Measurement of leaf biomechanical properties in studies of herbivory: Opportunities, problems and procedures

Leaf biomechanical properties have the potential to act as antiherbivore de fences. However, compared with studies on chemical defences, there are few studies that have demonstrated that the physical or biomechanical structure of plants can prevent or influence herbivory. This difference in focus by ecologists may relate to the dominant paradigm of plant chemical defences i n ecological research and the perceived difficulties that ecologists have w ith the engineering principles embedded in biomechanics. The advantage of u sing materials engineering concepts is that each property is precisely defi ned and quantifiable, although the latter may be difficult in leaves becaus e of their composite and anisotropic nature. Most herbivory studies have us ed simple penetrometers to measure leaf properties, often termed 'toughness '. As defined in materials engineering, the measured properties are 'force to fracture' and 'strength', not toughness. Measurement of strength, the re sistance to crack initiation, is relevant to understanding herbivory. Measu rement of 'toughness' as defined by materials engineering is also relevant. Toughness is the resistance to crack propagation and is a measure of the e nergy required to fracture the leaf. This requires more sophisticated equip ment than simple penetrometers because it requires a simultaneous measure o f the punch displacement. In addition, purists would argue that a punch can not be used to measure true toughness because the crack is not controlled a nd plastic deformation is also involved. However, it may be the only method that allows detection of fine-scale pattern in mechanical properties acros s a leaf surface at a scale that is relevant to herbivory. There is very li ttle work on the scale at which these properties vary, particularly with re gard to different sized herbivores. In addition, few studies have investiga ted a broad range of relevant biomechanical properties in relation to herbi vory. Therefore, it is not possible yet to be definitive about the relative merits of the various types of tests. A single test might show a pattern i n relation to herbivore damage at a gross level. However, to really underst and the functional and ecological significance of leaf texture in relation to herbivory, a more reductionist approach is needed. Only then can we move on to the larger scales of pattern that many ecologists are seeking.