© 2006 Heron Publishing—Victoria, Canada
Transpiration-induced axial and radial tension gradients in trunks of Douglas-fir trees
J.-C. Domec (1, 2), F. C. Meinzer (3), B. L. Gartner (1) and D. Woodruff (3, 4)
1. Department of Wood Science and Engineering, Oregon State University, Corvallis, OR 97331, USA / 2. Corresponding author (jc.domec@oregonstate.edu) / 3. USDA Forest Service, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331, US / 4. Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA / Received January 11, 2005; accepted June 11, 2005; published online December 15, 2005
Summary
We determined the axial and radial xylem tension gradients in trunks of young Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Axial specific conductivity (ks-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 ks-a but also of the driving force for water movement. The Js:ks-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 ks-a and its divergence from the theoretical ks-a determined by the Hagen Poisseuille equation. The ratio of ks-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 (ks-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 ks-a and ks-r in determining in situ partitioning of Js in the axial and radial directions.
Keywords:
sap flux density, specific conductivity, xylem anatomy, xylem embolism.
