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

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.