Importance of non-stand-replacing fire for development of forest structure in the Pacific Northwest, USA

Old-growth forests have been the subject of much political concern and forestry research in the 20th century. Evidence that many old-growth stands of the western Oregon Cascade Range have experienced multiple low- to moderate-severity fires has led to the hypothesis that such fires have sustained structural complexity in certain temperate coniferous forests. Multiple regression analysis was used to assess the role of historical non-stand-replacing fire for 90 mature and old forest sites on a 450-km2 landscape of mixed-severity fire regime in western Oregon. The occurrence of non-stand-replacing fire was inferred from the relative frequency of fire scars on Pseudotsuga menziesii. Results suggest that in the central western Cascades, the occurrence of non-stand-replacing fire has favored variability in tree diameter sizes, larger trees, and a greater component of shade-tolerant tree species, after accounting for the effects of elevation, slope aspect, and stand age. However, the fire scar variable explained a relatively small percentage of the variance not explained by these effects, with partial R2 values ranging from 0.05 to 0.16. A path analysis suggested that the effects of stand age on forest structure can be partitioned into direct effects (gap dynamics and successional processes in the absence of disturbance) and indirect effects (older stands have had more time to experience non-stand-replacing fire), with the indirect effects explaining a large component of forest structural attributes that are typically associated with the old-growth seral stage. While the cumulative and long-term effects of non-stand-replacing fire in this region are likely complex, it would seem prudent to include such disturbances as a key ecosystem process, where management practices seek to emulate natural disturbance regimes or facilitate the development of old-growth forest structures.