© 2003 Heron Publishing—Victoria, Canada
Incorporation of transfer resistance between tracheary elements into hydraulic resistance models for tapered conduits
Peter Becker (1, 2), Ronald J. Gribben (3) and Paul J. Schulte (4)
1. P.O. Box 128, Bunker, MO 63629, USA / 2. Author to whom correspondence should be addressed (wongbeck@yahoo.com) / 3. 25, Broomlands, Kelso, TD5 7PR, Scotland / 4. Department of Biological Sciences, University of Nevada, Las Vegas, NV 89154-4004, USA / Received November 27, 2002; accepted April 5, 2003; published online September 15, 2003
Summary
The model of West, Brown and Enquist (1999) shows that hydraulic resistance in trees can be independent of path length, provided
that vascular conduits widen sufficiently from tree top to base. We demonstrate that this result does not depend theoretically
on branching architecture or cross-sectional conductive area of the stem. Previous studies have shown that pit membrane resistance,
encountered when water moves between either tracheids or vessels, accounts for up to 60% of the total resistance in stem segments.
When pit membrane resistance, which is neglected by most whole-tree hydraulic models, was incorporated in hydraulic models
in three different ways, the near invariance of hydraulic resistance was preserved. If relative pit resistance was independent
of tracheid size or if tracheid dimensions were scaled to minimize wood resistivity, the minimum conduit taper required for
path length independence equaled that in the original model of West et al. (1999). Under the most realistic model, in which
relative pit resistance increased with tracheid radius, this value was doubled. Such taper is not possible within the typical
size range of tracheids over the entire length of moderately tall trees, but it might be possible for vessel-bearing trees.
Preliminary results indicated that although tracheid radius in the outer growth ring initially increased basipetally from
the top of an 18-m tall Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), it stabilized at mid-trunk. Also, conduit taper was not constant in this species, violating a key assumption
of the model of West et al. (1999), on which the invariance of hydraulic resistance depends.
Keywords:
Douglas-fir, flow path length, hydraulic limitation and compensation, pit membrane resistance, Pseudotsuga menziesii, tracheid dimensions.
