VARIATION OF SAPFLOW VELOCITY IN EUCALYPTUS-GLOBULUS WITH POSITION INSAPWOOD AND USE OF A CORRECTION COEFFICIENT

Sapflow sensors were used to investigate variation in sapflow velocity at different positions in the sapwood of three-year-old Encalyptus gl obulus ssp. globulus Labill. trees. Sapflow velocity was measured at 5 -mm intervals across the sapwood by moving two probe sets simultaneous ly on two opposite radii. Another probe set placed in a fixed position at right angles to the first two sets acted as a control. A sapflow v elocity ratio was defined as the velocity given by each moving sensor divided by that given by the static sensor. Correlation between observ ations of sapflow velocity at different positions exceeded 95%, and th e ratio of velocity between any pair of sensors was constant. We obser ved radial variation in sapflow velocity across the sapwood with the l owest velocities at the center of the tree. Variation due to sensor po sition was high implying the need for large numbers of sensors for acc urate estimates of sap flux. To overcome this need, we used a correcti on coefficient, namely a simple weighted average of the sapflow ratios with depth in the sapwood, for each fixed sensor. We recommend the us e of three probe sets to estimate the correction coefficient. Subseque ntly, two probe sets can be placed at two fixed positions for routine measurements of sap flux.