High-Severity Fire Selects for “Fast” Traits and Distinct Plant Communities 5 Years After a Mixed-Severity Fire

Questions: Wildfire in dry conifer forests of the western United States is becoming more severe, creating the potential for alternative ecosystem successional trajectories in the absence of intervention. We characterized understory plant communities across a burn severity gradient in a ponderosa pine (Pinus ponderosa) forest 5 years after a mixed-severity fire to ask whether community composition exhibits persistent change in response to high-severity fire. Location: Ponderosa pine understory community, Flagstaff, Arizona, USA. Methods: We evaluated differences in taxonomic composition and plant functional traits on 60 1-m² subplots measured annually in September from 2020 to 2024. Plants were identified to the species level, and species cover was visually estimated. We used six plant traits: specific leaf area (SLA; mm² g⁻¹), height (m), seed mass (mg), resprouting ability, nativity, and plant functional group (grass, forb, shrub, and tree). Results: Taxonomic and functional composition of unburned, low-severity, and high-severity understory communities were significantly distinct from each other 5 years post-fire. As burn severity increased, larger proportions of the understory were occupied by forbs and woody plants. Exotic plants increased dramatically with increasing severity, from < 1% relative cover in the unburned plots to a maximum of 75% in the high-severity plots. Plant species with resprouting ability were more abundant in unburned and low-severity than in the high-severity plots. High-severity communities also had the highest SLA. Conclusions: Our findings reveal that high-severity fire can overwhelm the resprouting ability of native plants, leading the understories in these areas to be colonized and dominated by fast-growing and exotic plants 5 years following wildfire. The persistence of novel understory communities following high-severity fire highlights the need for fuel management and maintenance of prescribed fire, as well as prompt restoration action after severe wildfires to maintain composition and structure in these communities.