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The leaf area to sapwood area ratio (A :A) of trees has been hypothesized to decrease as trees become older and taller. Theory suggests that A :A must decrease to maintain leaf-specific hydraulic sufficiency as path length, gravity, and tortuosity constrain whole-plant hydraulic conductance. We tested the hypothesis that A :A declines with tree height. Whole-tree A :A was measured on 15 individuals of Douglas-fir (Pseudotsuga menziesii var. menziesii) ranging in height from 13 to 62 m (aged 20-450 years). A :A declined substantially as height increased (P=0.02). Our test of the hypothesis that A :A declines with tree height was extended using a combination of original and published data on nine species across a range of maximum heights and climates. Meta-analysis of 13 whole-tree studies revealed a consistent and significant reduction in A :A with increasing height (P<0.05). However, two species (Picea abies and Abies balsamea) exhibited an increase in A :A with height, although the reason for this is not clear. The slope of the relationship between A :A and tree height (ΔA :A/Δh) was unrelated to mean annual precipitation. Maximum potential height was positively correlated with ΔA :A/Δh. The decrease in A :A with increasing tree size that we observed in the majority of species may be a homeostatic mechanism that partially compensates for decreased hydraulic conductance as trees grow in height.

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The hydraulic limitation hypothesis proposes that (1) reduced growth in taller trees is caused by decreased photosynthesis resulting from a decrease in hydraulic conductance promoted by a longer root-to-leaf flow path, and (2) this mechanism reduces stand productivity after canopy closure. This hypothesis was tested by comparing the phys-