A Century Of Wood Anatomy and 75 Years Of IAWA
Gartner, Barbara .
Why do coniferous trees produce juvenile and mature wood?
THE structure and properties of conifer xylem changes systematically from the pith outward at all heights in the bole, branches and roots. The juvenile wood (JW, produced by a young cambium, near the pith) has such different properties from mature wood (MW, produced by the older cambium) that the two wood types are often used and processed differently. JW has higher microfibril angle, shorter narrower tracheids, and often lower density wood than MW. I present evidence that these radial changes occur to fulfill changing hydraulic rather than mechanical roles. Arguments against the mechanical role include the observation that there is no known cost to having one microfibril angle or another, so there is no reason why a tree should not always produce wood with a low microfibril angle to confer higher material stiffness (MOE). Secondly, radial increases in MOE caused by the changes in microfibril angle and density are relatively minor; the large increase in tree stiffness results from the increase in the second moment of area (a function of radius to the fourth power) as trees grow. Arguments in favor of the hydraulic role are that JW appears to be able to withstand more negative water potentials before losing conductivity, a capacity that would be useful in plants or plant parts with inefficient or little access to stored water, such as young trees and tree tips. This positive argument purports that the properties seen in JW are actually useful there, rather than that the properties are not very good in JW but that they improve in MW. These ideas leave open why there is a radial decrease in microfibril angle, which could be a neutral trait tied developmentally to increases in tracheid length, or could be of importance in itself, possibly related to cell wall mechanics during drought.
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1 - Dept. of Wood Science and Engineering, Oregon State University, Corvallis, Oregon, 97331, USA
Presentation Type: Symposium or Colloquium Presentation
Date: Tuesday, August 1st, 2006
Time: 3:00 PM