Symbiotic Nitrogen Fixation and Aridity: Determining the Anatomical Differences of Nitrogen-Fixing Trees

Abstract

As climate change continues to threaten the natural world, biologists are concerned about how increased drought and subsequent nutrient deficiency might impact the nitrogen cycle, which influences primary productivity. The continued growth and survival of tree populations strengthen the global carbon sink by sequestering excess atmospheric carbon within their biomass. Diazotrophic bacteria (rhizobia) perform nitrogen fixation, the conversion of atmospheric N2 to a usable form for metabolic functions, and can exhibit symbiosis with species of the Fabaceae family of trees. Evidence shows a positive correlation between nitrogen-fixing tree abundance and aridity as fixers focus on plant survival over growth. Despite these phenomena, there has not been enough focus on the mechanisms that allow for this relationship. Utilizing field measurements of 6 wood traits across 325 tropical tree species in a Panamanian wet forest, this thesis aimed to determine the wood anatomical differences between nitrogen-fixing and non-fixing trees that can aid in the former’s increased water use efficiency during water stress. Nitrogen-fixers had significantly denser wood, a survival-related trait; however, the fixing species also had larger vessels in lower densities, which are traits that correlate to plant growth. These findings point to both the inaccuracy of using wood density to calculate plant growth as well as the growing influence that increased CO2 concentration, water limitation, and nitrogen availability has on hydraulic safety. This can be further incorporated into theoretical models to better predict the future of nitrogen-fixing tree abundance and the global nitrogen cycle in response to climate change.