Holocene vegetation and fire history of the Coast Range,western Oregon,USA

Pollen and high-resolution charcoal records from three lakes were examined to reconstruct the vegetation and fire history of the Oregon Coast Range for the last 9000 years. The sites are located along a north-to-south effective precipitation gradient and changes in vegetation and fire activity provided information on the nature of this gradient in the past. The relation of vegetation to climate change was examined at millennial timescales and the relation between fire and climate was examined on centennial timescales by comparing fire-interval distribution and fire synchrony between sites. The pollen data indicate more fire-adapted vegetation during the early-Holocene period ( c. 9000 to 6700 cal. yr BP), followed by a shift to forests with more fire-sensitive taxa in the mid Holocene (c. 6700 cal. yr BP to 2700 cal. yr BP) and modern forest assemblages developing over the last c. 2700 years. Comparisons of fire-interval distributions showed that the time between fires was similar between two of the three combinations of sites, suggesting that the moisture gradient has played an important role in determining long-term fire frequency. However, century-scale synchrony of fire occurrence between the two sites with the largest difference in effective precipitation suggests that centennial-scale shifts in climate may have overcome the environmental differences between these locations. Asynchrony in fire occurrence between the sites with more similar effective precipitation implies that local conditions may have played an important role in determining fire synchrony between sites with similar long-term climate histories.