Interpreting the variations in xylem sap flux density within the trunk of maritime pine (Pinus pinaster Ait.): Application of a model for calculating water flows at tree and stand levels

Sap flux density was measured throughout a whole growing season at different locations within a 25-year-old maritime pine trunk using a continuous constant-power heating method with the aim of 1) assessing the variability of the sap flux density within a horizontal plane of the stem section and 2) interpreting the time shift in sap flow at different heights over the course of a day. Measurements were made at five height levels, from 1.3 to 15 m above ground level. At two heights (i.e. 1.30 m and beneath the lower living whorl, respectively), sap flux density was also measured at four azimuth angles. Additionally, diurnal time courses of canopy transpiration, needle transpiration, needle and trunk water potential, and trunk volume variations were measured over 4 days with differing soil moisture contents. At the single tree level, the variability of sap flux density with respect to azimuth was higher at the base of the trunk than immediately beneath the live crown. This has important implications for sampling methodologies. The observed pattern suggests that the azimuth variations observed may be attributed to sapwood heterogeneity caused by anisotropic distribution of the sapwoods hydraulic properties rather than to a sectorisation of sap flux. At the stand level, we did not find any evidence of a relationship between the tree social status and its sap flux density, and this we attributed to the high degree of homogeneity within the stand and its low LAI. An unbranched three-compartment RC-analogue model of water transfer through the tree is proposed as a rational basis for interpreting the vertical variations in water flux along the soil-tree-atmosphere continuum. Methods for determining the parameters of the model in the field are described. The model outputs are evaluated through a comparison with tree transpiration and needle water potential collected in the field.

Duke Scholars

Author Jean Christophe Domec Environmental Sciences and Policy