A statistical evaluation of the relative influences of climate, vegetation, and prehistoric human population on the charcoal record of Five Lakes, Washington (USA)

Currently under debate is the extent to which prehistoric peoples influenced the size and frequency of fire on the North American continent. Previous studies have concentrated primarily on cultures that raised crops and had high population densities. The current study focuses, instead, on an area prehistorically inhabited by hunter-gatherer groups. Records of charcoal, pollen, and oxygen isotopes are used as proxies for fire, vegetation, and climate for the past 1000 years in the Rufus Woods Lake area of eastern Washington. This data set is compared to measures of prehistoric human population derived from archaeological data. Statistical analyses reveal that three factors—human population size, pine pollen abundances, and δ18O values—are significantly related to future charcoal influx values. The lag between climate and charcoal spans several centuries, indicating that the two are related through a series of climatically-controlled vegetation changes. Pine and human population abundances are related to charcoal deposition on shorter time intervals, pointing to more immediate mechanisms affecting fire through changes in ignition rates and/or available fuels. This work indicates that both natural and cultural processes have acted together in influencing fire in the study area over the past millennium.