Date of Award
Fall 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Ecology and Evolutionary Biology
First Advisor
Staver, Carla
Abstract
Ecosystem productivity is often constrained by the availability of essential macronutrients. Ecosystem processes that are vital to nutrient acquisition like nutrient cycling and microbial activity are expected to change under global warming, but prediction accuracy in the face of global change is challenging, as prediction requires a thorough mechanistic understanding of how plants acquire nutrients from their environments. These strategies include exploration by roots for resources like nutrients or water, releasing enzymes that decompose organic matter into plant available forms, or forming symbioses with soil bacteria or mycorrhizal fungi, which can have downstream impacts on ecosystem nutrient budgets, nutrient cycling, vegetation dynamics, and ecosystem resilience. In southern African savannas, we lack a basic understanding of how nutrients impact savanna vegetation growth, interspecific competition between trees and grasses, and investment in microbial mutualisms. Understanding these dynamics is especially challenging in savannas because precipitation variability, frequent fire, and herbivory strongly impact vegetation structure. In my dissertation, I have examined how savanna trees respond to changes in soil nutrients, drought, soil microbial communities, fire frequency, and grass competition. The three chapters of my dissertation are as follows: Chapter 2: Nitrogen and phosphorus availability alters tree-grass competition intensity in savannasNitrogen and phosphorus are known to influence savanna vegetation dynamics, along with rainfall, fire, and herbivory, but competition for nitrogen and phosphorus by the two distinct plant functional types in savannas (trees and grasses) is poorly studied. I examined tree-grass competition in response to varying levels of nitrogen and phosphorus in a fully factorial experimental design. Grasses utilized aggressive belowground nutrient foraging strategies, consistently outcompeting tree saplings for nutrients. In low-nutrient environments, tree saplings were nutrient limited, but in high nutrient environments, competition intensity increased, making grass competition more intense for trees when nutrients were more available. Understanding what these results mean for ecosystem responses to nutrient availability is not trivial but indicates that soil nutrients likely affect tree and grass growth and competition in savannas, suggesting future changes in nutrient cycling may have important effects on savanna vegetation. Chapter 3: Native soil microbes buffer savanna trees against resource limitation but are sensitive to droughtBelowground microbial processes can mediate resource acquisition. In savannas, plants experience pathogenic microbial effects and, conversely, can employ a range of symbioses with mycorrhizae and nitrogen fixers. In this chapter, we evaluated if native soil microbes provided positive or negative effects to trees by growing trees with or without soil inoculant, with varying levels of nitrogen and water in a fully-factorial experimental design. Soil microbes helped trees buffer against resource limitation, increasing mass gain across all resource treatments. Nodule biomass and mycorrhizal colonization increased in low nitrogen treatments, but soil inoculant was most beneficial in non-droughted water conditions, indicating that beneficial microbial symbionts may be negatively affected in droughted conditions. Chapter 4: Non-nodulating nitrogen fixation in Colophospermum mopane, a common southern African Fabaceae treeCertain plants (like legumes and actinorhizal species) can obtain nitrogen through nitrogen fixation, a symbiosis in which plants exchange carbohydrates for bacterially fixed atmospheric nitrogen. In recent years, diverse forms of nitrogen fixation have been discovered, suggesting that nitrogen fixation is more varied and widespread than previously assumed. However, the possibility of whether a widespread non-nodulating Fabaceae species of economic importance, Colophospermum mopane, is capable of nitrogen fixation has not been explored. Using isotopic tracer methods to measure nitrogen fixation rates and microbial community DNA sequencing in Kruger National Park, results show that C. mopane is capable of nitrogen fixation and that fixation rates depended on ecological variables, with higher fixation on sandy soils, with higher grass biomass, at intermediate fire frequencies, and at smaller tree heights. C. mopane associated with nitrogen-fixing Rhizobia and Frankia bacteria whose relative abundances were highly correlated with nitrogen fixation rates across ecological variables. Our results suggest that C. mopane utilizes nitrogen fixation in nutrient limited and early life stage contexts and introduces roughly 1.8 – 9.9 kg N ha-1 year-1, with a mean value of 5.8 kg N ha-1 year-1, which could have important implications for savanna nitrogen budgets. Overall, this dissertation demonstrates that nutrients and soil microbes are important determinants of savanna tree growth, suggesting that ongoing changes in nutrient cycling and microbial activity could substantially impact savanna vegetation. Soil nutrient availability and microbial facilitation can encourage biomass accumulation but are strongly regulated by local ecological conditions so that variables like drought, soil texture, grass competition, fire frequency, and tree size will influence their impact. Although savanna vegetation drivers are intertwined, a clear pattern emerges from the following chapters that suggests savanna trees depend on their microbial symbionts to provide benefits in nutrient-limited contexts. While these chapters provide empirical evidence and insight, soil nutrients and soil microbes remain understudied determinants of savanna systems.
Recommended Citation
Biro, Arielle, "Nutrient Acquisition and Plant-Microbe Interactions in Savanna Trees" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1111.
https://elischolar.library.yale.edu/gsas_dissertations/1111
