Hydraulic limitation of tree height: A critique

  • Becker P
  • Meinzer F
  • Wullschleger S
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Theoretical analysis indicates that the observed tapering of xylem conduits permits the total resistance of a tube running from tree stem to petiole to remain constant, regardless of path length (tree height), so that water supply to all leaves is comparable if the xylem's vulnerability to embolization is disregarded. Recently, however, Ryan, M. G. & Yoder, B. J. (BioScience (1997) 47, 235-242) reviewed 4 major hypotheses to account for the cessation of tree height growth with increasing age and concluded that hydraulic limitation was the most promising. They argued that as tree height increases so does xylem path length and thereby hydraulic resistance owing to friction. Although trees compensate for their increased size by producing xylem elements with increased permeability, this was presumed to be insufficient to prevent total resistance from increasing with height. Consequently, stomata on tall trees must close early in the day to prevent xylem water potential from reaching a level that would lead to runaway embolization and catastrophic failure of the transport system. Stomatal closure reduces both loss of water and uptake of carbon dioxide so that daily carbon assimilation by leaves decreases. If foliage on tall trees produces less photosynthate, then less wood growth will result because other carbon costs do not change much with tree height. The hypothesis that height growth, net primary production and wood production may be constrained by hydraulic properties of tall trees is increasingly the subject of both experimental and theoretical studies. The authors of this critique consider, however, that a whole-plant perspective, which also takes account of soil and atmospheric influences, is essential to understanding what controls water transport to tree crowns. They also consider that mechanistic explanations of plant growth (and ecosystem productivity) must be compatible with the operation of natural selection to maximize individual fitness. The formal hydraulic model of Whitehead, D., Edwards, W. R. N. & Jarvis, P. G. (Canadian Journal of Forest Research (1984) 14, 940-947) predicts that a homeostatic balance exists between transport capacity (sapwood cross-sectional area and permeability) and transpirational demand. Thus, any path-length effects on water transport could be fully compensated if this was advantageous to the plant. In this paper, it is shown that the available data generally do not support Ryan & Yoder's version of the hydraulic limitation hypothesis, and that leaf:sapwood area ratio, rather than path length, may dominate in determining whole-plant conductance. Next, evidence and arguments are presented that components of the soil-plant-atmosphere continuum, such as hydraulic resistance of the rhizosphere and leaves, and water storage in the stem, may reduce or eliminate the significance of axial resistance to water transport. It is argued that the height of most tree species is more constrained by genetic, rather than physical limitations. When height growth ceases to offer a competitive advantage through avoidance of shading, then (genetically programmed) resource allocation will be adjusted to enhance tree survival and reproduction, not necessarily wood production. Two comments on this critique follow later in this issue of Functional Ecology (Menuccini, M. & Magnani, F., 135-137; Bond, B. J. & Ryan, M. G., 137-140)

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  • P. Becker

  • F. C. Meinzer

  • S. D. Wullschleger

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