Burn Severity and Plant Species Recovery Two Decades Post-Fire in Mesic, Mixed-Conifer Forests of Northwestern Montana

Wildfires are becoming increasingly frequent and severe due to climate change and human activities, posing significant threats to forest ecosystems and altering fire regimes. Mesic, mixed-conifer forests, which contain both fire-adapted and non-fire-adapted species, are particularly vulnerable to these changes. This study examines plant community and landscape recovery over two decades following the 2003 Robert and Wedge Canyon fires in northwestern Montana, USA. Using a combination of satellite-derived burn severity metrics (Normalized Burn Ratio [NBR] and Differenced Normalized Burn Ratio [dNBR]) and field-based plant species inventories, we assessed burn severity change over time and space and its effects on plant diversity recovery. Our results reveal high spatial variability in initial burn severity, with the most intense burns concentrated on steep, south-facing slopes and ridges. Despite this heterogeneity, overall forest health and structure gradually returned to pre-fire levels within 20 years, though there was spatial variability in recovery. Generalized Linear Models indicate that species richness in severely burned plots (2003/04 dNBR) failed to recover to levels observed in less severely burned plots. However, other metrics of community composition, Shannon’s Index and Pielou’s Evenness, were not significantly affected by burn severity, underscoring a lasting legacy effect primarily on species richness. This pattern was driven by significant declines in tree and forb species in high-severity plots, while shrubs remained relatively stable. Species frequency patterns further illustrate these trends, where fire-adapted species dominated high-severity plots, whereas less fire-tolerant species, along with shallow-rooted forbs, exhibited lower densities in severely burned plots. Our findings suggest that high-severity fires create lasting disturbance legacies, altering successional trajectories and potentially reshaping long-term species composition. Given the extent of these fires, such changes may lead to widespread ecological transformations. This study underscores the importance of integrating satellite-derived burn severity assessments with field-based inventories to capture both large-scale recovery patterns and finer-scale shifts in community composition.