Holocene fire and vegetation history of the Oregon Coast Range, United States

The role of fire in seasonal rain forests is poorly known because such disturbances are relatively infrequent, the dominant species are long-lived, and dendrochronological methods of reconstructing fire history are limited by rapid wood decomposition rates. High-resolution charcoal and pollen analyses of lake sediments were undertaken at three lakes to reconstruct the fire and vegetation history of the Oregon Coast Range for the last 9000 years. The sites were located along a precipitation gradient and changes in fire-episode frequency and vegetation provided information on the nature of this gradient in the past. Comparison of sites allowed the examination of fire-vegetation-climate linkages on millennial as well as shorter time scales in different locations. Fire episodes were more frequent or near present rates during the early Holocene (ca. 9000 to 6500 cal yr B.P.) as a result of warmer drier summer conditions than at present. Forests composed of fire-adapted taxa, such as Pseudotsuga menziesii and Alnus rubra accompanied the increase in fire occurrence. As conditions became cooler and wetter in the late Holocene (ca. 6500 cal yr B.P. to present), fire return intervals lengthened and 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 ca. 8200 and 6900 cal yr B.P. and ca. 4500 and 3000 cal yr B.P., likely in response to centennial-scale drought. During these periods, fire episodes were synchronized at all sites, despite local climatic differences.; The paleoecological records suggest that fire has been an important disturbance in seasonal rain forests and variations in fire activity respond to changes in climate on multiple time scales. Given projections of warmer conditions and more fires in the future, as a result of increased greenhouse gases in the atmosphere, the response of fire to past variations in climate should be considered in forest management plans.; This dissertation includes previously published co-authored materials.