Changes in forest structure, community composition, and development in ponderosa pine forests following a mixed-severity wildfire in the Black Hills, SD, USA

Abstract

Fire behavior in ponderosa pine (Pinus ponderosa Laws.) forests of the interior West can vary along a continuum from low severity surface fire to high severity crown fire. Variations in fire behavior differentially impact forest vegetation and create a fine-scale mosaic of low, moderate, and high severity fire effects across the landscape. The immediate and longer-term impact mixed-severity wildfires have on forest overstory and understory structure and composition vary greatly in relation to fire severity and are not well understood. Here, we describe the long-term implications of the Jasper fire on overstory and understory forest development. In stands where low severity fire occurred, the fire had little impact on the structure of the overstory and understory vegetation. However, it did cause significant reductions in the forest floor litter and woody fuel layer reducing future fire hazard and potential fire severity. In contrast, stands impacted by moderate severity fire, resulting from passive crown fire, experienced significant changes throughout the forest overstory, understory, and forest floor. Substantial reductions in overstory tree density created an open, park-like stand with a grass/forb dominated understory that will persist until tree regeneration, recruitment, and canopy closure once again occurs. High rates of mortality, however lead to significant build-up of hazardous fuel which may increase future fire hazard and potential fire severity. Finally, stands impacted by high severity fire, which resulted from active, stand-replacing crown fire, led to a long-term change in cover type from a closed-canopied ponderosa pine forest to a grass and forb dominated meadow. Even with no overstory, these stands will continue to be susceptible to high severity surface fire well into the future due to the build-up of hazardous fuels resulting from the rapid snag-fall of fire-killed trees. While these mixed-severity wildfires are often viewed as catastrophic events, they are, in actuality, disturbance events that increase structural and compositional heterogeneity throughout an otherwise homogeneous landscape. In the case of the Black Hills where mixed-severity wildfires were part of the historic fire regime, recent mixed-severity wildfires actually restore many of the structural and compositional components present on the landscape prior to Euro-American settlement.