Evolution and Ecology of Temporal Variability in Annual Plants

Abstract

The study of life histories focuses on how evolution molds the life cycles of organisms and on the consequences of those life cycles for the demography and ecology of organisms. Plants exhibit a fantastic array of life history strategies for coping with environmental variability, including delayed germination that creates long-lived soil seed banks and years of vegetative growth followed by a single bout of flowering. In my dissertation, I revisit classic questions about the evolution and ecology of life histories in annual plants under temporal variability. Why do seed banks evolve? How does variability affect population dynamics? Does plant development alter life history strategies? I approach these questions with a variety of methods, from analyzing empirical data to simulations and theory. In Chapter 1, I develop statistical models to estimate seed mortality and germination from field experiments that ecologists regularly use to study the soil seed bank. In the next two chapters, I apply these models to empirical data to ask questions about the evolution of delayed germination and the consequences of temporal variability in demography. In Chapter 2, I test whether bet hedging explains patterns of germination in populations of the winter annual plant Clarkia xantiana ssp. xantiana. Delayed germination is predicted to act as a bet hedging trait via a trade-off between arithmetic and geometric mean fitness. Using 15 years of observations for per-capita reproductive success and estimates of seed survival and germination from a field experiment, I find that some, but not all, populations exhibit the expected trade-off. Across populations, observed germination rates are also lower than expected based on a density-independent bet hedging model. I do not find empirical support for the predictions of bet hedging theory, which suggests that understanding the evolution of delayed germination in C. xantiana ssp. xantiana will likely involve addressing factors such as density-dependence and plasticity in germination. In Chapter 3, I ask how temporal variability in demography shapes stochastic population dynamics across the range of C. xantiana ssp. xantiana. The `abundant center' hypothesis for geographic range limits predicts that vital rates and population growth rates will vary more through time in populations at the range edge than at the range center. I analyze observations from field surveys and experiments, and show that the variability of vital rates shows individualistic, vital-rate specific geographic patterns, but that variability in population growth rate is greatest at the range edge. I also conduct perturbation analyses that suggest variability has a bigger effect on population growth rate at range edges. In this chapter, I describe geographic patterns of variability and elucidate the processes that generate those patterns—closing this loop is central to understanding how life history mediates the effect of temporal variability on populations. In Chapter 4, I study the influence of plant development on the evolution of flowering time in variable environments. In plants, flowering is a critical event in the life cycle in which resources are re-allocated from growth to reproduction and meristems switch from vegetative to floral fates. I develop life history models that explicitly represent resource and meristem dynamics, and analyze the models with methods from optimal control theory. I show that both resources and meristems shape optimal flowering strategies when plants experience variability in season length. My dissertation contributes to the study of plant life histories and expands our empirical and theoretical understanding of the role of seed banks and plant development.