Indigenous impacts on North American Great Plainsfire regimes of the past millenniumChristopher I. Roosa,1, María Nieves Zedeñob, Kacy L. Hollenbacka, and Mary M. H. Erlickc

aDepartment of Anthropology, Southern Methodist University, Dallas, TX 75275-0336; bBureau of Applied Research in Anthropology, School ofAnthropology, University of Arizona, Tucson, AZ 85721; and cDepartment of Sociology, Social Work, and Anthropology, Utah State University, Logan, UT 84322

Edited by B. L. Turner, Arizona State University, Tempe, AZ, and approved June 26, 2018 (received for review March 27, 2018)

Fire use has played an important role in human evolution andsubsequent dispersals across the globe, yet the relative impor-tance of human activity and climate on fire regimes is controver-sial. This is particularly true for historical fire regimes of theAmericas, where indigenous groups used fire for myriad reasonsbut paleofire records indicate strong climate–fire relationships. InNorth American grasslands, decadal-scale wet periods facilitatedwidespread fire activity because of the abundance of fuel pro-moted by pluvial episodes. In these settings, human impacts onfire regimes are assumed to be independent of climate, therebydiminishing the strength of climate–fire relationships. We used anoffsite geoarchaeological approach to link terrestrial records ofprairie fire activity with spatially related archaeological features(driveline complexes) used for intensive, communal bison huntingin north-central Montana. Radiocarbon-dated charcoal layers fromalluvial and colluvial deposits associated with driveline complexesindicate that peak fire activity over the past millennium occurredcoincident with the use of these features (ca. 1100–1650 CE). How-ever, comparison of dated fire deposits with Palmer Drought Se-verity Index reconstructions reveal strong climate–fire linkages.More than half of all charcoal layers coincide with modest pluvialepisodes, suggesting that fire use by indigenous hunters enhancedthe effects of climate variability on prairie fire regimes. These re-sults indicate that relatively small, mobile human populations canimpact natural fire regimes, even in pyrogeographic settings inwhich climate exerts strong, top-down controls on fuels.

anthropogenic burning | bison hunting | pyric herbivory |climate–fire relationships | hunter-gatherers

Fire is a significant earth system process that has shaped theecological history of landscapes and biomes (1). Climate exerts

significant top-down controls on fire regimes, thereby influencingthe production of fuels and their drying for combustion (2). Infuel-limited grasslands and dry woodlands, wet periods are nec-essary to produce sufficient abundance and continuity of fuels toburn (3). This is particularly true for mixed, shortgrass, and fescueprairies of the North American interior, where fire activity cor-relates with wet phases (i.e., pluvial episodes) and correspondingincreases in grassland productivity (4, 5).Despite a wealth of documentary evidence of fire use for a

diverse range of purposes (6–10), the role of anthropogenicburning in the historical ecology of North American ecosystemsremains controversial (11). This is particularly so for low-densityhunter-gatherer populations, who are assumed to have minimallyimpacted ecosystems and their fire regimes (12). However,grassland burning by aboriginal hunters in Western Australia hasmeasurable impacts on fine-scale biodiversity, even as thelandscape-scale fire rotation is virtually identical betweenhuman-managed and unmanaged areas (13). Furthermore, thesehuman-managed areas are less vulnerable to climate-driven ex-treme fire years, suggesting that hunter-gatherer burning canbuffer impacts of climate variability on these fire regimes (14).This aligns well with fire history observations of human-managedlandscapes in similarly grass-fuel–dominated forests of theAmerican Southwest (15) and California (16).

In recent decades, two different narratives of human/climate/fire relationships have emerged (17). For many fire scientists,human activities and climate are mutually exclusive influences onfire regimes. From this perspective, the primary human impact isto dampen climate–fire relationships, producing fire historiesthat deviate from the expected “natural” regime (18, 19). Humanpopulation size or density are often used as a proxies for thepotential magnitude of human influence, with the assumptionthat smaller, mobile groups impact surrounding fire regimes lessthan larger, more sedentary populations (19). In this framework,demonstrable climate–fire correlations in paleofire records sug-gest that human activities had little or no influence on fire re-gimes (12, 18, 20, 21).From an ethnographic and archaeological perspective, anthro-

pogenic burning is abundant and diverse (6, 7, 22, 23), and bothforagers and farmers can measurably impact surrounding fire re-gimes, largely by buffering the impacts of climate variability (13–15, 24, 25). This effect is mostly unintentional, as human decision-making in these contexts probably prioritizes social and economicconcerns over suppressing climate effects (26, 27). Although thismodel acknowledges the abundant record of traditional fire uses,it is compatible with elements of the first perspective. Both suggestthat human contributions to fire regimes involve a suppression ordampening of the climate–fire relationship. However, ethno-graphic research also indicates that indigenous peoples are keenlyaware of climate impacts on fuels and may leverage this by lightingmore fires when fuels are abundant (27).By using an empirical approach, we demonstrate that at least

some Native American and First Nations foraging groups used fireto manipulate the location of herds for episodic mass harvesting of

Significance

The relative importance of human activities and climate inshaping fire regimes is controversial. In North American grass-lands, climate exerts strong top-down influences on fuels. Forcenturies before the introduction of the horse, Native Americanand First Nations hunters built and used landscape features onthese grasslands to harvest bison en masse. Charcoal layers as-sociated with drivelines indicate that fire was an important partof these hunting practices. Furthermore, correlation of dated firedeposits and climate records indicate that ancient bison huntersburned in response to favorable climate conditions. This studyindicates that climate and human activities are not mutuallyexclusive factors in fire histories; even relatively small groups ofhunter-gatherers can enhance climate impacts.

Author contributions: C.I.R. and M.N.Z. designed research; C.I.R., K.L.H., and M.M.H.E.performed research; C.I.R. analyzed data; and C.I.R., M.N.Z., and K.L.H. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Published under the PNAS license.1To whom correspondence should be addressed. Email: croos@smu.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1805259115/-/DCSupplemental.

Published online July 23, 2018.

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bison. This fire use was constrained by favorable climate condi-tions, thus aligning anthropogenic burning with the same climateprocesses driving natural fire regimes and enhancing climate ef-fects on landscape fire activity. We demonstrate this by combininglandscape archaeology with offsite geoarchaeology to reconstructpaleofire activity that is spatially isomorphic with the scale ofancient driveline hunting complexes and comparing these recordsvs. regional paleoclimate reconstructions.

Ecological and Archaeological ContextThe northwestern Great Plains is characterized by shortgrassprairie dominated by fescue (Festuca spp.). These grasslands arefuel-limited (28) but provide ideal winter forage for Americanbison (Bison bison) (29). Historically, naturally ignited fires oc-curred during the summer, whereas anthropogenic fires occurredpredominantly in the spring or the fall (SI Appendix, Fig. S9)(30). Lightning-strike densities in the fescue grasslands areamong the lowest on the continent (<4 flashes per km−2·y−1;National Lightning Detection Network), with the rate of light-ning fires less than 0.25 km−2·y−1 in neighboring shortgrassprairies (31). Nevertheless, lightning and anthropogenic fires areimportant in these ecosystems (30–32), with positive impacts onpostfire grassland productivity. Even low ignition rates can pro-duce substantial burn areas when climate produces sufficientlyabundant, continuous, and dry fuel (33). Festuca hallii (formerlyFestuca scabrella), the dominant grass species in these prairies, ismost productive after early spring burns followed by wet condi-tions, and burned patches are more productive than unburnedpatches in dry years as well (28, 34).In the northern RockyMountain foothills, fescue prairie uplands

are dissected with river valleys that provide the ideal setting for ahunting strategy that involves driving herds of bison over steepbluffs along the valleys (29, 35). Three key landscape features ofthis hunting strategy are the bison grazing area (i.e., gatheringbasin) (29) and the linear arrangements of rock cairns (i.e.,drivelines) that channel bison herds to the precipice (i.e., jump)(35). Together, gathering basins, drivelines, and jumps were part ofa linked strategy to increase returns by harvesting bison en masse.In the northern Plains, mass harvesting of bison by using

stone-lined drives and wooden corrals is first associated with

Late Archaic cultural complexes such as Bracken (800–100 BCE)(36, 37) and, more widely, with Besant (100 BCE to 500 CE) (37,38), when it may co-occur with increased fire activity, increasedcarrying capacity, and ultimately population growth and expan-sion into new areas (39). However, specialized bison hunting byusing drivelines is most characteristic of the Old Women’s phase(ca. 900–1750 CE) (38, 40). Old Women’s phase sites are foundin southern Alberta, southwestern Saskatchewan, and north-central Montana (29, 35, 37, 40). Recent research on theBlackfeet Indian Reservation in north-central Montana hasdocumented the timing and intensity of use of driveline com-plexes along the Two Medicine River (Fig. 1). Radiocarbondates from bone beds span approximately 900–1650 CE. Peakdriveline use from throughout the valley dates to 1400–1650 CE,although this record may be biased toward the latest periods ofuse (SI Appendix and SI Appendix, Fig. S1). Native populations inthe region may have been impacted by European diseases in the17th century, with strong evidence for smallpox epidemics in1730 and 1781–1782 CE reducing some northern Plains pop-ulations by as much as 50% over this interval and causing tem-porary regional depopulation (41, 42). However, at the end ofthe 18th century, European traders observed 150 individuals perhunting band and as many as 2,000 in aggregated winter campsalong the Rocky Mountain Front (43). These observations mayprovide a rough proxy for population in our study area during theOld Women’s phase.

ResultsTwo offsite stratigraphic exposures (KAP 2 and KAP 4; SI Ap-pendix and SI Appendix, Table S2) from tributary drainages tothe Two Medicine River have evidence of grassland burningassociated with driveline complexes (Fig. 1). The drainage basinfor each locality overlaps with the driveline and gathering basinfor specific bison jump complexes (SI Appendix, Figs. S2 and S5).These localities are on the north (Two Medicine/Schultz com-plex) and south (Stranglewolf complex) sides of the Two Medi-cine River, so charcoal deposits in each locality represent firesoverlapping specific driveline complexes and cannot representfires that spread from one locality to the other. Each exposurehad five to eight continuous charcoal layers interbedded within

Fig. 1. Map locating the study area in the NorthAmerican Great Plains prairie ecoprovinces (A) andMontana, USA (B). The primary map (C) situates thetwo geoarchaeological localities relative to the ma-jor driveline complexes and other archaeologicalfeatures.

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alluvium or colluvium. Charcoal-rich layers that extended >3 mlaterally (Fig. 2 and SI Appendix, Figs. S3 and S6) were sampledas evidence for elevated fire activity in the drainage basin for thatlocality (“fire deposit”). Eleven of these fire deposits were di-rectly dated via accelerator mass spectrometry 14C assays of ag-gregated grass charcoal (44, 45). Four charcoal samples fromsedimentary deposits or soils bracketing the fire deposits werealso radiocarbon-dated to provide constraining ages on theearliest and latest deposition of charcoal layers at each locality.Final radiocarbon measurements were calibrated by using aBayesian algorithm in BCal (https://bcal.sheffield.ac.uk/; ref. 46),with stratigraphic relationship between dated deposits used tofurther constrain the final posterior probability distributions.This Bayesian approach also permitted us to assign probabilitiesfor the ages of two fire deposits from KAP 4 that could not bedated directly. Posterior probability distributions for directly andindirectly dated charcoal deposits were summed for each locality togenerate a locality-specific history of enhanced fire activity (Fig. 2).Dated fire deposits indicate that unusually high prairie fire

activity occurred between 1100 and 1650 CE, approximately co-eval with the Old Women’s phase archaeology of the Two Med-icine valley (ca. 900–1650 CE; SI Appendix, Fig. S1). More thanhalf of the 13 deposits date to the period of most intensive use ofthe driveline complexes between 1400 and 1650 CE. It is impor-tant to note that, at both localities, there are alluvial and colluvialdeposits without distinct charcoal layers that postdate the OldWomen’s phase through at least the late 19th century. Further-more, stratigraphic evidence from both localities and a strati-graphic exposure adjacent to KAP 2 span at least 500 y before theOld Women’s phase but lack comparable fire deposits (SI Ap-pendix). The absence of charcoal deposits in strata that predateand postdate the Old Women’s phase suggests that the fire regimewas qualitatively different between 1100 and 1650 CE than

centuries before or after. The absence of dense charcoal depositsafter 1650 CE is particularly noteworthy because otherwise anal-ogous alluvial and colluvial deposits are common at our localities.Fires, both natural and anthropogenic, persisted after this date inthe historic record (30), and charcoal is commonly dispersed in thealluvium, colluvium, and soils that postdate this time. However,the unique circumstances that promoted the formation of dense,continuous charcoal layers exclusively dates to the Old Women’sphase; these are not found in strata that predate or postdate thisperiod in our records. Given the spatial proximity to drivelinecomplexes and the timing of fire activity relative to their use, weinterpret the radiocarbon and stratigraphic data as strong evidencefor anthropogenic burning as part of Old Women’s phase bisonharvesting technology.Although it is possible that fire was used to initiate bison drives

as well, we think it more likely that strategic grassland burning wasused to lure bison to the gathering basins of particular drivelinecomplexes. The seasonality of communal hunts, as indicated frombone bed faunal remains and ethnohistorical summaries (47),suggest that the drivelines were used from late fall to early spring.Spring burning (for a fall hunt) or fall burning (for a spring hunt)would have created attractive conditions for subsequent grazing bybison, who prefer to graze recently burned patches (30, 48). Incontemporary practice, this use of fire to manipulate grazer be-havior is called pyric herbivory (49). This practice would haveincreased the predictability of locating bison in the gathering basinfor a particular driveline complex, effectively enhancing encounterrates (13) as hunters-gatherers do across the globe (22).Although our radiocarbon records provide strong evidence for

anthropogenic burning during the Old Women’s phase, they alsoshow significant relationships with climate (Fig. 3). Six of thethirteen dated charcoal deposits coincide with decadal-scale plu-vial episodes (50), as would be expected even under “natural”

Fig. 2. Profile photos indicating directly and indirectly dated charcoal layers, bison bones, and other dated deposits for KAP 2 (A) and KAP 4 (B), and summedprobability distributions from Bayesian radiocarbon calibrations for fire and nonfire deposits for KAP 2 (C) and KAP 4 (D). Numbered peaks correspond tolabeled fire deposits in A and B.

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conditions. However, the remaining seven charcoal deposits cor-respond to modest or short wet periods, which should have lowerfire activity. Furthermore, although decadal-scale pluvial episodespersist after 1650 CE, comparable charcoal layers did not form ateither of these localities. Together, these patterns suggest thatanthropogenic burning during the Old Women’s phase leveragedpyric herbivory when there were sufficient fuels to use fire incommunal hunts, even amplifying the effects of short or modestwet periods on grassland fire regimes.The peak probabilities for individual fire layers for the two

localities indicate that the associated driveline complexes maynot have been used simultaneously. This supports the historicalview that use of specific complexes may have been rotated acrossthe landscape (51). Decadal-scale pluvial episodes were longenough to support burning at multiple driveline complexes(spaced at 8–15-km intervals along the river) over the course ofmost wet periods, so we would expect for the peak probabilitiesof more than 2 of the 13 fire layers to be synchronous if multipledrivelines were in use simultaneously at the decadal scale (Fig.2). This evidence of bison kill-site rotation, pyric herbivory, andfire regime change before the emergence of horse culture in the18th century has important implications for refining the eco-logical impact of bison hunters on northern prairie landscapes,which we discuss subsequently.

DiscussionAlthough many scholars treat climate and human activity asmutually exclusive influences on fire activity, our study indicatesthat human impacts on fire regimes are not so simple (52).Humans and fire both respond to climate influences, but notnecessarily in the same way or to the same effect. Climate vari-ability provides the context for human decisions to burn relativeto some specific goal, and that agency may result in human ac-tivities enhancing (53) or buffering (14, 15) climate impacts onfire regimes at different scales. For example, communal bisonhunts on the Great Plains may have primarily occurred duringwet periods (54), meaning that human activity and fire respondedto the same climate signals. In our study, the use of pyric herbivoryin communal bison hunts enabled indigenous foragers to leverageeven minor climate opportunities to burn, thus amplifying the

effects that these wet periods had on prairie fire regimes andcreating a stronger apparent climate–fire relationship. Within theannual cycle, communal bison hunts focused on bison winteringareas in the foothills (40, 55), contributing to differences in theseasonality between burning for pyric herbivory and naturalfires (30).Furthermore, our study indicates that hunter-gatherers can

have a significant impact on fire regimes (56) even at relativelylow population densities, suggesting that population estimates ormodes of economic organization may be useful, but not neces-sarily sufficient, proxies for human influence on fire activity (19).In our study, distinguishing the human role in fire history is onlypossible because of linkages between the fire record and thearchaeological record at comparable pyrogeographic scales (17,52). The visibility of the anthropogenic fire regime may havebeen facilitated by the relatively low rates of lightning ignitions,creating greater opportunity for anthropogenic ignitions togenerate distinctive fire patterns. Future paleofire studies shouldbe mindful that the presence of a strong climate–fire signal doesnot rule out the possibility of human influence, nor should ar-chaeologists expect that anthropogenic burning will necessarilybe independent of climate effects on local fire regimes.That relatively small populations of foragers can have impacts

on fire regimes has important implications for assessing the An-cient Anthropocene hypothesis (24, 56, 57). In this hypothesis, fireuse and land clearance associated with early agriculture generatedongoing, net-positive carbon releases to the atmosphere, stabiliz-ing Holocene climates (58). In part, this hypothesis assumes thatforagers before the invention or adoption of agriculture did notimpact fire regimes or carbon cycles. Although the pyric herbivorywe document for the Old Women’s phase was unlikely to producecarbon releases comparable to forest clearance, it indicates thathunter-gatherer burning can have meaningful impacts on fire re-gimes and needs to be considered as the Ancient Anthropocenehypotheses continues to be debated (56).Ethnographic (13, 59, 60) and ethnological research (7, 22, 23,

26, 61) makes clear that there is no universal “hunter-gatherer”use of fire. Past and present foragers vary widely in their homeenvironments, as well as their social, economic, and politicalstructures (62, 63). However, ethnographic diversity likely

Fig. 3. Summed probability distribution of radiocarbon dates for fire deposits from KAP 2 and KAP 4 (A) compared with decadal variability in PDSI by using10-y averages of antecedent moisture conditions (27), a proxy for fuel production (B). Decadal-scale pluvial episodes are indicated by vertical blue bars.Dashed vertical black lines indicate short, mild wet periods that correspond to peaks in the fire probability distribution.

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underrepresents hunter-gatherer diversity in the deep past. Bisonhunters occupied the Northern Plains before and after the OldWomen’s phase. However, at our localities, they did not have thesame impacts on prairie fire regimes. It is important to note thatthe Old Women’s phase involved intensified, communal bisonhunting using stationary landscape features and was associatedwith seasonal aggregation of hunting bands. This temporarilyelevated the population density, fostered increased economicand political complexity (29, 35), and concentrated the impactsof anthropogenic burning. With the introduction of the horse inthe early 1700s, hunters became much more mobile even as so-cial and economic complexity persisted. Northwestern Plainsforagers continued to use prairie fires as a tool and as a weaponbut did not generate the same type of fire record as during theOld Women’s phase. The greater mobility of equestrian hunters,and probably of pedestrian hunters before the Old Women’sphase, meant that fire use would have been much more widelydistributed on the landscape, rather than concentrated in asso-ciation with driveline complexes. Extensive fire use by mobileforagers may be less easily distinguished from natural fires,particularly if it also leveraged climate opportunities to burn.In this context, it is worth reiterating that changes in socio-

economic organization can drive abrupt, climate-independentfire regime changes (17, 64, 65). This appears to have occurred inour study area first with the transition to intensive, place-basedbison procurement and pemmican processing for trade (35, 66),followed by the adoption of the horse and transition to eques-trian bison hunting (67). Although simple dichotomies betweenforagers and farmers may not be useful for interpreting humanimpacts on past fire regimes, closer attention to particular so-cioeconomic patterns and transitions may be useful for dis-criminating changes in human impacts on fire regimes.It is also important to stress that our records do not represent

indiscriminate or broad, landscape-scale burning. Pyric herbivoryfor bison hunting would be useful only if the hunters couldpredict with a reasonable degree of certainty where bison weregoing to graze in the next hunting season. Burning too large anarea or in areas too far from driveline gathering basins wouldundermine this goal. Therefore, it is likely that the fire patterns wehave identified here are restricted to the uplands where drivelinecomplexes occur: a fairly large but not necessarily continuous areafrom central Montana into southern Alberta and southwesternSaskatchewan. The asynchrony of dated fire layers from our twolocalities suggests that, even at the spatial scale of the TwoMedicine River and the temporal scale of an individual pluvialepisode, anthropogenic burning was not uniform across the area.Finally, our study lends support to the growing body of work

that indicates that coupled human–natural fire regimes should beunderstood as integral parts of food webs (27, 49, 68). Humanuses of fire are largely, but not exclusively, related to their im-pacts on food resources (22). Changes in ecosystem structureassociated with human uses of fire have important impacts onbiodiversity (i.e., pyrodiversity) that have trophic consequences,only some of which have direct bearing on human economies anddiets at the time (69). Because of these important trophic con-sequences and their impacts on the historical trajectories of fire-prone environments, it is important that we understand the fullrange of human influences on historical fire regimes, even if theentanglements of human activities and climate are not readilyapparent at first glance. Understanding the full complexity ofhuman experiences with fire and climate will be essential ascontemporary communities plot a course to coexist with fire on aflammable planet (17, 52, 70).

MethodsFrom 2007 to 2012, M.N.Z. directed archaeological survey and excavations inthe Two Medicine River valley upstream from the confluence with BadgerCreek (Fig. 1). By using intensive survey methods, rock cairns were identified

and mapped by using GPS (20). Alignments of rock cairns that formeddriveline complexes were identified based on their spatial pattern, as dis-tinct from other features (e.g., Tipi rings, effigies). Test excavations (1 m ×1 m) were conducted at four bone bed sites associated with driveline com-plexes (Badger, Racine, Stranglewolf, Two Medicine/Schultz) to supplementthe regional survey. Stratigraphic excavations at the Kutoyis bison bone bedand processing site revealed five thermal features (66) and at least threediscrete layers of burned bison bone deposits. Bison bone, canid bone,charcoal, and blowfly pupae were radiocarbon-dated from the tested bonebeds and stratigraphically from Kutoyis bone bed and processing features (SIAppendix, Table S1). Dates from these strata were calibrated by using aBayesian algorithm in BCal (46) that used the radiocarbon dates and strati-graphic relationships as priors. Although the test excavations at regionalbone bed sites are probably biased toward the younger deposits, the sum-med probability distributions of these calibrated radiocarbon dates indicatethat driveline and bone bed use of the Two Medicine Valley spans at least900–1650 CE with peak probabilities between 1400 and 1650 CE (SI Ap-pendix, Fig. S1). Excluding a recent historic occupation, dates from theKutoyis site are more representative of its total use and span 1200–1800 CEwith peak probabilities between 1400 and 1650 CE (SI Appendix, Fig. S1).Luminescence dates from driveline cairns and rock rings demonstrate thatdrivelines, bone bed features, and campsites were coeval (71).

In 2010 and 2011, C.I.R. directed geoarchaeological fieldwork at four lo-calities associated with three driveline complexes (Kutoyis, Two Medicine/Schultz, and Stranglewolf). Two of these localities (KAP 2 and KAP 4) wereselected for further analysis based on the presence of dense charcoal depositsthat were interbedded within well-stratified alluvial (KAP 2) and colluvial (KAP4) sequences. The stream terrace at KAP 2 is inset into an older terrace (KAP 3).Although it did not evince any charcoal deposits, samples were collected fromKAP 3 to provide a longer-term context for KAP 2. In the field, erosional profileswere manually cleared by C.I.R. and K.L.H. to clarify the relationships betweendepositional strata and overprinted soils. Deposits were described, measured,and sampled for extracting charcoal for radiocarbon dating.

Deposits associated with elevated fire activity were identified based oncontinuous, laterally traceable (>3 m horizontal continuity), charcoal-richdeposits (elsewhere in this paper “fire deposits”). Charcoal was dispersedin alluvium, colluvium, and soils at both localities superposed above andbelow the fire deposits. Although undoubtedly associated with prairie fireactivity, dispersed or discontinuous charcoal deposits were treated as part ofthe background fire activity (30) and not as evidence of elevated fire activitythat is the subject of this study.

The glacial till that is parent material for the alluvial and colluvial sedi-ments in our study includes inherited ancient charcoal, thereby potentiallycontaminating bulk and aggregated radiocarbon dates and making themartificially older (SI Appendix). Because of the potential for contamination byolder carbon, the youngest dates on aggregated “grass” charcoal werepreferred for developing the fire chronologies. The accepted charcoal dateswere calibrated by using IntCal 13 (72) in BCal (46) using traditional andBayesian methods that include both stratigraphic relationships and the ra-diocarbon measurements as priors. Bayesian methods allowed us to estimatethe probability of the ages for the two undated charcoal deposits from KAP4, thus providing indirect dates. Traditional and Bayesian calibrations arepresented in SI Appendix, Table S2. Posterior probability density functionsfor each dated fire deposit were summed by using annual probability valuesto generate the summed posterior probability density functions for eachlocality (Fig. 2) and for both localities together (Fig. 3). Summed probabilitydistributions were then compared as time series vs. summed probabilitydistributions from the regional archaeological sites (SI Appendix, Fig. S1) andvs. the Palmer Drought Severity Index (PDSI) reconstruction for grid point 83in the North American Drought Atlas (NADA) version 2a (50), the nearestgrid point in NADA to our study area (Fig. 1B). As fire activity and fuelsrespond to previous moisture, decadal variability in the PDSI record wasmeasured by creating a 10-y antecedent average of annual PDSI values(PDSIant10). For the analysis, decadal-scale pluvial episodes were identified ascontinuous periods with positive PDSIant10 values for 10 y or longer.

ACKNOWLEDGMENTS. We thank Jesse Ballenger, who directed us to theselocalities; John Murray, who provided institutional support through theBlackfeet Tribal Historic Preservation Office; the Blackfeet Tribal BusinessCouncil and the Running Fisher family, who authorized archaeologicalresearch on their ancestral sites; and David Meltzer, Tom Swetnam, andthree anonymous reviewers for constructive comments on earlier drafts. Thisresearch was supported by National Science Foundation Awards BCS-0918081and BCS-1266118 (to M.N.Z.); a Marr Scholarship at Southern MethodistUniversity (to M.M.H.E.); and the Institute for the Study of Earth and Man, theDepartment of Anthropology, and the Dedman College of Humanities andSciences at Southern Methodist University.

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