Many western North American forest types have experienced considerable changes in ecosystem structure, composition, and function as a result of both fire exclusion and timber harvesting. These two influences co-occurred over a large portion of dry forests making it difficult to know the strength of either one on its own or the potential for an interaction between the two. In this study we used contemporary re-measurements of a systematic historical forest inventory to investigate forest change. The historical data opportunistically spanned a significant land management agency boundary, which protected part of the inventory area from timber harvesting. This allowed for a robust comparison of forest change between logged and unlogged areas. In addition, we assessed the effects of recent management activities aimed at forest restoration relative to the same areas historically, and to other areas without recent management.
Results/Conclusions
Based on analyses of 22,007 trees (historical: 9573; contemporary: 12,434) live basal area and tree density significantly increased from 1911 to present in both logged and unlogged areas. Both shrub cover and the proportion of live basal area occupied by pine species declined from 1911 to present in both areas, but statistical significance was inconsistent. The most notable difference between logged and unlogged areas was in the density of large trees, which declined significantly in logged areas, but slightly increased (non-significant) in unlogged areas. Recent management activities had a varied impact on the forest structure and composition variables analyzed. In general, areas with no recent management activities experienced the greatest change from 1911 to present. If approximating historical forest conditions is a land management goal the documented changes in forest structure and composition from 1911 to present indicate that active restoration, including fire use and mechanical thinning, is needed in many areas.