Understory plant dynamics following a wildfire in southern Patagonia

Abstract

Historically, wildfires have occurred infrequently in the Subantarctic forests on the island of Tierra del Fuego in southern Argentina: wildfires on the landscape are sporadic and exclusively human-caused. As a result of this, post-fire effects on native vegetation are largely unknown. In November of 2008, a wildfire started near a sawmill in the central part of the island. Six pre-established research plots located in Nothofagus antarctica (ñire) dominated forests burned, impacting a long-term study of understory vegetation dynamics. In 2008 (pre-fire), two 10 m long permanent transects were established per plot (n = 12), where we evaluated species richness and plant cover using a point intersection method at 20 cm intervals, resulting in 50 points per transect. After the fire, we continued sampling to evaluate post-fire understory response in burned and unburned plots. The first post-fire sampling occurred in January 2009, and was repeated semi-annually in burned and unburned plots (2010–2014, 2016, 2018, 2020, and 2021). Data were analyzed using GLMMs (Generalized Linear Mixed Models) and multivariate analyses (Nonmetric Multidimensional Scaling, Multi-Response Permutation Procedures). Total species richness and cover decreased the year following the fire, but recovered by 2010, eventually surpassing pre-fire levels. Increases in species diversity were due principally to an increase in exotic species richness (F = 4.73; p < 0.001) and cover (F = 51.59; p < 0.001). For native species, richness followed the same trend as total vegetation, but cover decreased drastically following the fire (F = 19.77; p < 0.001) and had not recovered to pre-fire levels by 2021. MRPP revealed that plant assemblage in burned plots differed from those in unburned plots the season following the fire (p < 0.001) and still differed 7–12 years post-fire (p < 0.001). Wildfire in Tierra del Fuego produces changes in understory dynamics that create non-forest plant assemblages dominated by exotic species; these changes last many years and pose a significant threat to the forests and native plant species of the island. Without active restoration, forests impacted by wildfire may be permanently lost along with the ecosystem services and habitat they provide.