Fire severity, feedback effects and resilience to alternative community states in forest assemblages

Abstract

Alternative stable state theory is often used to explain the occurrence of flammable vegetation adjacent to less flammable vegetation where fire regimes mediate the shift between states. In 2002 an extreme landscape scale fire burnt extensive areas of forests in eastern Australia, including rainforests that are rarely severely burnt. This unique event allowed us to test long-held assumptions that predict landscape scale change after major disturbance. We tested three assumptions for detecting alternative community states; (1) that the scale of the event was large enough to remove canopy dominants, (2) fire feedbacks, both positive and negative are present, and (3) shifts in the floristic composition of communities are detected. We also examined whether high severity fires resulted in a community shift from less flammable to more flammable vegetation (e.g. from rainforest to wet sclerophyll vegetation), by examining floristic composition of vegetation communities (rainforest, wet sclerophyll forest, and dry sclerophyll forest) when burnt at different fire severities (high and low). Conversely, we tested whether there was a state shift from flammable to less flammable vegetation communities in sclerophyll forests long unburnt. In addition, we determined if there was any evidence that antecedent fire regimes and fire severity influenced flammability feedbacks. Severe fire caused significant ongoing disruption to forest canopies and fire effects were still detectable some 7 years after fire. Whilst some pyrogenic environmental feedbacks were detected from historic fire regimes, we found no shifts in the floristic composition or pyrogenic traits of forests burned at high severity. Medium-term (30 year) fire exclusion did not result in the sclerophyll forest becoming more pyrophobic although some fire-cued species senesced in the absence of fire. Contrary to expectation, we found no evidence that the floristic composition of less flammable vegetation burnt at high severity became more similar to flammable vegetation burnt at low severity or that were unburnt. Conversely, with more than 30 years of fire exclusion there was no evidence that the sclerophyllous communities became more floristically similar to rainforest. We have shown that species assemblages in warm-temperate rainforest were resilient to a catastrophic fire event and propose that these forests are unlikely to represent alternative community states driven by fire alone.