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Abstract

The widespread loss of biodiversity expected under climate change is ultimately driven by how individual populations respond to climate. Predicting whether populations will persist under climate change first requires understanding how climate directly and indirectly influences the demographic rates (survival, growth, and reproduction) underlying population growth. In my dissertation, I paired a rainfall manipulation experiment in a natural plant community with mesocosm experiments and population modeling (integral projection models, count-based population viability analysis, and competition models) to understand the climatic and demographic drivers of population change under climate change in Garry oak savannas. Each chapter addresses a potential indirect effect of climate variation on population dynamics: in my first two data chapters, I examined how seasonal climate and reproductive trade-offs jointly influence population growth in a long-lived perennial, Primula hendersonii. Specifically, Chapter 2 examines how costs of sexual reproduction vary across climate and changes in flowering effort. Chapter 3 incorporates trade-offs between clonal and sexual reproduction with interactions with conspecific neighbors. In Chapter 4, I examine direct climate impacts and indirect effects of competitive interactions on demographic rates and population growth for two native annual forbs, Plectritis congesta and Claytonia perfoliata, and an annual invasive grass, Bromus sterilis. While I found demographic rate-specific responses to seasonal climate, rainfall manipulation, and reproductive trade-offs, I primarily found negative effects of increased spring temperature for two of the native species, positive effects of increased summer temperature for the perennial species, and positive effects of increased winter temperature on an increasingly common invasive grass. Overall, only density-dependence had a large indirect effect, where predictions of population growth rates and effects of climate were dependent on the local density of conspecifics. I did not see evidence for costs of reproduction influencing variation in population growth rate across time, despite observing such costs across multiple experiments. Interspecific competition was not influenced by rainfall manipulation or seasonal climate variation over time, and no focal species were competitively dominant. Thus, direct climate effects paired with density-dependent responses may be most important for understanding population persistence under climate change.