COS 99-2 - How do burn severity and short-interval fires affect tree regeneration of temperate forests in the Southern Hemisphere? A case study from Araucaria araucana forests in southern South America

Abstract: In temperate forest regions globally, warmer and drier conditions are increasing over recent decades, resulting in earlier snowmelt and longer fire seasons, altering fire regimes across forest landscapes, and reducing the capacity of many forest communities and tree species to resist and recover from wildfires. Following severe fires and reburns, a large group of fire-sensitive and fire-tolerant tree species have been experiencing regeneration failure in temperate forests of the Southern Hemisphere.
The goal of this work is to evaluate tree species regeneration patterns following single and repeated (13-year interval) wildfires in Andean Araucaria-Nothofagus forests in south-central Chile. Seedlings and resprouts abundance of the fire-resistant and threatened conifer Araucaria araucana (pehuén, monkey puzzle) and fire-sensitive Nothofagus (southern beech) species were surveyed within 120 plots (0.07 ha) across a gradient of burn severity in two study areas: one affected by a single mixed-severity fire, and the other reburned in a short-interval time. We used nonparametric statistical analyses to identify differences in tree seedling and resprout abundance across burn gradient as well as negative binomial generalized linear models to identify drivers of tree regeneration.
Our study shows that in severely burned plots, only a few Araucarias and no southern beech (N. alpina, N. obliqua, N. pumilio, N. dombeyi) trees survived. Consequently, we found poor seedling abundance of Araucaria and Nothofagus, and zero of the latter in reburned plots. Distance to the seed source (i.e., mature live trees) and burn severity were the main explanatory variables for the poor seedling response. Alternatively, asexual regeneration (via resprouts of A. araucana, N. alpina and N. obliqua) was generally successful in areas burned with a single high-severity fire. Although A. araucana is recognized for its epicormic postfire resprouting, the species was not able to resprout after two consecutive high severity fires in a period of 13 years, whereas one Nothofagus species (N. alpina) was able to resprout profusely.
In conclusion, our findings suggest that single high severity fires and/or reburns have the capacity for large-scale reorganization of Araucaria-Nothofagus forest, largely due to lack of seed availability in areas burned with high severity and reburned. If the occurrence of high-severity and/or short-interval fires increases in the coming decades, as predicted, we expect a change of forest structure and homogenization of the plant composition due to the potential dominance of shrubs, which is also associated with increased flammability and the risk of stabilizing fuel-vegetation feedbacks.