Aim Forest restoration in ponderosa pine and mixed ponderosa pine–Douglas fir forests in the US Rocky Mountains has been highly influenced by a historical model of frequent, low-severity surface fires developed for the ponderosa pine forests of the Southwestern USA. A restoration model, based on this low-severity fire model, focuses on thinning and prescribed burning to restore historical forest structure. However, in the US Rocky Mountains, research on fire history and forest structure, and early historical reports, suggest the low-severity model may only apply in limited geographical areas. The aim of this article is to elaborate a new variable-severity fire model and evaluate the applicability of this model, along with the low-severity model, for the ponderosa pine–Douglas fir forests of the Rocky Mountains.
Location Rocky Mountains, USA.
Methods The geographical applicability of the two fire models is evaluated using historical records, fire histories and forest age-structure analyses.
Results Historical sources and tree-ring reconstructions document that, near or before ad 1900, the low-severity model may apply in dry, low-elevation settings, but that fires naturally varied in severity in most of these forests. Low-severity fires were common, but high-severity fires also burned thousands of hectares. Tree regeneration increased after these high-severity fires, and often attained densities much greater than those reconstructed for Southwestern ponderosa pine forests.
Main conclusions Exclusion of fire has not clearly and uniformly increased fuels or shifted the fire type from low- to high-severity fires. However, logging and livestock grazing have increased tree densities and risk of high-severity fires in some areas. Restoration is likely to be most effective which seeks to (1) restore variability of fire, (2) reverse changes brought about by livestock grazing and logging, and (3) modify these land uses so that degradation is not repeated.