Modelling canopy conductance and stand transpiration of an oak forest from sap flow measurements
Granier, A.; Breda, N.
Annales des Sciences Forestieres 53(2-3): 537-546
In this study, transpiration was estimated from half-hourly sap flow measurements in a 35-year-old sessile oak stand (Quercus petraea) from 1990 until 1993 under various soil water conditions. The canopy conductance, calculated from the Penman-Monteith equation, was first analysed in relation to climatic variables: global radiation (R-g) and vapour pressure deficit (VPD). The maximum canopy conductance (g-cmax) was modelled with a nonlinear multiple regression over a period of nonlimiting soil water content, and of maximal leaf area index (LAI) with a r-2 apprx 0.80. Limitations of gc due to soil water deficit (relative extractable water (REW)) and canopy development (LAI) were then taken into account in the model by using multiplicative limiting functions of REW and LAI. A general canopy conductance model was then proposed. Finally, this relationship was re-introduced in the Penman-Monteith equation to predict dry canopy transpiration. Simulated transpiration was in good agreement with sap flow measurements during the year following the calibration (r-2= 0.92 in the control plot, 0.86 in the dry plot). The omega decoupling coefficient was close to 0.1 on a seasonal basis, indicating that transpiration was highly dependent on VPD.