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Indigenous impacts on North American Great Plains fire regimes of the past millennium

Christopher I. Roos,M. Zedeño,Kacy L. Hollenback,M. Erlick

Published 2018 in Proceedings of the National Academy of Sciences of the United States of America

ABSTRACT

Significance The relative importance of human activities and climate in shaping fire regimes is controversial. In North American grasslands, climate exerts strong top-down influences on fuels. For centuries before the introduction of the horse, Native American and First Nations hunters built and used landscape features on these grasslands to harvest bison en masse. Charcoal layers associated with drivelines indicate that fire was an important part of these hunting practices. Furthermore, correlation of dated fire deposits and climate records indicate that ancient bison hunters burned in response to favorable climate conditions. This study indicates that climate and human activities are not mutually exclusive factors in fire histories; even relatively small groups of hunter-gatherers can enhance climate impacts. Fire use has played an important role in human evolution and subsequent dispersals across the globe, yet the relative importance of human activity and climate on fire regimes is controversial. This is particularly true for historical fire regimes of the Americas, where indigenous groups used fire for myriad reasons but paleofire records indicate strong climate–fire relationships. In North American grasslands, decadal-scale wet periods facilitated widespread fire activity because of the abundance of fuel promoted by pluvial episodes. In these settings, human impacts on fire regimes are assumed to be independent of climate, thereby diminishing the strength of climate–fire relationships. We used an offsite geoarchaeological approach to link terrestrial records of prairie fire activity with spatially related archaeological features (driveline complexes) used for intensive, communal bison hunting in north-central Montana. Radiocarbon-dated charcoal layers from alluvial and colluvial deposits associated with driveline complexes indicate that peak fire activity over the past millennium occurred coincident with the use of these features (ca. 1100–1650 CE). However, comparison of dated fire deposits with Palmer Drought Severity Index reconstructions reveal strong climate–fire linkages. More than half of all charcoal layers coincide with modest pluvial episodes, suggesting that fire use by indigenous hunters enhanced the effects of climate variability on prairie fire regimes. These results indicate that relatively small, mobile human populations can impact natural fire regimes, even in pyrogeographic settings in which climate exerts strong, top-down controls on fuels.

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