Operational resilience in western US frequent-fire forests

Abstract

With the increasing frequency and severity of altered disturbance regimes in dry, western U.S. forests, treatments promoting resilience have become a management objective but have been difficult to define or operationalize. Many reconstruction studies of these forests when they had active fire regimes have documented very low tree densities before the onset of fire suppression. Building on ecological theory and recent studies, we suggest that this historic forest structure promoted resilience by minimizing competition which in turn supported vigorous tree growth. To assess these historic conditions for management practices, we calculated a widely-used measure of competition, relative stand density index (SDI), for two extensive historical datasets and compared those to contemporary forest conditions. Between 1911 and 2011, tree densities on average increased by six to seven fold while average tree size was reduced by 50%. Relative SDI for historical forests was 23–28% of maximum, in the ranges considered ‘free of’ (<25%) to ‘low’ competition (25–34%). In contrast, most (82–95%) contemporary stands were in the range of ‘full competition’ (35–59%) or ‘imminent mortality’ (≥60%). Historical relative SDI values suggest that treatments for restoring forest resilience may need to be much more intensive then the current focus on fuels reduction. With the contemporary increase in compounding stresses such as drought, bark beetles, and high-severity wildfire, resilience in frequent-fire forests may hinge on creating stands with significantly lower densities and minimal competition. Current management practices often prescribe conditions that maintain full competition to guide development of desired forest conditions. Creating stands largely free of competition would require a fundamental rethinking of how frequent-fire forests can be managed for resilience.