CO2-fertilisation enhances resilience to browsing in the recruitment phase of an encroaching savanna tree

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.

The relationship between herbivory and [CO2] is explored here using a widespread, strongly encroaching savanna tree, Vachellia karroo. Using greenhouse-grown seedlings under past—through to predicted future—[CO2] (180–1000 ppm) and field-grown seedlings under ambient [CO2], we assessed plant survival, growth, defence and palatability.

Increasing [CO2] improves the tolerance of greenhouse-grown seedlings to herbivory by stimulating growth and allowing a critical size threshold associated with survival to be reached earlier, thereby decreasing the probability of fatal herbivory during the vulnerable recruitment phase. Elevated [CO2] also decreases the time taken to reach a second size threshold linked to accelerated recovery of field-grown seedlings following herbivory. Seedling growth responses to increasing [CO2] are nonlinear, suggesting that historic growth and survival enhancements are smaller than those predicted for the future. Increasing [CO2] is associated with greater resistance to herbivores (more branched shoot architecture) but not leaf palatability (C:N ratio) or defence (leaf tannins and spine density).

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.