3000 years of fire history in the Western U.S

Background/Question/Methods   Fire regimes in the western U.S. have undergone major changes due to climate, grazing, fire suppression, and related human impacts during the past few centuries. Disentangling the effects, magnitude, relative importance of the controls, and even direction of the impacts has been difficult, however, due to the lack of historical information about human activities and about baseline fire regimes, particularly during the time before Euroamerican settlement. Fire history studies based on fire scarred trees have provided useful data on fire-history trends in some forest types, but coverage from such studies is limited across space and time. Here we present fire-history data from over 100 stratigraphically continuous sedimentary charcoal records that complement the fire scar data both geographically and temporally.

Results/Conclusions   Together these independent data sources show remarkable consistency; they indicate that biomass burning was variable from 1500 to 1600 A.D., increased sharply from 1600 to about 1850 A.D., and declined thereafter. Prior to those times, burning was particularly high in many areas, which raises important questions about climate conditions at this time and the extent of Native American burning. Comparisons of these data with additional records of human activities, climate, and vegetation changes help explain these variations, and highlight the existence of a distinct and widespread signal for the western U.S. that has implications for contemporary resource management issues, particularly relating to the carbon cycle.