Nitrogen-fixing plants provide critical nitrogen inputs that support the high productivity of tropical forests, but our understanding of the ecology of nitrogen fixers – and especially their interactions with herbivores – remains incomplete. Herbivores may interact differently with nitrogen fixers vs. non-fixers due to differences in leaf nitrogen content and herbivore defence strategies. To examine these potential differences, our study compared leaf carbon, nitrogen, toughness, chemical defence and herbivory for four nitrogen-fixing tree species (Inga oerstediana, Inga sapindoides, Inga thibaudiana and Pentaclethra macroloba) and three non-fixing species (Anaxagorea crassipetala, Casearia arborea and Dipteryx panamensis) in a lowland tropical rain forest. Leaf chemical defence, not nutritional content, was the primary driver of herbivore damage among our species. Even though nitrogen fixers exhibited 21.1% higher leaf nitrogen content, 20.1% lower C:N ratios and 15.4% lower leaf toughness than non-fixers, we found no differences in herbivory or chemical defence between these two plant groups. Our results do not support the common hypotheses that nitrogen fixers experience preferential herbivory or that they produce more nitrogen-rich defensive compounds than non-fixers. Rather, these findings suggest strong species-specific differences in plant–herbivore relationships among both nitrogen-fixing and non-fixing tropical trees.

Tropical plants have been suggested to have higher levels of mechanical, chemical and biotic defences than temperate plants. However, comparisons have usually included deciduous species within the temperate group, which confounds the analysis since deciduous species are predicted to have a different strategy with respect to investment, nutrition and defence than evergreen species. In this study we examined levels of defence and nutrition in five evergreen species of Eucryphia occurring along a latitudinal gradient in eastern Australia, grown under common conditions in a glasshouse. From the resource-availability hypothesis we predicted the opposite gradient in defence investment, i.e. that lowest levels of defence will occur in tropical species with potentially high growth rates and annual productivity. However, we found an increase in cell wall content, total phenolics and tannin activity, and a decrease in protein availability, with decreasing latitude and/or increasing mean annual temperature. Hence, there was a trend of increasing defence (although not in leaf toughness) and declining nutritional quality towards the tropics. These latitudinal trends were recorded in both mature and expanding leaves. The same trends were observed in leaves of two species collected from the field, indicating that the results were not peculiar to the experimental growth regime. The latitudinal trend in defence did not support our prediction based on the resource availability hypothesis and may indicate that herbivore pressure is providing an overriding selection pressure, although there are alternative explanations.

There is some evidence that traits of fresh leaves that provide structural or chemical protection (‘defence’) remain operational in the leaf litter and control interspecific variation in decomposition rate in or on the soil. We tested experimentally whether the negative relationship between foliar defence and litter decomposition rate is fundamental, i.e. whether it is seen consistently across higher plant species and life forms, and whether it is repeated in the floras of geographically and climatically distinct areas separated by an ocean. We employed the published results of two outdoor litter bag experiments, in which we simultaneously compared the relative mass losses (‘decomposibility’) of leaf litters of a wide range of plant species. One experiment was in Córdoba, Argentina, and included 48 Argentine species typical of the dry, subtropical landscapes along a steep altitudinal gradient. The other was in Sheffield, UK, and hosted 72 British species typical of the temperate–Atlantic landscape there. We linked the two experiments through a similar experiment in Sheffield that hosted litters of subsets of both the Argentine and British species. We also tested fresh leaves of all species from the same areas for tensile strength (‘toughness’) and relative palatability to generalist herbivorous snails in multi-species ‘cafeteria’ experiments. Both in Argentina and in Great Britain there were highly significant correlations between leaf palatability (r=0.61; 0.73) or leaf tensile strength (r=−0.60; −0.60) and litter mass loss across all species. These relationships could be explained by variation both between and within broad life-form groups. Specific leaf area (area[ratio ]dry mass) of fresh leaves was consistently correlated only with litter mass loss within British life form groups. We illustrated the possible ecosystem consequences of these relationships by comparing functional traits of British species differing in leaf habit. In comparison with deciduous species, evergreens generally had innately slow growth, which corresponded to their longer-lived leaves of lower specific leaf area, higher tensile strength and lower palatability to generalist invertebrate herbivores. Correspondingly, evergreens produced more resistant leaf litter. Thus, slow-growing evergreens might maintain their position in infertile ecosystems through leaf traits that help them to conserve their nutrients efficiently and to keep nutrient mineralization low, thereby not allowing potentially fast-growing deciduous species to outcompete them.