Transpiration-induced axial and radial tension gradients in trunks of Douglas-fir trees

We determined the axial and radial xylem tension gradients in trunks of young Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Axial specific conductivity (k(s-a)) and sap flux density (Js) were measured at four consecutive depths within the sapwood at a stem height of 1 m. By definition, at a given position in the bole, Js is a function not only of k(s-a) but also of the driving force for water movement. The Js:k(s-a) ratio was therefore used to estimate axial tension gradients and the radial gradients at a stem height of 1 m were calculated from the differences in axial tension gradients at each depth. Tracheid lumen diameter and tracheid length were used to predict differences in k(s-a) and its divergence from the theoretical k(s-a) determined by the Hagen Poisseuille equation. The ratio of k(s-a) (determined in the laboratory) to Js (measured in the field) varied with depth in the sapwood, resulting in non-uniform axial and radial tension gradients from inner to outer sapwood. Transpiration-induced axial tension gradients were in the range of 0.006-0.01 MPa m(-1) excluding the gravitational tension gradient. At a stem height of 1 m, radial tension gradients were in the range of 0.15-0.25 MPa m(-1) and were lower in the middle sapwood than in the inner or outer sapwood. Axial tension gradients were 44-50% higher in the outer sapwood than in the inner sapwood. At a stem height of 1 m, radial Js, calculated on the basis of radial tension gradients and measured radial specific conductivity (k(s-r)), was about two orders of magnitude smaller than axial Js. Our findings indicate that large radial tension gradients occur in the sapwood and clarify the role played by xylem k(s-a) and k(s-r) in determining in situ partitioning of Js in the axial and radial directions.