Leaf mechanical properties strongly influence leaf lifespan, plant-herbivore interactions, litter decomposition and nutrient cycling, but global patterns in their interspecific variation and underlying mechanisms remain poorly understood. We synthesize data across the three major measurement methods, permitting the first global analyses of leaf mechanics and associated traits, for 2819 species from 90 sites worldwide. Key measures of leaf mechanical resistance varied c. 500-800-fold among species. Contrary to a long-standing hypothesis, tropical leaves were not mechanically more resistant than temperate leaves. Leaf mechanical resistance was modestly related to rainfall and local light environment. By partitioning leaf mechanical resistance into three different components we discovered that toughness per density contributed a surprisingly large fraction to variation in mechanical resistance, larger than the fractions contributed by lamina thickness and tissue density. Higher toughness per density was associated with long leaf lifespan especially in forest understory. Seldom appreciated in the past, toughness per density is a key factor in leaf mechanical resistance, which itself influences plantanimal interactions and ecosystem functions across the globe.

Phenolics have been considered classic defence compounds for protecting plants from herbivores, ever since plant secondary metabolites were suggested to have evolved for that reason. The resource availability and carbon‐nutrient balance hypotheses proposed that variation in phenolic levels between and within plant species reflects environmental availability of nutrients and light, and represents a trade‐off in allocation by plants to growth and defence against herbivores. In contrast to these concepts, we suggest that (1) the main role of many plant phenolics may be to protect leaves from photodamage, not herbivores; (2) they can achieve this by acting as antioxidants; and (3) their levels may vary with environmental conditions in order to counteract this potential photodamage. We therefore suggest that patterns in phenolic levels, often used to support the concept of trade‐off between growth and herbivore defence in relation to resource availability, may actually reflect different risks of photodamage. We suggest that the level of many phenolics is low under some environmental conditions, not because resources to produce them are limited, but simply because the risk of photodamage is low and they are not required. If our photodamage hypothesis is correct, a reassessment of the ecological and evolutionary role of many phenolics in plant defence theory is required.

The use of model caterpillars to assess relative rates of predation has risen in popularity in recent years. Among the various benefits of the technique is its capacity to provide information on the identity of attackers through the impressions left in the modelling clay from which the model prey is constructed. However, there currently exists no detailed information on how to assign attack marks to particular predators. We aimed to address this gap by collating a comprehensive reference collection of the types of marks made by different predators to serve as a guide for researchers wanting to identify the predators responsible for attacks. To determine what level of resolution in identification may be considered reliable, we also tested the consistency of predator assignments made by different individuals. We found that predator identification at a coarse taxonomic level (i.e., bird, mammal, arthropod) was reasonably consistent. In contrast, when more fine‐scale identification was attempted, the level of consistency and therefore also confidence in the accuracy of an identification was dramatically reduced, reflecting the difficulty of distinguishing between attacks made not only by different arthropod groups but also by differently sized birds and mammals. We recommend that identifications be made at a coarse taxonomic level and, where possible, by multiple individuals. We also suggest that our collection of images of representative attack marks from each of the coarse predator categories, and descriptions of their defining characteristics, can serve as a guide to assist with identifications and this will be complemented by a good knowledge of the locally occurring and abundant predators.

Tannins are a diverse group of compounds which precipitate protein. The impact of tannins on herbivory has been difficult to assess because of diversity in tannin chemistry and in animal physiology. We have evaluated the effects of tannin on large ruminants (deer, sheep) using artificial diets containing well-defhred tannins, and have compared the results to those obtained with natural forages. The different effects of condensed tannins and gallotannins on herbivores are related to the chemical stability of the tannins. Commercial tannic acid does not have the same effects on herbivores as gallotannins in natural forages. Molecular weight apparently determines the metabolic fate of gallotannins from various sources. Tannins are a chemically diverse group of water soluble phenolits which bind proteins to form soluble or insoluble complexes (Bate-Smith and Swain 1962, Hagerman 1989). Tannins are widespread among dicotyledenous forbs, shrubs, and trees (Haslam 1979) and are thus ingested by many herbivorous mammals. Dietary tannin diminishes protein and dry matter digestibility in some mammals (Robbins et al. 1987a, 1987b) but does not decrease digestion in others (Driedger and Hatfield 1972). Tannin sometimes acts as a toxin rather than a digestion inhibitor (Mehansho et al. 1987a). The diversity of effects of tannin on digestion is due in part to differences in the physiological capabilities of animals to handle tannins and in part to differences in the chemical reactivity of various types of tannins. Recent work has demonstrated that several mechanisms are used by animals to counteract the effects of ingested tannins on digestibility. For example, tannin has little effect on digestibility in Research was funded by National Science Foundation grants BSR-8810233 and BSR-880S832.

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