The leaf size/number trade-off in trees

Using a sample of 24 common deciduous angiosperm trees of the Eastern Deciduous Forest region of North America, we tested the hypothesis that leaf size variation across species can be interpreted in terms of a trade‐off between individual leaf mass and the number of leaves produced.

The true nature of a resource allocation trade‐off is detectable only if variation in the total amount of growth is accounted for. We controlled for this effect by measuring all of the components of annual growth associated with leaf production at the individual terminal shoot level. Hence, number of leaves produced was expressed as ‘leafing intensity’, i.e. the number of leaves produced by newly emerged (current year's) shoots, divided by the total volume of these shoots.

Ninety per cent (r² = 0.90) of the variation in mean individual leaf mass across species, spanning two orders of magnitude, could be accounted for by proportional variation in mean leafing intensity, i.e. representing an isometric trade‐off, with a slope for log‐transformed data that did not deviate significantly from −1.0.

We suggest that this isometric relationship may represent a generalized trade‐off strategy for leaf deployment at the shoot level within temperate deciduous woody species. Following traditional interpretations, adaptation here may involve a fitness benefit associated with a particular leaf size. The present results also suggest an alternative, i.e. selection may instead favour high leafing intensity, with small leaf mass resulting not as a direct adaptation, but simply as a trade‐off.

According to this ‘leafing intensity premium’ hypothesis, the fitness benefits of higher leafing intensity are associated primarily with the fitness benefits of a larger pool of lateral meristems, because each leaf is usually associated with an axillary bud. This may in turn provide greater facility for wide phenotypic plasticity in the allocation of these meristems to vegetative vs. reproductive functions. This represents a plausible hypothesis, we suggest, in accounting for why most woody deciduous angiosperms, even some of the largest/tallest ones, have relatively small leaves.