Using fire history models to estimate proportions of old growth forest in northwest Montana, USA

Abstract

Mature and post-mature stands may be under-represented in many modern forest landscapes as a result of preferential harvest of these age classes. It is important for land managers concerned with protecting biological diversity to know the approximate proportion of old growth in forest landscapes prior to European interference. Negative exponential models, based on mean stand-replacing fire intervals taken from fire history studies, were used to estimate presettlement stand-age distributions for three areas in northwest Montana. Results were compared with empirical distributions calculated from stand maps prepared in 1937–1938, prior to significant timber harvest and effective fire suppression. The models predicted the observed proportion of 1937–1938 old growth (≥200 years) within 10% but were poor at predicting proportions in 20-year age classes. These results suggest that negative exponential models based on empirically determined estimates of fire interval can be used to obtain approximate estimates of presettlement old growth if local fire history studies have been done. Results of this study and numerous fire-history studies suggest that old growth occupied 20–50% of many presettlement forest ecosystems in the Northern Rockies.