Developmental and Structural Section
Domec, Jean-Christophe , Gartner, Barbara , Meinzer, Frederick .
Bordered pit structure and function determine spatial patterns of hydraulic thresholds in xylem of Douglas-fir trees: do ultimate xylem structural constraints come into play at species-specific height limits ?
WE used wood from Douglas-fir trees to determine whether xylem hydraulic function is linked directly to bordered pit functioning in roots, trunks and branches and at different vertical and radial locations as predicted by the air-seeding hypothesis, and whether height-related trends in bordered pit function exist. Dimensional characteristics of bordered pit structures were determined from light and scanning electron microscope images. Physiological ranges were determined from published values of xylem pressure and resistance to embolism. From roots to branches, tracheid margo resistivity increased significantly with height because of a decrease in the size and number of margo pores. Tracheids in branches also had fewer and smaller pits than those in roots, which decreased overall wood conductivity with increasing height. As a consequence, linkages between pit structure, resistance to embolism, and hydraulic conductivity were explained by the fraction of the tracheid radial wall represented by pits. Decreasing membrane flexibility and porosity at the top of the trees made tracheids less susceptible to air seeding but reduced conductivity with tree height. Our data suggested that pit resistivity would become infinite at a xylem pressure of about -3 MPa, which corresponded to a minimum margo pore diameter of 0.24mm. This predicted minimum xylem pressure is about 0.5 MPa lower than that measured in a 65-m-tall Douglas-fir. Therefore, our data suggest that xylem structural limitations may ultimately come into play at species-specific height limits in addition to physiological constraints associated with path length, and gravitational or hydrostatic components.
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1 - Oregon State University, Department Wood Science & Engineering, Corvallis, Oregon, 97331-7402, USA
2 - USDA Forest Service, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, Oregon, 97331, USA
Presentation Type: Oral Paper:Papers for Sections
Location: 312/Bell Memorial Union
Date: Monday, July 31st, 2006
Time: 2:15 PM