Early evidence of arrested succession in riparian forests after a large savanna wildfire

Climate change and human activities are increasing the frequency of megafires in savanna landscapes, affecting biodiversity and human livelihoods. In the savanna landscapes of the central Cerrado, within the iconic Chapada dos Veadeiros National Park, a megafire spread out-of-control during the 2017 extreme drought event, penetrating even the fire-sensitive riparian forests, and causing severe tree mortality. Here, we studied the impact and recovery of these riparian forests up to four years after the megafire. Using 36 field plots in burnt and unburnt riparian forests, we monitored vegetation and soil recovery, assessing structural and compositional changes over time. We tested the hypothesis that severely burnt forests could remain trapped in an open, grassy degraded state due to self-reinforcing feedbacks. Our results show that four years after the megafire, burnt flooded forests still showed structural and diversity losses relative to unburnt forests, with 80.1% lower tree density, 44.1% lower canopy cover, 64.5% lower aboveground biomass and 68.5% lower species richness. In these severely burnt forests, tree recruitment remained low, likely constrained by the increased cover of exotic grasses, which rose from 0.62% to 18.85%, and by invasive ferns, which increased from no recorded cover to 16.20% four years after fire. Analysis of tree composition suggest the process of biotic homogenization in burnt flooded forests, likely reflecting the novel sparse-tree habitats that persist in the riparian landscapes. Forest recovery was heterogeneous across study sites and primarily depended on fire severity. Our findings indicate that self-reinforcing feedbacks may have arrested the burnt riparian forests in an open degraded state after the megafire, due to a combination of soil erosion and persistent cover of invasive grasses and opportunistic ferns. Restoring forest successional trajectories requires interventions to disrupt these feedbacks and facilitate forest recovery. With the intensification of climate change, megafires will become more frequent in tropical savanna landscapes, threatening the persistence of riparian forests, with catastrophic social-ecological consequences.