Climate models predict that altered rainfall distribution coupled with warming may be an important component of climate change. Savannas may be especially responsive to these global change drivers as drought and competition are expected to impact tree establishment. In southern oak savanna in Texas, post oak (Quercus stellata) and juniper (Juniperus virginiana) are dominant C3 trees and little bluestem (Schizachyrium scoparium) the dominant C4 perennial grass. We warmed monoculture and tree-grass plots (+1.5 °C) and manipulated rainfall events to intensify summer drought and augment cool season rainfall compared to the long-term mean. Through three years of treatment, leaf-level photosynthesis and growth of the two tree species was reduced in response to rainfall redistribution and increased water stress. Only Juniper showed a positive warming response in height and diameter and leaf photosynthesis, mostly during the summer. Rainfall redistribution intensified tree-grass interactions during establishment. In contrast, warming resulted in reduced tree-grass competition, particularly during summer months. Tree-grass interactions switched from facilitation of aboveground tree growth in the wetter spring and autumn to competition in the drier summer, resulting in increased water stress and reduced aboveground tree growth. Increases in root length density and specific root length may play a role in enhancing the competitive ability of grass during tree establishment. However, grass competition appeared to decline with increased tree size by the third year of study. Reduced aboveground growth in the wet spring and dry summers coupled with a deciduous leaf habit, suggest that the early establishment phase of post oak may be suppressed to a greater extent than juniper in response to a shift in rainfall distribution. These trends suggest that juniper will increase in dominance and invasiveness in savanna grasslands with both climate warming and altered rainfall distribution.
