Logging exacerbates the effects of fire exclusion on stand density in ponderosa pine forests of the Northern Rockies over the long term

Recent increases of large wildfires in western forests are often attributed to the effects of increased stand density resulting from decades of fire exclusion and global warming. However, assumptions that fire exclusion is the primary cause of high stand density ignore the effects of other prominent anthropogenic disturbances on stand structure. We conducted a study to assess the relative influence of fire exclusion versus the long-term effects of historical logging practices on contemporary forest structure in ponderosa pine/Douglas-fir forests of the Northern Rockies. We documented stand characteristics for 24 paired, logged and unlogged sites across MT and ID where fire had been absent for 80 years. By combining our data with that of Keeling et al. (2006) we were able to assess changes in stand structure due to the absence of fire alone and combined with past logging. Our results show strong long-term effects of logging on forest structure, independent of logging type. Logged, unburned stands were four times denser than fire-maintained reference stands and twice as dense as unburned, unlogged stands. While fire exclusion increases stand density by promoting the growth of small, shade tolerant trees, logging greatly compounds this effect. These results provide valuable insight into the causes of departure in contemporary forest structure from historical reference conditions and confirm the long-term impacts of disturbance associated with various logging treatments. Our results caution that logging treatments for restoration or fuel reduction purposes may produce counterproductive effects over the long term.