Ecology theory and research results indicate that frequent fire forests with active fire regimes display similar structure and spatial pattern characterized by complex mosaics of individual trees, tree clumps, and openings. The importance of restoring these patterns is increasingly being recognized in order to increase the resilience and adaptive capacity of fire-prone ecosystems in the interior Western US. Our primary objective was to test the hypothesis that frequent fire forests from different geographic regions share a common, quantifiable envelope of structure and tree patterns. We reconstructed and mapped pre-fire suppression forest conditions in 48 x 2.5-6 ha plots in the Eastern slope of the Cascades of Washington, Kettle Crest in North Central Washington, the Blue Mountains in Northeast Oregon, and South Central Oregon. We assessed differences in pattern across a gradient of frequent fire forests, ranging from dry, lower productivity sites to more productive, mesic sites with abies species. We used a variety of spatial analysis methods, including a clump detection algorithm, the open space distribution, and a quadrat based approach. We also conducted a literature review of the relationships between spatial pattern and key aspects of resilience such as fire behavior, regeneration, insect behavior, snow retention, and wildlife habitat.
Results/Conclusions
The 48 plots displayed a common, definable envelope of pattern. A strong relationship between density, clumping, and open space was found. The 48 plots showed wide variation in pattern, ranging from low density (50 tph), open forests with relatively few clumps, to moderate density (250 tph) forests with 90% of trees in clumps. Plots with moderate to high proportions of trees in clumps had higher density levels, which allowed for moderate to large openings. Large clumps were often grouped into dense patches of 0.2 - 1 ha. More mesic, productive sites displayed a wider range of pattern and structure than drier sites, suggesting evidence of mixed severity fire behavior. Overall, these results from a large and geographically extensive dataset support the hypothesis that the disturbance regime, developmental process, and ecological functions associated with frequent fire forests are linked to a definable pattern of individual trees, clumps, and openings. Finally, we describe how this envelope of pattern is being used to inform design and monitoring of restoration treatments on multiple National Forests, including efforts to quantify and monitor tree patterns with LiDAR.