Population-level response to past and future climate change across the native range of California poppy

Increased aridity and temperatures are expected worldwide in the coming decades. Under predicted levels of climate change, threatened organisms that cannot move must either adapt, respond plastically, or depend on non-climatic factors to avoid extinction. Predicting how certain species will respond to climate change can be complicated by population-level differences in fitness-related traits, population growth rates and demography. We investigated 20 populations of Eschscholzia californica (California poppy) distributed across a fourfold gradient in annual precipitation. In a greenhouse and field common garden, plants received precipitation treatments approximating the wettest and driest sites, crossed with either the presence and absence of a 1.5°C warming treatment or home soil inoculum. We documented strong clinal variation across populations, with southern arid populations exhibiting the most drought tolerance. Populations, especially from arid sites, had higher growth rates, seed set, biomass and rates of survival to flowering under warm and wet conditions We also found that plants grown in home soil inoculation produced 10% more biomass than when grown in common garden soil; however, the influence of soil was small relative to the 13-fold variation across populations in fitness responses to drought. Assisted gene flow sourced from arid population would be most likely to allow populations to persist in the face of increasing warming and more extreme precipitation trends. We also investigated evidence of evolution of traits, over a 60-year period of climate change by comparing data sets from a 1958-1960 common garden and field collection by Stanton A. Cook with a 2017 re-survey. This data set included 74 populations of E. californica distributed across a sixfold gradient in annual precipitation. Seed diameter decreased over time, consistent with expectations based on increasing aridity over the past 60 years. While percent germination and stamen number did not change significantly over time. Our results suggest that plant traits may be responding to a complex suite of climatic and non-climatic changes over the past 60 years