Sap flow and water transfer in the Garonne river riparian woodland, France : first results on poplar and willow

This work is the first attempt at using Granier sap sensors on Populus nigra, Populus x euramericana cv I45/51 and Salix alba for the monitoring of sap flows in an active floodplain over two consecutive years. The main characteristic of these diffuse porous trees is their capacity to use several tree rings for xylem sap transfer. Results showed that the sap flux densities remained homogeneous on the external 4 cm of the trunk, then decreased with depth. For young trees, the active sapwood can represent half of the trunk. Results indicated that in the same environment and at the same age, daily differences existed between the two major native riparian tree species, the black poplar and the white willow. Their maximal sap flux density (2.6-3.6 dm 3 dm -2 h -1) was similar to other fast growing trees. The influence of age was the third important screened factor. Sap flow measurements over several months indicated that water uptake was variable throughout the season, depending on water availability, and was more pronounced for older trees. The sap flux densities for the planted poplar (I45/51) ranged from 2.2-2.6 dm 3 dm -2 h -1 (about 90 dm 3 day -1) in the wetter spring conditions and dropped to 1.6-1.7 dm 3 dm -2 h -1 (about 60 dm 3 day -1) in less favourable conditions. Under the worst conditions, e.g., the especially long drought in the summer of 1998, these values dropped to 1.0-1.2 (about 40 dm 3 day -1), and even to 0.35 dm 3 dm -2 h -1 (about 12 dm 3 day -1) for a few days. Complementary long-term studies are needed to better understand the complex sap flow changes and to be able to relate them to significant environmental factors. Priority should be given to the long-term monitoring of sap flows at different depths for a correct estimation of actual daily water uptakes by riparian softwood trees