Post-fire regeneration across a fire severity gradient in the southern Cascades

Abstract

Large scale, high-severity fires are increasing in the western United States. Despite this trend, there have been few studies investigating post-fire tree regeneration. We established a study in the footprint of the 2000 Storrie Fire, a 23,000ha wildfire that occurred in northern California, USA. We used a stratified sampling design to quantify post-fire vegetation dynamics across four levels of burn severity and three forest types on the Lassen National Forest nine and ten years following fire. Within each sampled stand, we recorded tree seedlings, forest overstory, shrub cover, and abiotic factors hypothesized to influence growth and establishment.Median conifer seedling densities varied substantially by burn severity: 1918 seedlings ha−1 in the Unchanged units; 4838 seedlings ha−1 in the Low-severity units; 6484 seedlings ha−1 in the Medium-severity units; and 710 seedlings ha−1 in the High-severity units. Increased burn severity was associated with greater shrub coverage: shrub cover in High-severity burns was more than three times those of lower burn severities. We calculated Shannon’s Species Diversity (H′) and Pielou’s Evenness (EH) indices to examine woody shrub and tree diversity. Abies spp. were by far the most abundant regenerating conifer species, which may be a concern for land managers; shrub cover after High-severity burns was dominated by Ceanothus spp. Although fir regeneration was prolific, the Storrie Fire generated diverse vegetative responses, potentially aiding in the reintroduction of the diverse landscape mosaic homogenized by a century of landscape-scale fire exclusion.