POST-MAZAMA CLIMATE, VEGETATION, AND FIRE: BLUE MOUNTAINS, OREGON

Analyses of fossil pollen, spores, algae, and charcoal from Lost Lake, Umatilla National Forest and from Twin Lakes, Wallowa-Whitman National Forest suggest changes in forest fire frequency and intensity during the period since the eruption of Mt. Mazama (6850 B.P.). At Lost Lake, layers of charcoal-rich sediment and microscopic charcoal deposited between 3400 and 2000 B.P. attest to frequent intense fires. At Twin Lakes charcoal-rich layers occurred between 6800-4800 B.P. whereas microscopic charcoal remained abundant until about 4000 B.P. At both sites declining charcoal and pollen from introduced or weedy species attest to effects of historical fire suppression and disturbance. These variations in charcoal abundance correspond to climate shifts reflected in variable water depths and forest composition. The most important of these occurs at about 4000 B.P.