EAGER: Tracking carbon allocation to unravel how a mutualism and its breakdown affect the carbon cycle

Board Of Regents, Nshe, Obo University Of Nevada, Reno, Reno NV

Investigators

Abstract

Trees are key players in the carbon cycle because they recycle atmospheric carbon dioxide into forms of carbon that living things can use. To be successful, most trees require animals or microbes to provide them with services, e.g., with defense against enemies or to supply them with nutrients. Trees reward these service providers, known as mutualists, with carbon. Tree mutualists thus probably have large impacts on the carbon cycle, but these impacts have rarely been quantified. This research seeks to determine how much carbon from trees is invested in mutualists, where that carbon comes from, and whether that carbon investment changes when mutualisms are disrupted by human-driven changes in the animal fauna. The project will leverage a mutualism between trees and defensive ants in Kenya that is uniquely well-suited to field experiments and landscape modeling, in order to determine how mutualists contribute to the carbon cycle. This research will be communicated to K-12 students throughout the country using a web-based education platform designed to teach biology through active learning. Symbiotic ant colonies protect Acacia drepanolobium trees against herbivores in Kenyan savannas in exchange for housing and carbohydrates, but this mutualism is disrupted by the disappearance of megaherbivores and by an invasive ant species. Ant mutualists reduce tree growth and reproduction in the absence of herbivory, but this apparent carbon cost of ants has never been investigated relative to other critical tree carbon sinks, including allocation to roots and microbial mutualists. The proposed work will investigate carbon pools and dynamics in A. drepanolobium in the context of variation in the tree's biotic interactions to discover how mutualists affect tree carbon allocation. Using field experiments to manipulate ant mutualists, megaherbivores, and invasive ants, this research will evaluate how much trees invest in their mutualists relative to tree carbon supply, and how new carbon assimilates are partitioned between mutualists and tree sinks. Carbon allocation will be assessed by measuring biomass, nonstructural carbohydrates, secondary metabolites, mutualist rewards, and by pulse labeling of new assimilates. Because A. drepanolobium is a monodominant tree across large areas of East Africa, these measurements will be important to predicting changes in ecosystem-level carbon storage. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.