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

Abstract

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 (J_s) 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, J_s is a function not only of k_s-a, but also of the driving force for water movement. The J_s: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 J_s (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 J_s calculated on the basis of radial tension gradients and measured radial specific conductivity (k_s-a), was about two orders of magnitude smaller than axial J_s. 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-a in determining in situ partitioning of J_s in the axial and radial directions.

Citation

Domec, J.C.; Meinzer, F. C.; Gartner, B. L.; Woodruff, D. 2006. Transpiration-induced axial and radial tension gradients in trunks of Douglas-fir trees. Tree Physiology. 26: 275-284