BELOWGROUND PLANT STRATEGIES AND PLANT-NUTRIENT FEEDBACKS: ECOSYSTEM PROPERTIES, BIOME PATTERNS, AND GLOBAL CHANGE

Abstract

Our ability to resolve belowground ecosystem processes has been limited by a lack of empirical data, and perhaps more critically, a lack of fundamental theoretical understanding. In this dissertation, I combine empirical and theoretical approaches to resolve ecosystem- and global-scale consequences of belowground plant strategies and plant-soil feedbacks.

Three major dimensions of variation make up the immense diversity of root strategies in nature: symbiotic diversity, spatial diversity, and anatomical diversity (Chapter 1). These three components further interact in complex ways that makes it important to address each of them separately (Chapter 2, 3, and 4), in order to form an ultimate synthetic picture of root strategies.

Chapter 2 presents a novel, first-principle modeling approach to resolve the complex organization of plant-symbiont strategies and nutrient cycles at the scale of biomes. This model, despite its simple structure, sheds light on the distribution of belowground symbioses worldwide, the sequence and timing of plant succession, the bistability of ecto- vs. arbuscular mycorrhizae in temperate and tropical forests, and major differences in the land carbon and nutrient cycles across biomes. In Chapter 3, I expand the approach developed in Chapter 2 to explicitly consider the vertical distribution of plant rooting systems. This chapter examines how plant-symbiont relationships and competition mediate the ways plants respond to rising atmospheric CO2. Importantly, this chapter identifies contrasting CO2 responses between arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) plants: AM plants proportionally invest more roots in deeper soils, while EM plants concentrate more roots to the surface soils. This difference in rooting responses ultimately translates into different projections of soil carbon storage.

In Chapter 4, I investigate the mechanisms that maintain the biome boundaries between the South African Fynbos and forests, using an in-situ nutrient manipulation/ seedling transplantation experiment, a quantitative comparison of belowground root traits, and a comprehensive data compilation of paired forest-Fynbos systems. The results reveal that the interaction between nutrients and fire is central to the maintenance of biome boundaries over both ecological and evolutionary timescales. The conclusion chapter synthesizes findings from previous chapters and gives a brief account of future research directions.