Climate warming and altered precipitation patterns accelerate change in tree establishment in post oak savanna grasslands

Background/Question/Methods   Climate change is predicted to alter tree establishment in savannas as tree and grass species differentially respond to warming and altered rainfall patterns. Yet, tree-grass interactions and linkages to tree establishment in response to climate change drivers remain largely unknown, but key to predicting vegetation change in a warming world. We investigated the growth and establishment of Eastern redcedar (Juniperus virginiana) and post oak (Quercus stellata) in competition with little bluestem (Schizachyrium scoparium), a C4 grass, all dominant species of post oak savanna. Using rainout shelters and infrared heaters, species monocultures and tree-grass mixtures were subjected to climate warming (ambient, +1.5 °C) and two precipitation treatments (long-term mean, summer redistributed) in factorial combination. The redistribution treatment reduced the amount of May-September rainfall by 40% compared to the long-term mean, lowering soil water content and amplifying summer drought.

Results/Conclusions   Throughout the 6-year period of seedling establishment, the tree species differed markedly in growth responses to warming and altered precipitation and effects were largely independent, and thus additive. Across treatment combinations, J. virginiana exhibited positive responses of growth to warming. In contrast, warming reduced growth of Q. stellata seedlings. In addition, Q. stellata growth was reduced in the redistributed compared to long-term mean rainfall treatment; whereas J. virginiana growth was minimally affected. Compared to Q. stellata, J. virginiana was a superior competitor with the grass, and owing to its faster growth rates, overtopped the grass canopy after 3 years. Tree-grass interactions were largely competitive and mediated by reduced soil water availability and species differences in drought resistance and response to rainfall events. In Q. stellata, the recovery of leaf gas exchange rates following intermittent rainfall events declined with summer drought and warming. In J. virginiana, recovery of leaf photosynthesis was maintained as drought progressed and enhanced by warming. These findings suggest that climate warming and altered precipitation will accelerate encroachment of J. virginiana in savanna grasslands. Coupled with reduced establishment of Q. stellata, these changes will likely result in altered tree composition and function of savanna woodlands in the future.