OOS 26-3 - Fire tended forests: Fire history and stand development of old-growth Douglas-fir forests in Oregon and Washington

Old-growth Coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) forests, hereafter Douglas-fir forests, are renowned for their tremendous biomass and compositional and structural diversity. Douglas-fir forests are found across a vast region west of the Cascade Range that extends from northern California, U.S.A. to British Columbia, Canada. Variation in climate, lightning ignitions, the frequency of severe fire weather, productivity, topography, and traditional burning practices by Native American tribes drove variability in the historical fire regime.

Traditional models of fire regimes in Douglas-fir forests hypothesize that an infrequent, high-severity fire regime is characteristic across much of the Douglas-fir region. In this “climate-limited” model, fire occurrence and spread is restricted to periods of severe drought and/or severe fire weather, and fire effects are predominantly stand-replacing. Douglas-fir forests develop over several centuries without fire. Competition among trees and endogenous disturbances facilitate the development of old-growth conditions. Landscapes are coarse-grained with large areas of forest having similar forest conditions and development history.

There’s growing recognition that a moderately-frequent, mixed-severity fire regime characterized the central and southern Cascades. Fires were not limited to exceptional drought and extreme fire weather. Thus fuels, topography, and ignitions were important drivers of fire frequency and effects. The resulting variability in fire resulted in several forest successional pathways that develop mature and old-growth variants with distinct structure and composition. In contrast to landscapes with infrequent high-severity fire, landscapes with mixed-severity fire have a finer-grained mosaic composed of forest patches with distinct fire-tended development histories.

Until recently, rigorous, and annually resolved reconstructions of historical fires were not available to inform pedagogical models and mappings of historical fire regimes in Douglas-fir forests. This presentation summarizes 103 new multi-century reconstructions of historical fire occurrence from the central Oregon Coast Range, and the Umpqua, Willamette, Mt. Hood, Gifford-Pinchot, and Mt. Baker-Snoqualmie National Forests. These reconstructions illustrate enormous diversity in fire frequency and effects across Douglas-fir forests over the past 8 centuries and defy expectation. For example, the central Oregon Coast Range has a history of frequent mixed-severity fire. Old-growth forests less than 20 km from the Pacific Ocean historically developed with non-stand-replacing fires. Much of the Douglas-fir region was characterized by moderately frequent, mixed-severity fire. High-severity fire effects occurred in the past 800 years at most study sites. However, there was little evidence of extensive patches of high-severity fire and coarse-grained landscapes. This is potentially because high-severity patches were diversified by multiple mixed-severity reburns.