Although mixed-severity fire regimes are widespread in the western United States, including a substantial portion of the productive Douglas-fir region west of the crest of the Cascade Mountains, a conceptual framework for this complex regime is lacking. We consider mixed-severity systems as those where burn severity in individual fires and over successive events is mixed at forest stand and landscape scales, thereby producing fine-scale mosaics of even-aged and multi-cohort stands with multiple developmental pathways and various successional roles for shade-intolerant and shade-tolerant species. To better understand the mixed-severity attributes of Douglas-fir forests of the Pacific Northwest and the topographic factors fostering those attributes, we conducted a study centered on two large (240-300 km2) watersheds in the central western Cascades of Oregon. Forest stand and age structure data, including ages of more than 3,250 trees, were collected in 124 transects. Ordination of transects by age structure variables produced a strong gradient representing the cumulative effects of fire on forest development. The ordination scores, as a proxy for site fire-proneness, were related to topographic variables by Non-Parametric Multiplicative Regression (NPMR), which allows the strength of topographic variables and interactions among them to vary across environmental space.
The majority (>75%) of stands had two or more distinct post-fire cohorts, with up to four cohorts in the most fire-prone sites. Shade-tolerant species either regenerated continuously in the absence of fire or formed distinct cohorts following low-severity fires that killed few canopy trees. Fires of higher severity gave rise to Douglas-fir cohorts. Areas of high relief exhibited the greatest fine-scale spatial variation in age structure and the strongest differentiation of age structure types by terrain shape and slope position. Areas of lower relief had a narrower range of age-structure types and weaker relationships between age structure and topography. The contribution of relief to spatial variation in fire effects, as inferred from age structure, most likely reflects stronger microclimatic variation and more pronounced temperature inversions in areas of high relief. For example, the least fire-prone sites were associated with concave terrain and high local relief, where cold-air pooling may maintain high fuel moisture. The multiple successional roles of shade-intolerant and shade-tolerant species and the contribution of topographic complexity to systematic spatial variation in fire effects clarify mixed-severity systems as more than a simple blending of the better-known low- and high-severity fire regimes.