Herbivory Constraints on Symbiotic N2-fixers in Young Recovering Tropical Rainforests

Abstract

Early succession tropical forests can serve as a significant carbon sink, but their growth, and therefore, carbon sequestration capacity may be constrained by 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. Fixers have evolved defense mechanisms, including tougher leaves, to deter herbivory, but the effectiveness and frequency of this anti-herbivore strategy remains unclear. We established herbivore exclosure experiments in young forests in Panama to resolve whether fixers experience more herbivory than non-fixers and whether herbivory rates correspond to leaf nitrogen content or leaf toughness. We observed higher rates of herbivory with fixers and a positive relationship between leaf nitrogen content and herbivory, suggesting that herbivores preferentially selected leaves for nutritional value. Leaf toughness was not a unique trait of fixers and there was no consistent correlation between toughness and herbivory rate. There may be more specialist herbivores that invest in fixers than non-fixers because fixers present a reliable source of nitrogen through space and time. Leaf damage from these herbivores may detract from growth advantages accrued by fixers from their access to supplementary nitrogen. Fixers may be outcompeted by non-fixers, which would indirectly reduce the forest carbon sink by decreasing nitrogen available to non-fixers. Our findings suggest that herbivory may constrain fixers, and in subsequently, may constrain the extent to which tropical forests can act as a carbon sink.