The ecology of mixed severity fire regimes in Washington, Oregon, and Northern California

Forests characterized by mixed-severity fires occupy a broad moisture gradient between lower elevation forests typified by low-severity fires and higher elevation forests in which high-severity, stand replacing fires are the norm. Mixed-severity forest types are poorly documented and little understood but likely occupy significant areas in the western United States. By definition, mixed-severity types have high beta diversity at meso-scales, encompassing patches of both high and low severity and gradients in between. Studies of mixed-severity types reveal complex landscapes in which patch sizes follow a power law distribution with many small and few large patches. Forest types characterized by mixed severity can be classified according to the modal proportion of high to low severity patches, which increases from relatively dry to relatively mesic site conditions. Mixed-severity regimes are produced by interactions between top-down forcing by climate and bottom-up shaping by topography and the flammability of vegetation, although specific effects may vary widely across the region, especially the relation between aspect and fire severity. History is important in shaping fire behavior in mixed-severity landscapes, as patterns laid down by previous fires can play a significant role in shaping future fires. Like low-severity forests in the western United States, many dry mixed-severity types experienced significant increases in stand density during the 20th century, threatening forest health and biodiversity, however not all understory development in mixed-severity forests increases the threat of severe wild fires. In general, current landscapes have been homogenized, reducing beta diversity and increasing the probability of large fires and insect outbreaks. Further loss of old, fire tolerant trees is of particular concern, but understory diversity has been reduced as well. High stand densities on relatively dry sites increase water use and therefore susceptibility to drought and insect outbreaks, exacerbating a trend of increasing regional drying. The need to restore beta diversity while protecting habitat for closed-forest specialists such as the northern spotted owl call for landscape-level approaches to ecological restoration.