Estimating transpiration of apple tree branches from leaf stomatal conductance measurements - a first assessment of RATP model on apple trees

The aim of this study was to estimate water flux of apple tree branches, from measurements of transpiration and stomatal conductance, made at the leaf scale. For this purpose, we used the model RATP (Radiation Absorption, Transpiration and Photosynthesis) which simulates the spatial distribution of solar radiation and leaf-gas exchange within tree crowns as a function of canopy structure, microclimate within the canopy and physical and physiological leaf properties. In the present application, the model was used to estimate the transpiration of branches, at one-hour time steps and model outputs were compared with field measurements. Field measurements were carried out on six-year-old trees of 'Fuji'. Two trees were digitised and, on each tree, two branches were selected in eastern and western parts of the crown, respectively. Their transpiration was measured at ten minute intervals with sapflow sensors. Stomatal conductance was measured with a portable photosynthesis system LICOR 6200, on leaves sampled on similar trees of the same plot. Global and diffuse radiation were collected in the field while temperature and relative humidity were recorded at a near by meteorological station. At the branch scale, the measured transpiration was highly correlated to air temperature and air vapour pressure deficit (VPD). On the contrary, a poor correlation was found between the stomatal conductance of individual leaves and these climatic factors. A first assessment of RATP, was done by comparing estimated and measured transpiration rates on one tree, for three successive days with differing climatic conditions. The transpiration rates, estimated at one hour time steps, were consistent with the climatic conditions and followed the variations of PAR. But, the model over-estimated transpiration, in particular during the morning and for the most sunny days. These results appear to indicate a weakness of the sub-model for modelling stomatal regulation of apple tree and in the climatic data measurements.