Nitrogen-fixing tree species are much less abundant than expected in tropical forests, despite the benefits of nitrogen fixation. The recent observation that some tropical nitrogen-fixing tree species face a high herbivory cost may explain why nitrogen-fixers represent < 15% of Neotropical forest trees, but it remains unclear if high herbivory is general across fixer species. Under consistent herbivory pressure, species have evolved varying leaf nutrient contents and defence mechanisms to deter herbivores, even within functional fixer and non-fixer groups. Thus, it’s not clear whether fixer species consistently have higher herbivory than non-fixers, or if, across a wide array of species, no clear differences emerge between the two groups. We here combine (1) measurements of herbivory from the 22 fixer and 21 non-fixer tree species at the seedling stage in a Panamanian tropical rain forest with (2) data on leaf traits, including nutrient content and leaf toughness. Generalized mixed effects modelling was used to determine if herbivory widely varies between the two functional groups and to examine what traits or strategies may drive differences in herbivory.
Results/Conclusions
Mixed effects models indicated that fixer species were 23% more likely to have undergone herbivory (p = 0.01) and lost 27% more leaf area compared to non-fixers (p = 0.01), but variation in nutrient content and defense investment in both functional groups also affected herbivory. We found a significant positive correlation between leaf nitrogen and herbivory for all species, which drove greater incidence of herbivory and leaf area loss for fixers, but fixer species also had higher leaf concentrations of tough, defensive fibres, like cellulose, which negatively correlated with herbivory. Accordingly, mean herbivory per species varied widely across both groups, but with consistently higher values for fixers than non-fixers. We therefore present two related findings. First, despite variation in herbivory, our results support recent evidence that tropical fixers undergo greater herbivory than non-fixers driven by high leaf nitrogen; and, second, as a result of high herbivory, fixer species invest more in physical leaf defence than non-fixing species. High herbivory and consequential investment in defence over growth or reproduction can explain why nitrogen-fixing species are found at relatively low abundances across tropical forests, despite the advantage of fixing atmospheric nitrogen. Thus, trophic interactions may have shaped tropical forest communities over ecological and evolutionary time, with profound consequences for nitrogen cycling and the tropical forest carbon sink.