Young, M. L. 2025. From plants to microbes: understanding community responses to experimental climate change. Dissertation, Michigan State University, East Lansing, MI.

Climate change is reshaping ecosystems worldwide, altering both abiotic conditions and biotic interactions in ways that influence the structure and function of plant and microbial communities. Yet our understanding of these responses is mostly limited to the direct and individual effects of climate stressors, leaving a gap in our understanding of how complex interactions among multiple abiotic and biotic factors shape ecological responses. Furthermore, soil microbial communities, which play essential roles in nutrient cycling, plant health, and ecosystem resilience, remain relatively understudied in the context of climate change. This dissertation explores the ecological impacts of climate change and biotic interactions on early successional plant and soil microbial communities in Michigan, USA, through a series of complementary experiments that integrate field manipulations and a greenhouse study. By combining multi-year in situ treatments with a controlled experiment, this work aims to uncover responses across above- and belowground communities, providing insight into the mechanisms driving ecosystem change under climate stress.

In Chapter 1, I apply long-term in situ warming and herbivory reduction treatments to two distinct early successional communities, measuring a range of plant traits and community-level properties annually over seven years to assess differential responses across environments. I demonstrate that warming and herbivory can have strong direct effects on plant communities, but that their interactive effects are limited in these early successional systems. These treatments had stronger effects in some years than others, highlighting the need for multi-year experiments. This work ultimately underscores the importance of incorporating biotic factors into climate change experiments to predict vegetation dynamics under future climates. Chapter 2 builds on these findings through a greenhouse experiment using field-conditioned soils from the in situ experiment in Chapter 1 to examine the microbe-mediated indirect effects of warming on plant growth and traits. I detected limited effects of these microbe-mediated indirect effects but argue that a lack of detection does not always mean lack of response, and considering these plant-soil interactions should be prioritized in future studies focusing on plant and soil microbial communities. In Chapter 3, we extend the field-based approach by applying in situ warming and drought treatments to an early successional community over multiple years, focusing on soil bacterial and fungal community responses to evaluate initial responses and how prolonged exposure to multiple climate stressors influences soil microbial dynamics. This work shows that soil microbial communities are shifting their composition and diversity in response to climate warming and drought as individual stressors. Importantly, the combination of both warming and drought can interactively affect the soil microbial community in ways that are different than when climate stressors are considered independently. Notably, these responses become more pronounced after multiple years of exposure to climate treatments, highlighting the importance of long-term observation.

This dissertation demonstrates how a range of plant traits and community-level characteristics respond to climate warming, while also considering the role of plant-soil interactions. It also reveals how soil bacterial and fungal communities react to multiple climate stressors, including both warming and drought. Together, these findings underscore the intricate and interconnected nature of above- and belowground ecological responses to environmental change. Crucially, this research highlights the importance of temporal scale and ecological context, emphasizing that long-term data is essential for understanding and predicting ecosystem dynamics under a changing climate.

Associated Treatment Areas:

• MCSE Main Cropping Systems Experiment

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