Stomatal control of transpiration in the canopy of a clonal Eucalyptus grandis plantation

Predawn leaf water potential, stomatal conductance and microclimatic variab les were measured on 13 sampling days from November 1995 through August 199 6 to determine how environmental and physiological factors affect water use at the canopy scale in a plantation of mature clonal Eucalyptus grandis Hi ll ex-Maiden hybrids in the State of Espirito Santo, Brazil. The simple "bi g leaf" Penman-Monteith model was used to estimate canopy transpiration. Du ring the study period the predawn leaf water potential varied from -0.4 to -1.3 MPa, with the minimum values observed in the winter months (June and A ugust 1996), while the average estimated values for canopy conductance and canopy transpiration fell from 17.3 to 5.8 mm s(-1) and from 0.53 to 0.18 m m h(-1), respectively. On the basis of all measurements, the average value of the decoupling coefficient was 0.25. During continuous soil water shorta ge a proportional reduction was observed in predawn leaf water potential an d in daily maximum values of stomatal conductance, canopy transpiration and decoupling coefficient. The results showed that water vapour exchange in t his canopy is strongly dominated by the regional vapour pressure deficit an d that canopy transpiration is controlled mainly by stomatal conductance. O n a seasonal basis, stomatal conductance and canopy transpiration were main ly related to predawn leaf water potential and, thus, to soil moisture and rainfall. Good results were obtained with a multiplicative empirical model that uses values of photosynthetically active radiation, vapour pressure de ficit and predawn leaf water potential to estimate stomatal conductance.