Comparative efficiency of conifer vs angiosperm tree wood

The data show that conifer wood can be less efficient hydraulically than angiosperm wood (Figure 4.2B, compare conifer vs. angiosperm closed symbols). Yet conifers remain competitive in certain habitats with angiosperm trees, suggesting that there are other considerations. The Murray's law definition of efficiency is a narrow one: maximum hydraulic conductance per unit volume of conducting cells. The difference between conifers and angiosperms could be much less if the definition of efficiency included the nonconducting volume required for mechanical support. By doing double duty, conifer tracheids may save on the sum of the tissue required for both hydraulic and mechanical functions when compared with the fiber vs. vessel cell types in angiosperm wood.

A theoretical analysis of this more comprehensive definition of efficiency is beyond the scope of this chapter. However, an approximation is to convert the Murray law conductances per fixed conducting volume to total volume (conducting plus nonconducting) by multiplying by the ratio of cross-sectional conducting area to total xylem area. This is equivalent to the analogous volume ratio for the same network dimensions, as long as the area ratio is the same for each branch rank. Doing this for the freestanding trees in Figure 4.2B (solid symbols) results in equivalent conductances per total volume across the wood types (Figure 4.2B, gray symbols). This preliminary calculation suggests that while angiosperm tree wood can pack more conductance into a given volume than conifer wood, when the additional volume required for fibers to support the tree is added, the two wood types can become more similar. Indeed, there is considerable overlap in xylem conductivity per total wood cross-sectional area between conifers and angiosperms

[47,48], although angiosperms as a group reach a higher maximum. Conifers are often competitive in stressful habitats [49] where avoidance of cavitation may limit vessel size, keeping conductivity per total wood area in angiosperms from significantly exceeding the conifer range.

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