Herbivory constraints on symbiotic N2-fixers in young recovering tropical rainforests

Background/Question/Methods

Early succession tropical forests can serve as a significant carbon sink, but their growth, and therefore, carbon sequestration capacity may be constrained by the nutrient nitrogen.  Some tree species have evolved symbiotic relationships with nitrogen-fixing bacteria and help facilitate a carbon sink through a nutrient-carbon feedback mechanism.  Yet, the extent to which these N₂-fixers can provide the nitrogen needed to support a carbon sink may be constrained by leaf tissue lost to herbivores that prefer nitrogen-rich leaves.  Some tree species have evolved defense mechanisms, including tougher leaves, to deter herbivory, but whether fixers utilize this strategy and whether it is effective remains unclear.  We established herbivore exclosure experiments on 42 nitrogen-fixing trees (Inga cocleensis, Inga thibaudiana) and 44 non-fixing trees (including species Vernpa patens, Miconia argentea, Xylopia frutescens) and collected leaf samples from 60 fixers (I. cocleensis, I. thibaudiana) and 232 non-fixers (Byrsonima crassifolia, Vismia baccifera, Conostegia xalapensis, Xylopia frutescens, M. argentea, V. patens) in young forests in Panama to resolve whether fixers experience more herbivory than non-fixers. In addition we evaluated mechanisms that may attract or deter herbivores, including nitrogen content and leaf toughness. 

Results/Conclusions

We observed higher rates of herbivory with fixers and a positive relationship between leaf nitrogen content and herbivory, suggesting that fixers experienced more herbivory than non-fixers and that herbivores preferentially selected leaves for nutritional value.  Fixers had a herbivory rate that was three times higher than that of non-fixers (3.0 vs. 0.9% leaf removed month^-1) and a cumulative herbivory that was five times higher than that of non-fixers (22.3% vs. 4.2% leaf removed).  Leaf toughness was not a unique trait of fixers and there was no consistent correlation between toughness and herbivory rate.  Although fixers experienced the highest rates of herbivory, they still have high growth rates in these young forests and are able to withstand large losses of nitrogen due to herbivory, likely because of their ability acquire atmospheric nitrogen via nitrogen fixation.  Our findings suggest that herbivory may constrain nitrogen-fixation and the abundance of nitrogen-fixers, and may moderate the extent to which tropical forests can act as a carbon sink.