Fire effects on diversity patterns of the understory communities of Araucaria-Nothofagus forests

Short-interval and high-severity fires combined are emerging as a catalyst of major reorganization of understory plant communities. In temperate forests of south-central Chile, concern exists about the resilience of threatened Araucaria-Nothofagus forests, including its understory community following extensive and severe fires. In this study we use an array of diversity indices and functional traits as proxies of community resilience. We ask if the species and communities are affected by wildfires, and how these responses are mediated by burn severity and frequency, and other biophysical variables. First, we explore the hypothesis that fire is the major driver of community changes, and that burn severity is the main factor that shifts compositional attributes of communities. Secondly, we hypothesize that a reburn will lead to a greater shift than a single burn in understory compositional attributes, where resprouting species replace obligate seeders, reducing local diversity. We established 120 field plots across a burn severity gradient in two study sites: one affected by a single (burned 2015), and the second by two fires (burned 2002 and 2015), where vascular plant species abundance, among other biotic, abiotic, and topographic variables were estimated. We found that burn (high) severity is the main driver of post-fire understory assemblages, resulting in communities less competitive and heterogeneous, with an increasing number of exotic species. Also, post-fire responses are resulting in communities in which the high abundance of flammable taxa and post-fire resprouter species at the early-seral stage may lead to large-scale transitions from mesic forest ecosystems to dry, open forest and fire-prone shrublands in reburned areas. Our results highlight the ecological importance of short-interval and severe wildfires as leading factors in the transition of post-fire understory communities of Araucaria-Nothofagus forests to a system dominated by post-fire obligate resprouters, where tree species are less represented compromising the recovery of these ecosystems. These findings improve the understanding of the current post-fire processes that affect flammability feedbacks and contribute to a baseline of the current patterns in a world of altered fire regimes.