Increased fire severity triggers positive feedbacks of greater vegetation flammability and favors plant community-type conversions

Abstract

Questions: Increased wildfire activity is resulting in plant community-type conversions worldwide. In some regions, fire-sensitive forests are being replaced by flammable fire-resilient communities, increasing the likelihood of reburning due to positive fire feedbacks. Here we evaluated whether fire severity affects post-fire plant community flammability attributes that lead to community-type conversions and changes in the likelihood of reburning. Specifically, we assessed how fire severity, the dominant pre-fire vegetation, and distance to unburned remnant forest may shape post-fire community properties and flammability trajectories in northwestern Patagonia. Location: Northwestern Patagonia, Argentina. Methods: We repeated sampling of Nothofagus pumilio, Nothofagus dombeyi, and Austrocedrus chilensis forests and native shrublands (composed of multiple shrub species) two and 18 years after fires that burned at low- to very high-severity levels, and measured community structure and flammability attributes. Results: Eighteen years after fire, forests that burned at moderate to very high severity were unable to recover and were replaced by more flammable shrublands and grasslands. Following low-severity fire, fine-fuel density was lower and forest recovery was enhanced by greater survival of remnant seed trees. Burnt shrublands increased in abundance across all severity classes but attained highest fine-fuel production after moderate- to very high-severity fire. Conclusions: Low fire severity, by enabling regeneration of forests that are less flammable than shrublands, diminishes the likelihood of reburning, thus establishing a negative feedback favorable to forest perpetuation. Conversely, moderate to very high fire severity leads to a positive feedback by promoting conversion to shrublands and greater fine-fuel accumulation. This increases the likelihood of reburning, reinforcing the persistence of pyrophytic communities and favoring landscape-scale loss of fire-sensitive forests. This fire severity-mediated positive feedback may in many regions worldwide further amplify warming-related wildfire activity increases, posing serious threats to the persistence of fire-sensitive ecosystems.