Annals of Botany

The proof of the postulate is in the eating

An obvious disadvantage of being rooted to the spot is the inability to run away from predators. Plants have thus evolved many different forms of defence against herbivores. The role of tough leaves in defence has been studied by a large multinational team working in tropical lowland rain forests in several different locations who have produced two comprehensive and informative papers (Dominy et al., pp. 1363–1377 and Grubb et al., pp. 1379–1389). Space does not permit a discussion of all their results and so we focus on the relationship between herbivory and leaf toughness in shade-tolerant monocots and dicots. Leaf toughness was measured as punch-strength using a penetrometer or as fracture-resistance using automated scissors. Herbivory was assessed by measuring loss of leaf area and by direct presentation of leaves to potential predators. Monocot leaves were tougher than dicot leaves at all stages of development; indeed it was especially noted that monocot leaves can be tough during the expansion phase, a phenomenon not seen in dicots. Further, the authors invite us to broaden our mental picture of ‘tough leaves’. Although it is true that in some monocot groups, such as palms, tough leaves fit the stereotype of being stiff with relatively low water content, others (mainly seen in the Zingerales) have large, non-stiff leaves with high water content that quickly roll up in dry conditions. There is a strong negative correlation between toughness (measured by either technique) and the extent of herbivory. This is reflected in the field observations made by the authors: monocots were much less prone to herbivory than dicots; for both groups, losses were mainly confined to the leaf expansion phase. However, the extent to which leaf toughness is involved in protection is variable and the authors challenge the botanical community to undertake further studies of herbivory in relation to monocot and dicot abundance in lowland tropical rain forests.