Transpiration of a 64-year-old maritime pine stand in Portugal .1. Seasonal course of water flux through maritime pine

The transpiration, sap flow, stomatal conductance and water relations of Pinus pinaster were deter mined during spring and summer in a 64-year-old stand in Ribatejo (Portugal). The transpiration of the pine canopy was determined from sap flow or eddy covariance techniques. Canopy conductance values (g(c)) were estimated from inversion methods using eddy covariance or sap flow data, respectively, and from scaling-up methods using stomatal conductance values measured in the field and leaf area index (LAI) values. The transpiration was closely controlled by the stomatal conductance of pines (Omega was 0.05-0.15). For wet soil conditions, the various estimates of g(c) showed reasonable agreement. g(c) peaked in the morning at 0.01 m x s(-1), exhibited a midday depression and showed a secondary peak in late afternoon. This behaviour could be predicted simply on the basis of the stomatal sensitivity to air vapour pressure deficit. On a seasonal basis, monthly average values of g(c) decreased from 4 x 10(3) m x s(-1) in spring to 1.7 x 10(-3) m . s(-1) in late summer. Accordingly, the transpiration peaked at 3 mm x d(-1) on wet soil in May. It decreased progressive ly during the summer drought to 0.8 mm x d(-1) at the end of August. The minimal value of needle water potential was maintained at -1.9 MPa but predawn values de creased from -0.6 MPa in May to -0.9 MPa in July. It may have reached lower values in August. The amount of water stored in the trunk accounted for a 12% (10 kg x tree(-1) x day(-1)) of the daily transpiration in spring. The storage capacity of the canopy was within the same order of magnitude. The trunk storage increased to 25% (13 kg x tree(-1) x day(-1)) of the daily transpiration at the end of summer under drought conditions. The sap flow beneath the crown lagged accordingly behind transpiration with a time constant estimated between 26 min in spring and 40 min at the end of summer.