Control of transpiration in an irrigated Eucalyptus globulus Labill. plantation

Stomatal conductance and transpiration were measured concurrently in an irr igated Eucalyptus globulus Labill. plantation. Canopy stomatal conductance, canopy boundary layer conductance and the dimensionless decoupling coeffic ient (Omega) were calculated (a) summing the conductance of three canopy la yers (g(c)) and (b) weighting the contribution of foliage according to the amount of radiation received (g(c)'). Canopy transpiration was then calcula ted from g(c) and g(c)' for Omega = 1 (E-eq), Omega = 0 (E-imp) and by weig hting E-eq and E-imp using Omega (E-Omega). E-eq, E-imp and E-Omega were co mpared to transpiration estimated from measurements of heat pulse velocity. The mean value of Omega was 0.63. Transpiration calculated using g(c) and assuming perfect coupling (12.5 +/- 0.9 mmol m(-2) s(-1)) significantly ove restimated measured values (8.7 +/- 0.8 mmol m(-2) s(-1)). Good estimates o f canopy transpiration were obtained either (a) calculating E-Omega separat ely for the individual canopy layers or (b) treating the canopy as a single layer and using g(c)' in a calculation of E-imp (Omega = 0). The latter ap proach only required measurement of stomatal conductance at a single canopy position but would be unsuitable for use in combined models of canopy tran spiration and assimilation. It should however, be suitable for estimating t ranspiration in forests regardless of the degree of coupling.