Mid-term (17 years) post-fire dynamics in mixed broadleaf lowland forests: Tree mortality and natural regeneration patterns

Abstract

Increasing wildfire activity following ongoing land-use and climate change is altering forest ecosystems worldwide. Post-fire forest recovery and resilience depend on the survival of existing trees and successful regeneration. This study presents a 17-year post-fire stand development of a mixed broadleaf forest in southern Switzerland, subjected to post-cultural natural processes and multiple transient stressors, including two wildfires of variable severity and size (i.e., 2006 and 2018), biotic disturbances (e.g., Asian chestnut gall), and increasing browsing pressure. A systematic permanent plot network with a mesh size of 75 × 75 m was established across 55 ha of the 2006 burn area and adjacent unburnt area. Forest structure and regeneration dynamics were surveyed at 3-, 11- and 17-years post-fire to study stand development and related biotic and abiotic driving factors. Tree regeneration occurred in most plots, but few species reached the pole stage. In unburnt and low-severity areas, canopy gaps created by factors probably unrelated to fire provided light conditions that favored the growth of shade-tolerant late-successional native species. Conversely, moderate- to high-severity plots experienced high tree mortality, favoring colonization by pioneer species with a significant shift toward non-native woody species. Ungulate browsing strongly reduced native regeneration success, leading to major changes in stand structure and species composition, favoring non-native pioneer species, particularly in high-severity plots. Our study shows that mixed-broadleaf forests can regenerate after a fire, but chronic browsing pressure hinders native species establishment and alters successional trajectories. Mitigating browsing is essential to maintain diverse, resilient forests under increasing disturbance regimes.