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

1. CO2-fertilisation is implicated in the widespread and significant woody
encroachment of savannas due to CO2-stimulated increases in belowground
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. 2. 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. 3. 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 non-linear, 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). 4.
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.