Raubenheimer, S.L. and Ripley, B.S. 2022. CO2-stimulation of savanna tree seedling growth depends on interactions with local drivers. Journal of Ecology. 

This publication was awarded the Journal of Ecology Editor's Choice Award.

We explore how simulated drought, herbivory and grass competition impact the growth & survival of Vachellia karroo, one of the most prominent woody encroachers in southern African savanna. We demonstrate that elevated CO2-induced C3 stimulation in encroaching savanna species such as V. karroo will be inconsistent across time and space, translating into spatial and temporal variation in seedling responses to eCO2 and CO2-driven woody encroachment, explaining some of the variability observed in woody encroachment across geographical regions and resource and herbivore gradients.

Raubenheimer, S.L., Simpson, K.J., Carkeek, R., Ripley, B.S. 2022. Could CO2-induced changes to C4 grass flammability aggravate savanna woody encroachment? African Journal of Range and Forage Science: Special Edition on Fire. 

Fire plays an important role in maintaining the savanna tree-grass balance by limiting the recruitment of heat-sensitive tree seedlings. However, fire behaviour may change under increasing CO2 concentrations, due to altered flammability of the grassy layer. We present a novel look into how plant physiological responses to rising CO2 concentrations can affect plant flammanbilty and show the effect that these changes have on fire simulations. This manuscript is the first of its kind and highlights the need to incorporate plant physiological responses into fire modeling efforts. 

Ripley, B.S., Raubenheimer, S.L., Perumal, L., Anderson, M., Mostert, E., Kgope, B.S., Midgley, G.F., Simpson, K.J. 2022. CO2‐fertilisation enhances resilience to browsing in the recruitment phase of an encroaching savanna tree. Functional Ecology.

CO2-fertilisation is implicated in the widespread and significant woody encroachment of savannas due to CO2-stimulated increases in below-ground reserves that enhance sapling regrowth after fire. However, the effect of CO2 concentration ([CO2]) on tree responses to the other major disturbance in savannas, herbivory, is poorly understood. Herbivory responses cannot be predicted from fire responses, as herbivore effects occur earlier during establishment and are moderated by plant palatability and defence rather than below-ground carbon accumulation. Increasing V. karroo densities already constitute a major land management problem in southern African savannas. However, encroachment by this species, and likely other savanna tree species, may be greatly exacerbated under future [CO2], as tolerance to herbivory at the recruitment stage is further enhanced.