The Impacts of Environmental Change on Plant and Microbial Communities: A Turf Transplant Experiment in the Colorado Rocky Mountains

Abstract

Climate change is rapidly altering temperature and moisture regimes, reshaping species distributions, interactions, and ecosystem processes worldwide. High elevation mountain ecosystems are especially vulnerable due to accelerated climate change and limited opportunities for upslope range expansion, particularly as low elevation competitors move in. Responses to climate change are driven by a complex interplay of direct climatic effects and indirect species interactions. As a result, predicting how communities and ecosystems will change remains challenging, especially across biological scales.To address this, I established a reciprocal whole-community turf transplant experiment in the Colorado Rocky Mountains, relocating intact plant and soil communities across an elevational gradient characterized predominantly by shifts in temperature and moisture. This design allowed me to assess how community composition, flowering phenology, functional traits, and ecosystem functioning respond to environmental change. By transplanting both up and down elevation I tested for asymmetries in ecological sensitivity to climate shifts. Turfs transplanted downhill and exposed to a warmer environment had rapid, directional shifts in community composition, which were stronger in soil bacteria/archaea and weaker in plants and soil fungi. In contrast, turfs transplanted uphill and exposed to a cooler environment had slower, weaker, and more variable changes in community composition, with limited turnover. These asymmetrical responses suggest that upslope colonization by low-elevation species is more likely than downslope persistence, increasing the vulnerability of high-elevation communities to novel competitors and altered interactions. Flowering phenology shifted rapidly with both uphill and downhill transplantation, suggestive of strong plastic responses to both warming and cooling that mirrored natural elevational patterns. Community-level flowering and production converged with the destination sites within five years, despite differences in species composition. However, phenological shifts were only weakly associated with reproductive output and unrelated to species dominance, suggesting that timing differences are consistent responses to climate, but are poor predictors of broader ecological outcomes. Plant functional traits also shifted rapidly under warmer conditions, changing from traits associated with fast growth and resource use to traits linked to resource conservation and survival, similar to those found in the lower elevation, warm-adapted communities. In contrast, cooling led to slower and more inconsistent changes. These shifts were driven primarily by plasticity and, to a lesser extent, changes in species dominance. Notably, carbon and water fluxes increased under warming despite more conservative trait profiles, which suggests context-dependent links between local conditions, traits, and function. In summary, transplantation to warmer sites drove rapid reassembly of high elevation communities and altered ecosystem functioning, while transplantation to cooler sites overall led to slower, less consistent responses. These asymmetries indicate a heightened vulnerability of high elevation ecosystems to climate change, and the potential for upslope establishment of lower elevation species. While some community responses shifted predictably, others were context-dependent, indicating the complexity of ecosystem responses to environmental change. This dissertation reveals that complexity by empirically demonstrating the range of ecological responses to environmental change and exploring the mechanisms driving divergence. Although many ecological models and management plans depend on generalizable, predictable patterns, this work shows that in subalpine ecosystems, responses are often nonlinear, variable, and shaped by local context. In a time of urgent environmental decision-making, understanding this ecological complexity is essential for building informed strategies for a changing climate.