Eco-evolutionary Responses to Climate Change in Relict Ecosystems: Global Patterns and Plant-Soil Dynamics

Understanding how ecosystems persist or disassemble under climate change requires frameworks that capture eco-evolutionary dynamics across multiple levels of biological organization. In this dissertation, I use the natural laboratory of isolated Sky Islands (SIs) and connected Mountain Chains (MCs) across the western U.S. to explore and compare how long-term climate exposure and geographic isolation shape genetic, phenotypic, and microbial responses in a foundational riparian tree species, Populus angustifolia. In Chapter I, I review the Sky Island literature from around the world and build on previous studies to highlight the SI-MC comparison within a global context. This review shows that SIs are evolutionary hotspots characterized by reduced gene flow, increased environmental filtering, and lineage divergence across taxa. In Chapter II, I show that SI populations exhibit phenotypic divergence in traits related to reproduction and productivity, shaped by both natural selection and genetic drift. In Chapter III, I demonstrate that SI soil microbial communities are compositionally distinct from MCs with more heat- and drought-tolerant taxa – supporting the idea of a core climate-change microbiome. Finally, in Chapter IV, I test how variation in microbial communities feeds back to plant performance using a reciprocal feedback experiment. I find that SI and MC populations differ not only in microbial conditioning capacity, but in how feedbacks influence traits tied to water use efficiency and growth. Together, these findings illustrate how relict ecosystems offer critical insights into the evolutionary and microbial mechanisms that drive ecosystem resilience. By integrating genetics, microbial ecology, and trait-based plant physiology, this work advances an eco-evolutionary framework for understanding plant persistence under climate change.