How does forest recovery following moderate-severity fire influence effects of subsequent wildfire in mixed-conifer forests?

Abstract

Background: Given regional increases in fire activity in western North American forests, understanding how fire influences the extent and effects of subsequent fires is particularly relevant. Remotely sensed estimates of fire effects have allowed for spatial portioning into different severity categories based on the degree of fire-caused vegetation change. Fire effects between minimal overstory tree mortality (< 20%) and complete (or nearly complete) overstory tree mortality (> 95%) are often lumped into a single category referred to as moderate severity. In this paper, we investigated how burned areas in this broad category of moderate-severity fire fared when reburned by a subsequent fire. Specifically, we examined the influence of forest structure, tree species composition, and shrub cover 9–17 yr following moderate-severity fire on the severity of a subsequent large wildfire event. We used plot-based measurements of trees and shrub cover to develop 15 forest structure and composition variables to attempt to explain observed reburn severity. Results: Only live Abies Mill. species basal area and dead standing biomass were identified as significant predictors of reburn severity using conditional inference tree analysis, both of which were positively related to reburn severity. Conclusion: Our findings emphasize that the wide range of fire effects in the moderate-severity category can contribute to highly variable responses to subsequent wildfire.