Estimation of conduit taper for the hydraulic resistance model of West et al.
The model of West, Brown and Enquist (1999) showed that total hydraulic resistance in trees can be independent of path length, provided that vascular conduits taper sufficiently. The original model defined the coefficient of taper, a bar, in terms of conduit radius and segment number, but segments can be difficult to distinguish in old stems. Adhering strictly to the definitions and assumptions of West et al., we derived several equations for the estimation of a bar based on measurements of tracheary radius and flow path length. However, an additional parameter, b, was introduced to permit different models of variation in the length of segments along the flow path besides the volume-filling model of West et al., including a fixed-length model and intermediate versions. One of the new equations was fitted to previously published data for two tree species to illustrate the estimation of conduit taper from path-length relations, which proved quite sensitive to variation in b. Sampling protocols for improving such analyses are suggested.