| The southwestern U.S. is a hot spot of extreme climate change, where increasing temperatures and limited water has resulted in more frequent and severe heatwave, drought, and fire events, subsequently affecting species traits, distributions, interactions, and prevalence across the landscape. These climate extremes, combined with competition from exotic species, has resulted in the degradation of many native ecosystems, including large pine forests and riparian corridors throughout the west. In this dissertation, I examine the impact of climate change in the southwestern United States on the ecological and evolutionary processes influencing species occurrence and traits at the plant community, population, and genotype level. First, I analyze the natural climate change event of severe drought in the Northern Arizona cinderfields to understand the response of whole plant communities to the combined effects of drought-stress and overstory tree mortality. In this long-term observational study, I combine traditional taxonomic metrics with community phylogenetic measures. In the next two studies, I utilize an experimental model of climate change through the use of replicated common garden experiments, where populations of Fremont cottonwood were transplanted into environments ranging from 12°C hotter and 10°C colder compared to their origin sites. Using this design, I first explore the degree of genetic (G), environment (E), and GxE effects on two phenology traits, spring bud flush and fall bud set, within and across the three common gardens. The use of cloned genotypes allowed for the study of phenotypic plasticity for these traits, and the degree of genetic variation, and thus evolutionary potential, for plasticity. Lastly, I compared the among-population quantitative variation in phenology, growth, and leaf traits found in each common garden to their neutral genetic variation using QST-FST analysis to understand the role of climate-driven natural selection and the degree of local adaptation in shaping large phenotypic divergences across the species range. Together, these studies can help inform how important native species will respond to extreme climate events, and help shape management guidelines to ensure healthy and productive ecosystems are preserved. |
