A quicker fix: Herbivory may stimulate nitrogen fixation rates in tropical forests

Will Barker, Sheila Palmer, Oliver Phillips, David Ashley and Sarah A. Batterman, School of Geography, University of Leeds, Leeds, United Kingdom, Klaus Winter, Aurelio Virgo and Jorge Aranda, Smithsonian Tropical Research Institute, Panama

Background/Question/Methods
Recent evidence suggests that nitrogen-fixing trees fulfil a critical role in promoting tropical forests regrowth after disturbance and that high herbivory may impose a significant constraint on the trait of fixation. This high herbivory may limit the amount of nitrogen fixed in young forests which could affect the regrowth of tropical forests recovering from disturbance. However, how high herbivory for fixers affects the fixation rates of individual plants remains unclear. Leaf nutrients lost to herbivory could alter patterns of fixation by affecting plant nutrient demand - depending on the level of herbivory, soil nitrogen availability and species antiherbivore strategy. Species may utilise different strategies in response to herbivory which could affect fixation, for example upregulating fixation to produce nitrogen based chemical defences or downregulating fixation to conserve carbon for compensatory growth. Therefore, we conduct greenhouse experiments on 200 seedlings from five Inga species to test three hypotheses: (1) that herbivory governs fixation rate, (2) that this effect is governed by an interaction between herbivory level and soil nitrogen availability, and (3) that the herbivory-fixation relationship is mediated by species antiherbivore strategy.
Results/Conclusions
We find up to tenfold increases in fixation rates following simulated herbivory (two-way anova, p = <0.05), but, as hypothesized, this effect depends on the amount of leaf area lost to herbivory, soil nitrogen availability and species antiherbivore strategy. This strong positive relationship between herbivory and fixation may facilitate faster forest regrowth after disturbance and, as herbivory temporarily increased nitrogen demand, could explain the prevalence of high fixation rates in tropical forests, despite high soil nitrogen. However, recent evidence suggests that herbivory may reduce the abundance of fixers in tropical forests. In addition, more work is needed to determine how general fixation responses to herbivory are across fixer species and life stages. Our findings therefore identify a key role for herbivory in governing tropical nitrogen fixation, with likely consequences for the tropical forest carbon sink.