The Costs of Leaf Support in Understory Palms: Economy Versus Safety

Abstract

Biomass allocation to nonproductive leaf-support structures represents an inescapable compromise between economy and safety. In the bifid-leaved understory palm species Geonoma cuneata and Asterogyne martiana, the costs of leaf support increase disproportionately with leaf size and crown size. Greater leaf longevities of larger leaves are insufficient to compensate for these escalating costs. Increases in leaf size also entail increases in stresses acting on the petiole, resulting in lower factors of safety in the largest leaves. In Geonoma congesta, ontogenetic changes in leaf morphology from bifid to split leaves at larger leaf sizes result in reduced costs of leaf support relative to bifid juvenile leaves. Despite differences in maximum leaf size, the leaves of reproductive individuals in all three species have similar factors of safety, indicating that mechanical constraints may be limiting leaf size in these species. Although reproductive G. congesta leaves have the greatest leaf-support efficiency (leaf area per unit of support biomass), total leaf area per whole-plant biomass is lowest. In the reproductive stage, plants of G. cuneata, the smallest species, have the highest leaf-support efficiency at leaf and crown levels and the highest total leaf area per whole-plant biomass. These characteristics, largely a consequence of small leaf and plant size, enable G. cuneata to exploit the most heavily shaded microsites within the rain-forest understory.