Daily Variations of Water Use with Vapor Pressure Deficit in a Plantation of I214 Poplars

Daily rates of whole tree water use were measured in a homogeneous 7-year-old plantation over 84 days. Two trees were selected in the two most contrasting ends, i.e., with mean water table at -1.5 m and -3.5 m. Results showed that the mean tree water use was close to 80 dm day with a small mean difference between the two sites (5.4 dm day, about 7%). Higher daily variations were observed on a single tree over the season (from 17 to 138 dm day). These fluctuations could be modeled using linear regressions on the vapor pressure deficit (VPD). Results also indicated decreasing sap flux densities with radial sapwood depth and that the whole tree water use could be estimated from the sap flux densities measured at the 0–2 cm depth.