulse velocity, tissue heat balance (Cermak-Type), and radial flowmeter (Granier-Type). In the EUROFLUX programme, aiming at analysi ng 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. P rincipal problems of the methods are linked with the influence of natu ral thermal gradients in the trunks and with effects of heat storage a nd conduction within the tissue. Sapflow probes can be typically left in place during one vegetation period, without any apparent modificati on 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 scalar s. In a homogeneous, untreated stand, the appropriate sample size is u sually about ten but increases depending on species, conducting type o f the xylem and spatial heterogeneity of the site. It is recommended t o combine sapflow measurements with eddy covariance techniques in orde r to separate tree transpiration from total forest water vapor flux an d to examine spatial heterogeneity of fluxes within forest stands. ((C ) Inra/Elsevier, Paris.).