Effect Of Height On Tree Hydraulic Conductance Incompletely Compensated By Xylem Tapering

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Abstract
Summary: The hydraulic limitation theory proposes that the decline of forest productivity with age is a consequence of the loss of whole‐plant and leaf‐specific hydraulic conductance with tree height caused by increased friction. Recent theoretical analyses have suggested that tapering (the broadening of xylem vessel diameter from terminal branches to the base of the stem) could compensate completely for the effect of tree height on hydraulic conductance, and thus on tree growth. The data available for testing this hypothesis are limited, but they do not support the implication that whole‐tree and leaf‐specific hydraulic conductance are generally independent of tree height. Tapering cannot exclude hydraulic limitation as the principle mechanism for the observed decline in growth. Reduction of the leaf‐to‐sapwood area ratio, decreased leaf water potential, loss of leaf‐cell turgor, or osmotic adjustments in taller trees could reduce the effect of increased plant hydraulic resistance on stomatal conductance with height. However, these mechanisms operate with diminishing returns, as they infer increased costs to the tree that will ultimately limit tree growth. To understand the decline in forest growth, the effects of these acclimation mechanisms on carbon uptake and allocation should be considered.