Spatial Variation in Forest Age Structure and its Association with Topography in the Central Western Cascades Of Oregon

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 events and over successive fires 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 and vegetative factors fostering those attributes, we conducted a study centered on two large (240-350 km2) watersheds in the central western Cascades of Oregon. Forest stand and age structure data, including ages of more than 3,300 trees, were collected in 124 transects. Age structure data were related to topographic variables by Nonparametric Multiplicative Regression, which allows the strength of topographic variables and interactions among them to vary across the sample space. The majority (\u3e75%) 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 slope position. Areas of lower relief had a narrower range of age-structure types and weaker relationships between age structure and topography. The uncovering of systematic variation in forest age structure and the topographic factors driving that variation clarifies mixed-severity systems as more than a simple blending of the more well-known low- and high-severity regimes