PARTITIONING EVAPOTRANSPIRATION INTO TREE AND UNDERSTOREY COMPONENTS IN 2 YOUNG PINUS-RADIATA D DON STANDS

The tree and understorey components of latent heat flux density (LE) in two young Pinus radiata D. Don stands at different stages of commercial management were measured on two consecutive fine autumn days in each stand when there was no soil water deficit. LE was determined from the difference between available energy (A) and sensible heat flux density (H) measured using eddy covariance. Contributing LE from understorey (LEu) and trees (LEt) was estimated using small weighing lysimeters and the Penman-Monteith equation, respectively. There was generally good agreement between (A-H) and (LEt+LEu). LEt was strongly determined by stomatal conductance, vapour pressure deficit and leaf area index (at). Tree canopy conductances were of a similar magnitude in the two stands but were an order of magnitude less than corresponding values of aerodynamic conductance. In both stands, the daily contribution of LEu to forest LE was considerable. LEu was nearly equal to LEt in a 4-year-old stand which had not been pruned or thinned since establishment, but in a nearby 7-year-old stand, LEu was 30% less than LEt. In the 7-year-old stand, the presence of woody debris from pruning and thinning operations covering c. 60% of the understorey was responsible for the comparative reduction in LEu. Equilibrium LEu was estimated in the 7-year-old stand and overall was 43% less than measured LEu. Vertical wind statistics, determined near ground level in the relatively open 7-year-old stand, indicated that gusts regularly penetrated from above the forest to the understorey. These results suggest that forest management effects on the available energy and turbulence regimes below the tree canopy can be important determinants of LEu. © 1990.