Synergistic interactions between fire and browsing drive plant diversity in a forest understorey

Abstract

Questions: Does browsing by large herbivores affect forest understorey diversity by modifying assemblage dominance? Does fire interact with browsing to affect forest understorey diversity? Does this interaction occur via a numerically mediated or functionally moderated pathway? Location: Booderee National Park, Jervis Bay Territory, south-eastern Australia. Methods: We tested the interactive effects of fire and browsing by native herbivores on understorey plant diversity using a randomized blocked experiment in an open eucalypt forest. We monitored the percentage cover of every vascular plant species in 24 sites over four experimental blocks. We applied a different treatment to each of the six sites in the four blocks. Treatments were a factorial combination of three levels of herbivory treatment (open, partial exclosure, full exclosure) and two levels of prescribed fire treatment (burned, unburned). Results: Browsing increased plant community dominance and reduced evenness and diversity, but only in burned sites. Heavy browsing following fire created an understorey dominated by an unpalatable, fire-resistant fern species (bracken, Pteridium esculentum). This fire-browsing interaction was driven by both numerically mediated and functionally moderated pathways: Fire both increased local browsing intensity, and amplified the per-unit effect of herbivores on the plant community. Conclusions: The altered competitive environment after fire, combined with heavy post-fire browsing created a depauperate understorey, dominated by bracken. The ability of bracken to suppress the establishment of other plants means that, once established, this fern-dominated understorey may be difficult to reverse. Our results demonstrate the key role of fire-browsing interactions in forest vegetation dynamics and highlight the importance of integrating large herbivore management with fire planning in forest ecosystems.