The Mediating Influence of Plant Communities on Alpine Soil Respiration in a Changing World

Soil respiration is a critical carbon flux from the terrestrial biosphere to the atmosphere. It is influenced by various ecological conditions including temperature, plant community composition, and insect herbivory. Global changes are altering these conditions through climate warming, the proliferation of invasive species, and promoting insect outbreaks. Understanding how these global changes influence soil respiration is essential for accurate modeling of the carbon cycle. However, the role of biotic communities as mediators of these global changes remains unclear. To address this uncertainty, I examine soil respiration dynamics through field experiments, an observational study, and a global meta-analysis. My objectives were to: 1) determine the direct and interactive influence of warming and plant community change on peak soil respiration rates across diverse alpine communities; 2) examine the influence of warming and plant community change on the effect of warming on soil respiration; 3) determine if an invasive plant modifies the effect of aboveground insect herbivore effects on soil respiration; and 4) explore potential plant-mediated mechanisms by which insect herbivores affect soil respiration.

I show that plant community composition significantly modifies how soil respiration responds to warming during the height of the growing season. Specifically, I found that soil respiration in sites with high woody plant cover exhibited a higher relative sensitivity to warming across widely distributed alpine sites. I also found evidence that plant community composition modifies the temperature sensitivity of soil respiration among plots in the Colorado Rocky Mountains, United States of America. However, unlike the previous analysis, the most influential plant functional group in this study was graminoids. Across both of these studies, my results reveal plant functional groups were highly influential mediators of the relationship between temperature and alpine soil respiration on both local and global scales. I also examined how increasing insect herbivory affects soil respiration and how an invasive plant species modifies this relationship in a New Zealand alpine grassland. Insect herbivore abundance was linked to increased soil respiration, but I found no significant difference between native and invasive host plants. This investigation also revealed that soil microbial communities, particularly bacterial abundance, mediate the effects of insect herbivores on soil respiration. Results from my global meta-analysis suggest that plants regulate these mediation effects through multiple mechanisms including modifications to soil communities, plant litter composition, and plant resource allocation. Among these, belowground resource allocation showed a particularly strong relationship to herbivory. This plant-mediated response may be the dominant process by which insect herbivores alter soil microbial communities, and thereby influence soil respiration. These findings further highlight the crucial role of plants in mediating the impacts of global changes on soil respiration. By integrating studies with local and global perspectives into a single narrative, I provide substantial evidence that plant communities regulate how global changes will influence soil respiration dynamics. I show that consideration of these communities is essential for refining ecosystem carbon models, particularly as climate change and plant invasions rapidly alter plant functional group representation among alpine ecosystems. The insights in this thesis also provide direction for further studies into the mechanisms underpinning plant-microbial interactions and their implications for soil respiration. Ultimately, this knowledge will improve ecosystem carbon models and our understanding of carbon dynamics in an era of rapid ecological change.