Grasslands are an important ecosystem with potential to help stabilize food security and reduce greenhouse gas levels. As global temperatures rise, weather patterns are predicted to drastically change. The resulting increase in intensity, duration, and frequency of drought in important grassland areas will not only affect agricultural production, but also increase grassland susceptibility to fire, disease, and soil erosion. Thus, developing ways to sustainably promote grassland health and production is essential to increase food security and reduce environmental strain. Microbes in the phyllosphere, or aerial surface of plants, promote host fitness through phytohormone and nutrient production, increased stress tolerance, and protection against pathogens. However, what drives phyllosphere community assembly is not well understood. Because plant-health is reliant on microbial communities, understanding the ecological impacts of climate change on plant-microbe relationships is important to develop strategies to counteract the associated negative effects. The effects of drought on microbial community assembly on tropical and temperate grass hosts is described in this dissertation through four different research studies. We found that phyllosphere communities on the leaf surface are dynamic and are strongly selected for by their host species, that selection increased throughout the experiments driving communities to become more distinct from each other over time, and we found that microbial communities were significantly impacted by drought stress. Microbial communities responded to drought stress before measured plant traits showed a response to stress. Additionally, we found that changes in microbial community structure correlated with traits and stress response strategies of the host. Furthermore, we found that phyllosphere communities fix nitrogen and that this process is stable over time and under the stress condition of water reduction. Understanding the roles of phyllosphere bacteria in plant health and global biogeochemical cycles will allow us to leverage plant-microbe relationships to promote sustainable farming practices and reduce greenhouse gas emissions in the future.