Characterising sclerophylly: some mechanical properties of leaves from heath and forest

Abstract

Although sclerophylly is widespread through the world and is often the dominant leaf-form in mediterranean climates, the mechanical properties of sclerophyllous leaves are poorly understood. The term "sclerophyllous" means hard-leaved, but biologists also use terms such as tough, stiff and leathery to describe sclerophyllous leaves. The latter term has no precise definition that allows quantification. However, each of the former terms is well-defined in materials engineering, although they may be difficult or sometimes inappropriate to measure in leaves because of their size, shape or composite and anisotropic nature. Two of the most appropriate and practically applicable mechanical properties of sclerophyllous leaves are "strength" and "toughness", which in this study were applied using punching, tearing and shearing tests to 19 species of tree and shrub at Wilson's Promontory, Australia. The results of these tests were compared with leaf specific mass (LSM) and a sclerophylly index derived from botanists' ranks. Principal components analysis was used to reduce the set of mechanical properties to major axes of variation. Component 1 correlated strongly with the botanists' ranks. Overall, leaves ranked as sclerophyllous by botanists were both tough and strong in terms of punching and tearing tests. In addition, tough and strong leaves typically had high toughness and strength per unit leaf thickness. There was also a significant correlation between component 1 and LSM. Although more detailed surveys are required, we argue that sclerophylly should be defined in terms of properties that have precise meanings and are measurable, such as toughness and strength, and that relate directly to mechanical properties as implicit in the term.

Leaf palmate venation and vascular redundancy confer tolerance of hydraulic disruption

“…The multiple 1°veins in palmate leaves are expensive, entailing higher mass allocation to the veins relative to photosynthetic mesophyll (11), and a less efficient mechanical support per mass investment in these veins (5). We expect that the damage tolerance provided by palmate venation should be most advantageous for thin or short-lived leaves, because thick, tough leaves would already be protected against damage regardless of their venation architecture (52,53). The more expensive protection provided by general thickening would presumably be most beneficial in long-lived leaves subject to higher lifetime levels of herbivory (53,54).…”

Comparação da morfologia foliar de espécies arbóreas de três estádios sucessionais distintos de floresta ombrófila densa (Floresta Atlântica) no Sul do Brasil

“…Baseadas nas características morfológicas e nos baixos teores de fósforo e nitrogênio foliares, muitas dessas formações vegetais foram classificadas como esclerófilas. Apesar da controvérsia sobre o termo esclerofilia, que no senso estrito significa "folhas duras" (Roth 1984), o conceito mais aceito (ou pelo menos mais utilizado) é aquele que relaciona plantas esclerófilas com deficiências hídricas sazonais, baixos teores de nutrientes no solo, defesa contra herbivoria ou mecanismos de proteção da longevidade das folhas, aumentando assim o carbono foliar por unidade de investimento (Edwards et al 2000). Essas hipóteses não são excludentes e somam-se para explicar a presença da vegetação esclerófila nas formações florestais tropicais (Tanner & Kapos 1982, Medina et al 1990, Turner 1994.…”

Variabilidade morfológica foliar de Miconia sellowiana (DC.) Naudin (Melastomataceae) em diferentes fitofisionomias no Estado do Paraná

“…Estes resultados corroboram alguns estudos que relacionaram a AFE com vários fatores com intensidade de luz, indicando que as folhas expostas a altas intensidades luminosas (Edwards et al 2000, Kun-Fang 2000, Mendes et al 2001) assim como aquelas que se desenvolveram em solos com baixa retenção de água e concentração de nutrientes (Meziane & Shipley 1999, Navas & Garnier 2002, como o solo da EGL e FODAM, possuem maior grau de esclerofilia. Entretanto, é importante ressaltar que existem diferenças ambientais entre EGL e FODAM (tabela 1) e que estas se manifestam através das diferenças estruturais encontradas nas folhas destas duas fitofisionomias.…”