The potential contribution of biomechanical properties to anti-herbivore defence in seedlings of six Australian rainforest trees

Abstract

Biomechanical properties of leaves, such as strength and toughness, may contribute to anti-herbivore defence by making it physically difficult or energetically costly for animals to eat them. We investigated leaf strength and toughness in seedlings of six rainforest trees from eastern Australia and their potential contribution to defence. Strength and toughness (work to fracture) were measured at the scale of the whole leaf and of different leaf parts. Resources for herbivores (water and nitrogen) and potential chemical defences (phenolics, alkaloids and cyanogenic glycosides) were also investigated.

Leaves of Nothofagus moorei (F.Muell.) Krasser, Ceratopetalum apetalum D.Don and Doryphora sassafras Endl. were generally the toughest and strongest of those studied. Toona ciliata M.Roemer showed high concentrations of nutritive resources and little investment in the defences investigated, consistent with its shorter leaf lifespan and higher growth rate. There were no significant correlations of mechanical properties with palatability, but the softest leaf (T. ciliata) was associated with high levels of leaf damage in bioassay trials. There was also evidence that some leaves that are well-defended mechanically (involving carbon-rich cell wall) may invest less in competing carbon-based chemical defences, such as phenolics.