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Sclerophylly in two contrasting tropical environments: low nutrients vs. low rainfall¹

²School of Biological Sciences, Monash University, Victoria 3800, Australia; ³Institut Agronomique néo Calédonien, CIRAD UPR 22 Gestion intégrée de la faune, Station de Port-Laguerre, BP 73, Païta 98890, New Caledonia; and ⁴IRD–Laboratoire de Botanique et d'Écologie Végétale Appliquée, Institut de recherche pour le développement, Centre de Nouméa, BP A5 Nouméa 98848, New Caledonia

ABSTRACT

The defining characteristics of sclerophylly are mechanical (e.g., hardness, toughness, stiffness), but little is known about how they vary in contrasting environments and contribute to the adaptiveness of sclerophylly. Here we investigate how the degree and nature of sclerophylly in terms of leaf mechanics differ between vegetation of two contrasting stressful environments, maquis on nutrient-deficient, moist sites and dry forest on moderate-nutrient, drier sites. We measured toughness, strength, and stiffness at the level of the whole leaf ("structural") and per unit thickness ("material"). Leaves of maquis plants were on average structurally stiffer, stronger, and tougher than those in dry forest. There was little difference in material properties between habitats, and leaf thickness was the main contributor to differences in structural mechanical properties between habitats. Flexural stiffness varied most among species and habitats, correlating strongly with leaf mass per area and thickness. We suggest that having thicker leaves allows efficient packaging of biomass to reduce branching costs in sunny but stressful environments, with subsequent impacts on structural mechanical properties. Sclerophylly is probably a complex phenomenon, however, with its mechanical constitution arising from both evolved mechanical properties that confer protection or resistance to stress and nonadaptive mechanical consequences of adaptation to stressful environments.

Key Words: dry forests • ecobiomechanics • leaf toughness • plant biomechanics • sclerophylly • ultramafic vegetation

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