The influence of biotic interactions on carbon allocation in a dominant savanna ant-plant

Abstract

Terrestrial plants are the main source of energy available to most land-dwelling organisms. How plants partition fixed carbon (C) can have impacts ranging from individuals to entire ecosystems. This dissertation explores how biotic interactions influence plant C allocation. Specifically, it addresses how plant competition, herbivory, and mutualism influence C allocation across life stages of a dominant savanna ant-plant, Acacia (Vachellia) drepanolobium.Chapter One focuses on how aboveground plant competition imposes a C limitation in saplings, and how this impacts C allocation to above- and belowground biomass, storage, and defense. We simulated grass shade in a screenhouse and destructively harvested saplings that experienced shade or full sun. We found that shaded saplings increased relative C allocation to belowground biomass and storage and decreased relative allocation to aboveground biomass. Our results are consistent with possible adaptive allocation strategies that buffer impacts of fire and herbivory, highlighting the essential role of belowground reserve for future regrowth. Chapter Two investigates the viability of an isotope tracer to study C allocation in a tropical ant-plant in situ. Isotope tracer methods may not be conducive to tropical trees because of methodological constraints: heat and humidity inside a labeling chamber may inhibit the uptake of an isotope tracer. In this study, we piloted a 13C pulse-labeling method to test if trees could withstand labeling conditions and assimilate 13CO2. Trees successfully took up 13CO2 and 13C was detected in all sampled C sinks, including ant mutualists. Our results demonstrate that isotope tracer studies can be used to track C allocation in mature tropical trees in situ. Chapter Three applies 13C labeling methods established in Chapter Two to examine how changes to the biotic environment of a tree influences C allocation patterns. The tropical ant-plant A. drepanolobium is facing two major changes to its biotic environment: megaherbivore decline and the loss of its ant defensive mutualists due to invasion by the ant Pheidole megacephala. Using a factorial combination of invasion and herbivory, we found that trees invaded by P. megacephala rely on older C for stem metabolism and/or growth, and that uninvaded trees protected from herbivores allocate less new photosynthate to ant rewards (nectar) than trees with herbivores present. Our results emphasize the importance of biotic interactions in determining tree C allocation. In summary, this dissertation builds on our understanding of how biotic interactions influence tree C allocation. Notably, it incorporates an understudied biotic interaction, mutualism, into tree C dynamics. It examines biotic interactions across ontogeny and shows how plant competition, mutualism, and herbivory can shape C allocation patterns in a dominant savanna ant-plant.