Fire and herbivory are not substitutable: evidence from regrowth patterns and changes in physical and chemical defences in A cacia seedlings

Questions What are the relative effects of burning and herbivory on regrowth and physical and chemical defences in A cacia seedlings? Are effects of herbivory consistent with predictions of the resource availability hypothesis? Do plants growing with adequate resources regrow lost material while those with low resource availability defend themselves to minimize effects of herbivory? Location A rboretum, P ietermaritzburg, S outh A frica. Methods We performed a two‐factor, completely randomized experiment on seven indigenous A frican A cacia ( A cacia erioloba, A . haematoxylon, A . karroo, A . nilotica, A . robusta, A . sieberiana and A . xanthophloea ) with simulated burning using a blowtorch and simulated herbivory by cutting at the first node. We measured seedling regrowth, thorn length and number, and tannin content. Results New stems were produced for all species. In all seven species, the control (no fire, no herbivory) produced longer stems than other treatments. Simulated herbivory increased thorn abundance among A cacia species, while burning often led to decreased overall thorn length. Tannin analysis revealed positive and negative responses of individual species to burning and herbivory. Conclusions We demonstrate that A cacia seedlings are tolerant of disturbance events such as herbivory and burning but these effects cannot be assumed to be substitutable. Importantly, there was a positive response to simulated herbivory and a negative response to fire in most species. The mechanisms behind these changes appear to be mediated by induction of defence responses (resulting in a positive response to herbivory) and a cost for root storage (resulting in reduced defences after fire). Only A . robusta displayed regrowth and defence responses consistent with predictions of the resource availability hypothesis. In contrast, the two arid‐adapted species invested heavily in regrowth but not in defence, while four of the five mesic species invested in increased defence.