Legacies of Indigenous land use shaped past wildfire regimes in the Basin-Plateau Region, USA

The study addresses how Indigenous land use, specifically prehistoric farming and associated burning practices, influenced high-elevation fire regimes in the western United States. While climate—especially drought—has been viewed as the dominant driver of fire activity, recent work suggests human-caused ignitions may have significantly shaped forest composition and fire regimes, even during cooler, wetter periods. There remains controversy regarding the scale of Indigenous impacts on North American fire regimes, particularly in mountainous regions often assumed to be dominated by lightning ignitions. This research examines the past 1200 years on the Fish Lake Plateau, Utah, to determine the relative roles of climate and human land use in shaping fire activity and forest structure.

Prior research documents increased wildfire activity in recent decades in the western U.S., often linked to climate change, fuel accumulation, and human ignitions. However, studies in the Sierra Nevada and elsewhere indicate that human-caused fire can better explain forest composition changes during prolonged cool/wet periods, implying a significant Indigenous role in shaping fire regimes. The extent of pre-European Indigenous fire use remains debated, with a persistent paradigm that low-density populations had minimal landscape-scale impacts, especially in remote mountains where lightning is considered the main ignition source. Nonetheless, archaeological and ethnographic evidence across the Great Plains, Great Basin, Sierra Nevada, and Rockies demonstrates Indigenous use of fire for farming, foraging, hunting, travel, and resource enhancement, potentially altering fire frequency, fuels, and forest structure. Applied historical ecology integrating paleoecology and archaeology is necessary to disentangle human versus climatic controls on past fire regimes.

Study area: Fish Lake, a high-elevation (2700 m) lake on the Fish Lake Plateau at the Great Basin–Colorado Plateau boundary (south-central Utah), surrounded by mixed-conifer to spruce-fir forests with aspect-driven species distributions including Engelmann spruce, subalpine fir, Douglas-fir, aspen, ponderosa pine, piñon, juniper, and mountain mahogany. Sedimentary proxies and chronology: Two sediment cores (freeze core FLFC2_17_14, 49 cm; UWITEC core D14, 11 m) were retrieved through ice (Feb 2014) in 32 m water depth, 5 m apart. Cores were extruded at 0.5 cm intervals; loss-on-ignition at 550 °C was used to correlate cores and build a composite age-depth model using 210Pb (D14) and two AMS 14C dates (FLFC). Radiocarbon ages were calibrated with IntCal13 and modeled with BACON. All proxy time series were standardized to a common 30-year binning interval (constant median sampling interval from charcoal influx) by averaging within bins. Fire activity: Sieved sedimentary charcoal accumulation rates (CHAR; pieces cm−2 yr−1) were used as a proxy for regional (<40–50 km) biomass burning. Higher CHAR indicates more biomass burned. Vegetation: Sedimentary pollen influx (grains cm−2 yr−1) was used to infer regional (>10 km) vegetation composition, summarized as arboreal pollen (AP), non-arboreal pollen (NAP), and paleoethnobotanically significant taxa. Human activity: Summed probability distributions (SPDs) of calibrated radiocarbon dates were generated from 308 dates across 46 archaeological sites in Sevier County using IntCal13 in the rCarbon package. A 100-year moving average and 100-year binning were applied to reduce calibration and sampling biases, serving as a proxy for local prehistoric population density and land use intensity. Climate: Local summer (May–Aug) drought was reconstructed using multiple regression models relating historical PDSI to tree-ring width chronologies (including newly collected and ITRDB site ut549), achieving R2 = 0.57 with sufficient depth back to 800 CE. ENSO variance over the past millennium was taken from a published reconstruction. Statistical modeling: Multivariate generalized additive models (GAMs; mgcv in R) with Poisson error, log link, and quasi-likelihood estimation were used to assess effects of predictors (Population SPD, drought PDSI, fuel AP:NAP ratio, ENSO variance) on charcoal influx. Population smoother used k = 4. Model fit statistics included estimated degrees of freedom, F-statistics, p-values, and proportion deviance explained. Residual temporal autocorrelation was checked with autocovariance functions. Pairwise Pearson correlations among variables were also computed.

- Fire-human synchronicity: High fire activity (elevated CHAR) between 900–1400 CE coincides with a peak in Indigenous human activity (SPD), then both decline after ~1400 CE and remain low until ~1900 CE when fire activity increases with Euro-American settlement and the end of the Little Ice Age. - Climate context: Drought extremes (PDSI) occurred between 1100–1300 CE; ENSO variance declined during the Medieval Climate Anomaly (MCA; more La Niña-like) and increased during the Little Ice Age (LIA) to present (more El Niño-like). Despite this, during peak farming, fire activity was not significantly related to drought or ENSO variability. - Vegetation signals: During ~1050–1200 CE (peak farming), both AP and NAP influx increased; NAP increased relative to AP, and paleoethnobotanically significant taxa peaked, suggesting disturbance-adapted herbaceous plants thrived. Post-1900, AP increased dramatically relative to NAP, reflecting conifer densification with fire suppression and altered land use. - GAM results: Prior to European settlement, climate (drought, ENSO) and fuels (AP:NAP) together explained over 74% of deviance in fire activity, yet only human activity had a significant effect (F = 8.652; p = 0.00109). Drought (PDSI), fuels (AP:NAP), and ENSO showed negligible effects (non-significant p-values). - Correlations: Fire activity (CHAR) was positively correlated with population density (r ≈ 0.88) and negatively with AP:NAP (r ≈ −0.62), with negligible correlations to PDSI and ENSO. - Post-1850 changes: After establishment of the Fish Lake Forest Reserve and fire suppression (late 1890s), conifers, sagebrush, and piñon-juniper increased while grasslands and aspen declined. Since the 1970s, CHAR increased roughly fourfold compared to the previous 1200 years; overall a threefold increase in CHAR is evident in the modern era. Since 1984, ~16 large (>400 ha) fires within 50 km occurred, including a 682 ha fire within the watershed (2002) and Utah’s largest wildfire (141,143 ha) ~70 km away (2007). - Interpretation: During 900–1400 CE, frequent low-severity, human-set fires likely increased edible plant resources and maintained more open, ruderal-rich conditions, masking climate’s influence on high-elevation fire regimes. After ~1400 CE, with farming abandonment and lower population densities, climate regained dominance over the fire regime under cooler-wetter LIA conditions.

The findings demonstrate that Indigenous land-use practices, particularly during the Fremont farming apex (900–1400 CE), were the primary driver of high-elevation fire activity on the Fish Lake Plateau, overriding climatic influences such as drought and ENSO. Archaeobotanical evidence of edible ruderal taxa, presence of maize, and faunal remains, in conjunction with elevated CHAR and increased paleoethnobotanical pollen relative to other herbs, indicate intentional and frequent low-severity burning to enhance resource yields. This anthropogenic burning likely reduced surface fuels and limited the size of climate-driven fires despite periods of extreme drought variability during the MCA. Following the abandonment of agriculture around 1400 CE, decreased human-caused ignitions and cooler-wetter LIA conditions lowered biomass burning and promoted denser, more arboreal fuels, enabling a shift toward infrequent, higher-severity fire regimes characteristic of modern subalpine forests. In the 20th century, fire suppression, fuel accumulation, warming, and increased ignitions produced unprecedented CHAR levels relative to the prior millennium. Overall, the study challenges the assumption that high-elevation mixed-conifer fire regimes are purely climate-driven and highlights the capacity of human fire use to restructure forests and modulate climate-fire interactions.

This study integrates sedimentary charcoal and pollen records, archaeological radiocarbon-derived population proxies, and tree-ring climate reconstructions to show that Indigenous land use, particularly frequent low-severity burning associated with prehistoric farming, substantially shaped high-elevation fire regimes on the Fish Lake Plateau over the past 1200 years. Human activity exerted the only significant modeled effect on fire activity during the farming apex, with climate becoming dominant only after farming ceased around 1400 CE. In the modern era, combined human and climatic influences have produced fire activity exceeding historical variability. Management implications include considering the reintroduction of frequent, low-severity Indigenous-style burning to reduce fuel loads and mitigate large, severe wildfires under ongoing warming. Future research should refine temporal resolution of multiproxy datasets, expand regional comparisons, and further disentangle human versus climate drivers across different elevations and forest types.

- Temporal resolution: The 30-year binning and inherent temporal uncertainties in paleo proxies may mask short-term climate variability and transient fire-climate relationships, potentially limiting detection of climate-fire correlations. - SPD proxy biases: While mitigated via 100-year binning and smoothing, summed radiocarbon probability distributions can be influenced by sampling intensity, calibration curve structure, and site-specific dating practices. - Spatial representativeness: Macro-charcoal integrates signals over regional scales (~40–50 km), potentially missing small, local, low-severity fires common at lower elevations during the LIA. - Inference of causality: Despite strong associations, attributing specific ignition sources or exact burning practices relies on indirect evidence from multiproxy correlations and archaeological context.