ECOLOGY AND EVOLUTION OF PLANT PHYSIOLOGICAL STRATEGIES USING LEAF STABLE ISOTOPES
Goud, Ellie M
Determining the mechanisms that shape biodiversity is a central question in ecology and evolutionary biology. Presumably, environmental challenges exert important influences on organismal growth and survival, leading to diverse ecological strategies. One such challenge that terrestrial plants face is how to gain carbon for photosynthesis without losing too much water. This challenge arises from the fact that as carbon dioxide (CO2) diffuses into leaves, water vapor simultaneously diffuses out, resulting in a tradeoff between leaf-level carbon gain and water loss. The need to maximize carbon gain while minimizing water loss has long been presumed to drive the evolution of diverse strategies by which plants adapt to variable environments. In this dissertation, I explore the macroevolution and ecological consequences of variation in plant physiological strategies, with a focus on leaf-level carbon and water exchange. In the first chapter, I introduce the concept of ‘integrated metabolic strategy’ (IMS) to describe the ratio between leaf carbon isotope composition (13C) and oxygen isotope composition above source water (18O). IMS is a novel way of representing leaf carbon-water tradeoffs that are integrated over the lifespan of a leaf, thus avoiding problematic comparisons between instantaneous point measurements of metabolic fluctuations. I tested how metabolic strategies evolve among closely related yet ecologically diverse milkweed species, and subsequently addressed phenotypic plasticity in response to water availability in species with divergent strategies. In the second chapter, I asked whether metabolic strategies vary among co-occurring species in a successional old field community in Ithaca, NY. I found considerable variation in 13C, 18O, and IMS values among 18 perennial angiosperm species. Changes in species abundance over two years suggested that temporal variation in water availability (i.e., inter-annual precipitation) may be an important mechanism structuring functional diversity and species composition in this community. In the third chapter, I formally tested the hypothesis that inter-annual variation in growing season precipitation promotes metabolic diversity among old field Asteraceae species. Through the use of rainout shelters, I subjected the community to five water treatments to simulate the range of growing season water availabilities based on the long-term average in the region. Species differentially responded to variation in growing season water availability and, importantly, how they responded could be explained by differences in metabolism. Water-conservative species grew best in the dry treatments and had their minimal growth in wet treatments. Carbon-acquisitive species displayed the opposite pattern, with maximal growth in wet treatments and steep declines in dry treatments. Metabolic differences among co-occurring species may help explain temporal variation in growth, and could provide an underlying physiological mechanism for long-term dynamics that promote biodiversity. In the fourth chapter, I assessed the relative roles of phylogenetic history and environment on patterns of leaf δ13C and nitrogen stable isotope ratios (δ15N) as integrators of physiological processes in a diverse group of Ericaceae species native to North America. The signal of phylogeny was generally stronger than that of the local environment, suggesting that close relatives have similar physiological strategies across this plant family. Examining ecological and evolutionary patterns of leaf stable isotopes across plant clades and within communities illustrates a previously under-appreciated role of metabolism for species distributions and community diversity.
community ecology; comparative phylogenetics; physiological ecology; plant biology; plant traits; stable isotopes
Sparks, Jed P.
Agrawal, Anurag; Geber, Monica A.
Ecology and Evolutionary Biology
Ph. D., Ecology and Evolutionary Biology
Doctor of Philosophy
dissertation or thesis