Thresholds and alternative states in a neotropical dry forest in response to fire severity.

Neotropical xerophytic forest ecosystems evolved with fires that shaped their resilience to disturbance events. However, it is unknown whether forest resilience to fires persists under a new fire regime influenced by anthropogenic disturbance and climate change. We asked if there is evidence for a fire severity threshold causing an abrupt transition from a forest to an alternative shrub thicket state in the presence of typical post-fire management. We studied a heterogeneous wildfire event to assess medium-term effects (11 years) of varying fire severity in a xerophytic Caldén forest in central Argentina. We conducted vegetation surveys in patches that were exposed to low (LFS), medium (MFS), and high (HFS) fire severities, but had similar pre-fire woody canopy cover. Satellite images were used to quantify fire severity using a delta Normalized Burning Ratio (dNBR) and to map pre-fire canopy cover. Post-fire total woody canopy cover was higher in low and medium than high severity patches, but the understory woody component was highest in HFS patches. The density of woody plants was over three times higher under HFS than MFS and LFS due to the contribution of small woody plants to the total density. Unlike LFS and MFS patches, the small plants in HFS patches were persistent, multi-stemmed shrubs that resulted from the resprouting of top-killed Prosopis caldenia trees, and more importantly, from young shrubs that probably established after the wildfire. Our results suggest that the Caldén forest is resilient to fires of low to moderate severities but not to high severity fires. Fire severities with dNBR values > ~600 triggered an abrupt transition to a shrub thicket state. Post-fire grazing and controlled fire treatments likely contributed to shrub dominance after high-severity wildfire. Forest to shrub thicket transitions enable recurring high severity fire events. We propose that repeated fires combined with grazing can trap the system in a shrub thicket state. Further studies are needed to evaluate if the relationships between fire and vegetation structure examined in this case study represent general mechanisms of irreversible state changes across the Caldenal forest region and if analogous threshold relationships exist in other fire-prone woodland ecosystems. This article is protected by copyright. All rights reserved.