Graduate Thesis Or Dissertation
 

Hydraulic properties of compression wood in branches and reoriented shoots of Douglas-fir (Psuedotsuga menziesii)

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  • Xylem anatomy is a strong determinant of water transport efficiency and is therefore an important component of the overall hydraulic strategy of any woody plant. However, in addition to its role in water transport, xylem also serves in mechanical support, and these two functions may represent conflicting design requirements. To further our understanding of how mechanical requirements might affect water transport in woody plants, this research evaluates the efficiency with which compression wood, which is specialized to function in mechanical support, conducts water. Using young Douglas-fir (Pseudotsuga menziesii) trees, two modes of compression wood formation are considered: normally-growing branches and branches that take over as leaders. Lower branch halves, which contained large amounts of compression wood, were significantly limited in their ability to conduct water relative to upper halves. This is likely a result of their anatomy, as lower halves were denser and had shorter tracheids with narrower cell lumens than upper halves. Compression wood samples from branches that had taken over as leaders were also limited in their ability to conduct water, relative to samples from the wood formed opposite and to the sides of compression wood. In addition, compression wood samples from branches that had taken over as leaders were less permeable than lower branch halves. Despite the poor transport efficiency of lower branch halves, the amount of compression wood visible on the end of each branch segment did not explain any significant variation in whole segment hydraulic properties. This lack of relationship between compression wood and whole segment properties suggests that branches may make up for the lost conductive capacity in some way. In branches that replaced a missing leader, there was no evidence that xylem formed elsewhere in the segment (i.e., "opposite wood") made up for the loss through a concomitant increase in permeability. Instead, branches that replaced leaders increased their transport efficiency relative to normal branches through an increase in conducting cross-sectional area. Given the reduced permeability of compression wood, future work should address how woody plants minimize or counteract the detrimental effects of this mechanically-specialized xylem to fulfill their water transport needs.
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