Continuous measurements of water tensions in the xylem of trees based on the elastic properties of wood

According to the cohesion theory for the ascent of water in vascular plants, significant tensions should develop in the water columns of transpiring trees. These tensions cause small but detectable changes in the diameter of the xylem as a consequence of adhesive forces between water molecules and the inner xylem walls. The diurnal time course of tension in the water columns in the xylem of the trunk of mature Scots pine (Pinus sylvestris L.) was measured during the summer of 1995 by means of a displacement transducer mounted on a rigid steel frame. The apparent elastic modulus of Scots pine wood in the radial direction (E'(r)) was determined in the laboratory and then used to estimate tensions from the measured displacement. Laboratory measurements on logs indicated that only the sapwood contributed to dimensional changes of the xylem. Corrections for thermal expansion of the system were included. Water tensions fell by 0.19 MPa over the course of the day, when needle water potentials fell by 0.50 MPa. Such data are consistent with the cohesion theory, and with the view that the hydraulic resistances to flow in above- and below-ground plant parts are of similar magnitude.