Herbivore effects on nutrient cycling and the tropical forest carbon sink

Summary

Nitrogen-fixing trees hold a central role in providing the nitrogen needed to support a tropical carbon sink during periods of rapid biomass accumulation. New research suggests that nitrogen-fixing trees pay a major cost to herbivory with potential consequences for ecosystem nutrient and carbon cycles, yet data are restricted to a few species of trees in very young tropical forests. This project will use experimental and observational techniques to determine how widespread the herbivory cost is across tropical tree species and across forests of different ages, and to evaluate the ecosystem consequences for nitrogen fixation and tropical forest carbon sequestration.

Background

Nitrogen-fixing trees provide large quantities of nitrogen required to support high rates of biomass accumulation in tropical lowland rainforests, such as following disturbance by agriculture (Batterman et al. 2013) or potentially in response to rising atmospheric CO2 (Gerber et al. 2010). Yet despite this benefit to tropical trees, only a small fraction (5-10%) of the forest community has the potential to fix nitrogen, raising the observation that there must be costs associated with fixation to explain why more fixers do not become more abundant in tropical forests over ecological or evolutionary time scales.

Our recent findings suggest that in addition to costs associated with the metabolism and maintenance of fixation that have previously been considered, fixers face a major penalty due to the reduction in leaf tissue and photosynthesis by animal herbivores (Costa et al., in prep). When compared to non-fixers, herbivores prefer the nitrogen-rich leaves of fixers, which results in an additional and large carbon cost. This carbon cost may reduce the competitive advantage of fixers in tropical forests, leading to lower abundances of fixers in the forest community, lower nitrogen fixation rates and reduced carbon sequestration.

While these findings provide a first glimpse into the way that herbivores could shape biogeochemical carbon and nitrogen cycles, they are based on species of trees belonging to most abundant taxa of nitrogen fixers across the Neotropics and that fix the majority of the nitrogen in very young recovering Panamanian forests. It remains unclear how widespread this effect is in tropical forests.

This PhD project will take a major next step in expanding our understanding of the herbivore costs to nitrogen fixers and the ecosystem consequences. It will determine how widespread the herbivory cost is across species of fixers and non-fixers in Panamanian rainforests by considering the evolutionary history of species and any changes with tree and forest ages. The project will also consider herbivore effects on the recruitment of fixers and shifts in community composition of seedlings using a herbivore exclosure experiment.

Aims and approach

The project will address the following questions with a number of experimental and observational techniques:
1.Do herbivores shift the community composition of fixers and non-fixers via differential effects on their recruitment into the seedling/sapling community? Establish a herbivore-exclosure experiment on communities of seedlings that include fixers and non-fixers.
2.Does herbivory influences fixation rates and carbon accumulation rates? Using the herbivore exclosure experiment in #1, the researcher would measure fixation rates and biomass accumulation rates in the seedlings and scale up to the ecosystem/plot level using the abundance of fixer seedlings and allometric equations.
3.How widespread is the difference in herbivory rates between fixers and non-fixers? Is this phylogenetically constrained and/or determined by nitrogen content across species? Measure nitrogen content and herbivory across fixer and non-fixer species by targeting species across the fixer and non-fixer phylogenies, including sister taxa (i.e., legume fixers and legume non-fixers).
4.How constant is the herbivory cost as trees get older and forests age? Quantify nitrogen content and herbivory rates across tree and forest ages.