Control of transpiration in an irrigated Eucalyptus globulus Labill. plantation

Stomatal conductance and transpiration were measured concurrently in an irrigated Eucalyptus globulus Labill. plantation. Canopy stomatal conductance, canopy boundary layer conductance and the dimensionless decoupling coefficient (Ω) were calculated (a) summing the conductance of three canopy layers (g(c)) and (b) weighting the contribution of foliage according to the amount of radiation received (g(c)'). Canopy transpiration was then calculated from g(c) and g(c)' for Ω = 1 (E(eq)), Ω = 0 (E(imp)) and by weighting E(eq) and E(imp) using W (E(Ω)). E(eq), E(imp) and E(Ω) were compared to transpiration estimated from measurements of heat pulse velocity. The mean value of Ω was 0.63. Transpiration calculated using g(c) and assuming perfect coupling (12.5 ± 0.9 mmol m-2 s-1) significantly overestimated measured values (8.7 ± 0.8 mmol m-2 s-1). Good estimates of canopy transpiration were obtained either (a) calculating E(Ω) separately for the individual canopy layers or (b) treating the canopy as a single layer and using g(c)' in a calculation of E(imp) (Ω = 0). The latter approach only required measurement of stomatal conductance at a single canopy position but would be unsuitable for use in combined models of canopy transpiration and assimilation. It should however, be suitable for estimating transpiration in forests regardless of the degree of coupling.