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

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