Characterizing the leaf size / number trade-off at different scales

Abstract

A novel interpretation of leaf mass variation in plants has been recently proposed based on its relationship with variation in leafing intensity – number of leaves produced per unit remaining above-ground dry mass. It is now widely established that between species variation in these two traits is strongly, negatively (and isometrically) correlated for woody and herbaceous species. Possible fitness advantages of higher leafing intensity (requiring smaller leaf size) can be interpreted in terms of the greater concentration of axillary meristems (larger ‘bud bank’), which may provide greater potential deployment flexibility for growth or reproduction. However, no research to date has addressed how these two traits are related at different scales, specifically at the between-individual (within-species) level, or between sexes in dioecious species. For the majority of 24 herbaceous angiosperms studied here, between-individual (within species) co-variation in leaf size and leafing intensity displayed as an isometric trade-off. For the few species with allometric trade-offs, plants with smaller body size produced disproportionately more leaves, indicating a possible mechanism for promoting reproductive economy when plant size is suppressed (e.g. by competition). For two out of three woody dioecious species, and two out of three herbaceous dioecious species, males tended to produce more, smaller leaves than females, possibly promoting pollen dispersal through a more broadly-spaced floral display (developing from more numerous axillary meristems). In contrast, the larger (but fewer) leaves in females may serve to maximize local photosynthetic resources available for axillary fruit and seed maturation. Males and females did not differ in branching rates associated with differences in leafing intensities, but in one herbaceous species, higher leafing intensity in males was associated with higher flower production than in females. In the larger 24-species data set, there was no evidence that species with higher leafing intensity (providing more axillary meristems) was associated with either a greater magnitude or a greater flexibility in branching or flowering intensity. Additional studies are required to explore the possibility that large ‘bud banks’ of axillary meristems (conferred by high leafing intensity) may promote effective mechanisms for tolerating environmental variability, responding to damage (e.g. from herbivory), and/or maximizing reproductive economy.