Short-interval reburns drive shifts to early species composition at a high-elevation forest site

Background More frequent wildfire ignitions, increased drought conditions, and historical forest management practices are leading to shorter intervals between fire events (e.g., < 30 years) in some forest ecosystems. When these short-interval reburns occur in high-elevation forest ecosystems that are adapted to infrequent and stand-replacing wildfires, impacts on vegetation regeneration can include ecological changes in forests such as reduced tree densities, shifts in community composition, and in some cases regeneration failure. The goal of this study is to characterize forest regeneration at a high-elevation forest site affected by a single recent wildfire (2017) and a short-interval reburn (1992 and 2017). Results We surveyed vegetation that experienced varying combinations of burn severity (e.g., low severity, low severity followed by high severity) to compare regeneration. Our results indicate that successive wildfires (i.e., short-interval reburns) impact regenerating tree and understory diversity differently and can lead to species-specific reductions in regenerating tree counts (seedlings and saplings) and understory plant cover. Forest composition was largely unchanged following a single wildfire; however, reburned areas exhibited divergent compositional shifts, with regenerating tree communities responding differently than understory plant communities. In regions that experienced the greatest disturbance (e.g., high severity wildfire followed by high severity wildfire), few to no coniferous species typical of unburned forests at this elevation were observed; however, an increased presence of pioneer species such as lodgepole pine ( Pinus contorta Douglas ex Loudon) and deciduous species was detected. Conclusions Shifts in tree and understory composition following a reburn suggest a potential departure from historical regeneration pathways. Additionally, our findings highlight the importance of distinguishing between the responses of tree and understory regeneration to reburns and burn severity. These patterns may have implications for forest resilience under repeated fire and warrant further consideration in the context of forest fire management planning.