Shrub succession on eight mixed-severity wildfires in western Montana, northeastern Oregon, and northern Idaho

Abstract

The response of 28 shrub species to wildfire burn severity was assessed for 8 wildfires on 6 national forests in the northern Rocky Mountains, USA. Stratified random sampling was used to choose 224 stands based on burn severity, habitat type series, slope steepness, stand height, and stand density, which resulted in 896 plots measured at approximately 2-year intervals from 1 to 11 years after the fire. For each species, mathematical models were developed to predict the probability of occurrence on 1/300-acre plots, cover on the plot, and average height. The association between burn severity and the probability of occurrence was generally negative. For most species, occurrence increased with increasing time since the wildfire. Cover decreased with increasing burn severity, except for some species that are dependent on seed for establishment. The longer it took the species to become established on the plot, the less cover could be expected. The best variable to predict shrub height was the cover of that species on the plot. Height decreased with increasing basal area and burn severity. These results can help natural resource managers predict the effects of wildfire on vegetation succession as a function of burn severity, site factors, and time. Of particular importance is the finding that percent cover of shrubs is generally low (less than 10 percent for almost all species) when the time between the wildfire and when the species was first observed on the plot is more than about 2 years. This result could be verified in other studies, either by a designed prospective study or reanalysis of previous studies where succession was followed over time.