Fire severity in endangered Araucaria-Nothofagus forests: Variable regeneration success dependent on regeneration strategy

Abstract

High-severity fires increasingly threaten the resilience of forests by disrupting regeneration mechanisms. Post-fire resilience mechanisms of temperate Araucaria-Nothofagus forests in southern South America, recently affected by severe fire, are poorly understood. We evaluated the effect of burn severity on stand structure and regeneration density (seedlings and resprouts, < 4 cm DBH) after fires. We surveyed 117 plots, seven years post-fire, in pure stands of A. araucana, Nothofagus obliqua, and Nothofagus pumilio. Araucaria araucana is fire resistant except during intense fires, whereas the two Nothofagus are easily killed by fire. Araucaria araucana and N. obliqua resprout following fire whereas N. pumilio does not resprout and instead depends on new seedling establishment to regenerate following fire. We quantified stem survival (stems with green foliage crown, > 4 cm DBH), shrub and grass cover, and topographic characteristics (e.g., slope, aspect). Using GLMs, we modeled seedlings and resprouts abundance as a function of these variables. We found that high-severity fires caused high stem mortality (98 %) and promoted shrubs and grass growth. Higher post-fire surviving live stems increases seedlings abundance for all species and A. araucana resprouts. High-severity fires do not threaten N. obliqua regeneration due to its resprouting ability. However, A. araucana and N. pumilio stands burned at high-severity may struggle to regenerate naturally, leading to replacement by shrublands or grasslands. Our results imply that successful recovery of Araucaria-Nothofagus forests in these areas require restoration protocols that integrate information on fire severity and topographic variability affecting sites suitable for post-fire regeneration.