Wildfire is an important ecological process within the mixed conifer forests of the western United States. Scientists and land managers are currently concerned because wildfires have increased in frequency and size over recent decades and these trends are expected to continue under a warming climate. Assessments of burn severity enable managers to assess impacts of fires on ecosystem goods and services, however, long term (> 10 years) assessments of plant community composition and structure at a variety of burn severity levels is largely unexplored. This study compares a satellite-sensor based index of burn severity (dNBR) at four wildfires from the 2003 fire season in western Montana to field measurements including tree density, tree regeneration, fuel loadings, ground cover components, and understory species composition. A non-parametric multi-response permutation procedure (MRPP) and the univariate Kruskal-Wallis sum rank test were used to evaluate the significance of burn severity level (low, moderate, high) on understory species composition, wildland fuels, and tree density post-fire. We evaluated species diversity and richness along a dNBR gradient using a non-linear regression.
Results/Conclusions
Multivariate analysis indicates that the major difference between burn severity classes were density of live and dead trees, char on ground cover components, and species richness, while no significant differences were detected for ground cover components (green or non-photosynthetic vegetation, soil, or rock cover), woody fuels, sapling density, or shrub cover. Ten years post-burn, no live trees were sampled in the high severity burn areas. Seedling regeneration was higher in moderate and high severity burns, compared to low severity. Species diversity and richness showed a nonlinear relationship with the remotely sensed burn severity index dNBR, with a maximum observed at moderate burns severity levels. Under current climate conditions, we conclude that the mixed-severity fire regime, characterized by a pattern of patches burned at variable severity, contributes to ecological resilience by increasing gamma diversity and heterogeneity in the mixed conifer forests of the Northern Rockies region. However, we concur with other researchers stating that frequent reburns or otherwise altered fire regimes could affect future forest composition and structure.