HOLOCENE FIRE, VEGETATION, AND CLIMATE HISTORY OF THE SEASONAL RAIN FORESTS OF WESTERN OREGON USA

The role of fire in seasonal rain forests of northwestern North America is poorly known, because such disturbances are infrequent, the dominant species are long-lived, and wood decomposition rates are high. High-resolution charcoal and pollen analyses of lake sediments in western Oregon were used to reconstruct the fire and vegetation history of the past 9000 years. Sites were located along a latitudinal precipitation gradient in order to examine the nature of this gradient during periods when climate was different than today. The comparison of sites allowed us to consider the linkages between fire, vegetation, and climate on millennial and shorter time scales in different locations. Fire episodes were more frequent than present at the driest site and similar to present at a more mesic site during the early Holocene (10,000 to 6500 cal yr BP), when summer climate conditions were warmer and drier than present. Forest composition was dominated by fire-adapted taxa such as Pseudotsuga menziesii and Alnus rubra. The frequency of fire episodes decreased as climate conditions became cooler and wetter in the late Holocene (ca. 6500 cal yr BP to present). As a result, fire-sensitive taxa, such as Tsuga heterophylla, Thuja plicata, and Picea sitchensis, increased in abundance. The records also indicate periods of heightened fire activity between 8200 and 6900 cal yr BP and 4500 and 3000 cal yr BP, likely in response to centennial-scale drought. During these periods, the fire regimes were apparently synchronized at all sites, overcoming the environmental differences between sites. These records are consistent with other seasonal rain forest locations in northwestern North America and suggest that fire occurrence responded to variations in climate at millennial and centennial time scales throughout the ecoregion.