UC Davis

Plants vary in their phenological and morphological responses to abiotic and biotic conditions along environmental gradients like elevation and latitude, and this variation often reflects local adaptation. However, rapid climate change poses new challenges for many biological systems, threatening population viability, species persistence, and diversity. In this dissertation, I investigated phenological, morphological, and fitness responses to climate and herbivory across elevations in the native California wildflower, Streptanthus tortuosus. In Chapter 1, I explored relationships between snowmelt timing, reproductive phenology, and fitness along a steep elevation gradient at Lassen Volcanic National Park. My findings show that flowering phenology generally tracks snowmelt timing but suggest that populations may be approaching their limits in plastic phenological responses to snowmelt timing. Furthermore, fitness is generally maximized at earlier snowmelt times unless warm, wet conditions extend the growing season. In Chapter 2, I explored responses to herbivory and drought in 8 populations that vary in elevation, climate, and herbivory pressure at their home environment. I conducted a common garden experiment manipulating simulated herbivory and drought and measured phenological and fitness responses. I found that individuals from lower elevations display higher herbivory tolerance, especially when water availability is high. These patterns are consistent with local adaptation along an elevation gradient and are best explained by herbivory pressure and growing season length experienced by populations in the field. In Chapter 3, I studied variation in morphological traits along elevation and latitudinal gradients across the species range. Using phenotypic and genomic data collected and sequenced from 20 populations across climatic and elevation gradients grown in a common garden, I found that morphological traits vary in tandem along multiple parallel elevation gradients. Specifically, individuals at low elevations tend to be taller with thinner leaves and fewer branches, while plants at high elevations are shorter with thicker leaves and more branches. These results are concordant with a large body of literature describing a fast-slow continuum in morphological traits across elevations. Overall, my dissertation research demonstrates that S. tortuosus populations display variation in phenological, morphological, and fitness responses to precipitation and herbivory regimes across elevations and reveals patterns consistent with local adaptation. Under continued climate change, low elevation S. tortuosus populations may be better equipped to cope with herbivory and drought than high elevations, though high elevation populations may be robust to expected changes in the short term. The presence of extensive genetically based intraspecific trait variation may aid S. tortuosus in adaptive evolutionary response to climate change; alternatively, a lack of gene flow from warm-adapted low elevation populations may hinder evolutionary rescue as populations at high elevations experience warmer temperatures and increased drought stress. Accurate and robust predictions of responses to ongoing changes in environmental conditions will require further research investigating intraspecific phenological, morphological, and fitness responses in variable environments.