The relationship between tree height and leaf area:: sapwood area ratio

The leaf area to sapwood area ratio (A(1):A(s)) of trees has been hypothesized to decrease as trees become older and taller. Theory suggests that A(1):A(s) 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(1):A(s) declines with tree height. Whole-tree A(1):A(s) 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(1):A(s) declined substantially as height increased (P=0.02). Our test of the hypothesis that A(1):A(s) 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(1):A(s) with increasing height (P<0.05). However, two species (Picea abies and Abies balsamea) exhibited an increase in A(1):A(s) with height, although the reason for this is not clear. The slope of the relationship between A(1):A(s) and tree height (Delta A(1):A(s)/Delta h) was unrelated to mean annual precipitation. Maximum potential height was positively correlated with Delta(A)1:A(s)/Delta h. The decrease in A(1):A(s) 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.