Developmental and Structural Section
Lachenbruch, Barbara , Domec, Jean-Christophe , Meinzer, Frederick , Woodruff, David R. , Warrren, Jeffrey M. , McCulloh, Katherine A. .
Ultimate height of Douglas-fir trees appears related to water transport safety vs. efficiency through the bordered pits.
It is unclear what factors control the maximum heights that trees can attain, although it is clear that as they become taller, their xylem experiences more negative water potentials due to the greater path length and the longer water column upon which gravity is acting. We explored the hypotheses that a) the xylem at the tops of tall trees is constructed to withstand more negative water potentials than lower in the trunk and b) that the structural modifications that confer this drought resistance at tree tops is associated with decreased hydraulic conductance, which may ultimately limit water transport, and therefore growth, at the tops of trees. We studied bole and branch wood from mid and upper crown of Douglas-fir trees ranging in height from 6 to 85.5 m. From morphological measurements of xylem we estimated air-seeding pressure (=P50; the pressure difference between tracheids causing half of the original conductivity to be lost) and pit hydraulic conductance. With increasing height, air-seeding pressure increased because the pit aperture diameter decreased steadily, but the torus diameter remained relatively constant. This pattern resulted in a greater pressure difference needed at height to pull the torus sufficiently through the aperture that air-seeding could occur through the margo at the torus’ edge. The decrease in aperture diameter with height, in turn, caused a sharp decrease in pit conductance. Extrapolations of conductance vs. height suggested that pit conductance would approach zero at heights of 109 m (99-123 m, 95% CI) for boles and 138 m (131-145 m; 95% CI) for branches, values that are similar to the maximum tree heights recorded historically for this species of 100-127 m. These results suggest that the ultimate height of Douglas-fir trees may be limited in part by the conflicting requirements for water transport and water column safety.
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1 - Dept. of Wood Science and Engineering, Oregon State University, Corvallis, OR, 97331, USA
2 - Forestry and Environmental Resources, 3120 Jordan Hall, Raleigh, NC, 27695, USA
3 - USDA Forest Service, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, Oregon, 97331, USA
4 - Pacific Northwest Research Station, USDA Forest Service,, 3200 SW Jefferson Way, Corvallis, OR, 97331, USA
5 - Environmental Sciences Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN, 37831, USA
Presentation Type: Oral Paper:Papers for Sections
Date: Monday, July 28th, 2008
Time: 11:15 AM