Wildfires are a major disturbance process in the western United States, shaping many ecosystems. Wildfire activity has dramatically increased in recent decades due to factors like human settlement, human-caused climate change, and past fire suppression. While climate, especially drought, is often considered the primary driver, research suggests that human actions, including Indigenous fire practices, may have significantly altered past fire regimes. This study focuses on the Great Basin-Colorado Plateau region, specifically the Fish Lake Plateau in central Utah, where a unique history of maize agriculture provides an opportunity to examine the interplay between climate, human activity, and fire. The Fish Lake Plateau offers a valuable case study because of the combination of existing records of past fire activity, vegetation change from sedimentary archives, tree-ring reconstructions of drought, and a well-documented archaeological record. Understanding the extent of past Indigenous fire use and its long-term impacts is crucial for effective modern fire management strategies in the region.

The legacy of Indigenous fire use is increasingly recognized in various regions, with evidence suggesting pre-European fire practices had a more significant impact on modern flora than previously believed. However, the influence of pre-European Indigenous populations on North American fire regimes remains debated due to a lack of integrated studies examining fire-human-climate interactions. A prevailing paradigm suggests that low-density human populations had minimal landscape-scale impacts, especially in remote mountainous areas. However, growing archaeological and ethnographic evidence from various regions, including the Great Plains, Great Basin, Sierra Nevada, and Rocky Mountains, supports the role of human activities alongside climate in shaping fire regimes. By altering ignition patterns for various purposes (clearing vegetation, enhancing plant regrowth, driving game), Indigenous fire use may have significantly altered fire frequency and fuel availability, subsequently impacting forest composition and structure. The integration of paleoecological and archaeological data, a methodology known as applied historical ecology, is vital for separating the influences of human-caused and natural fire regimes. This requires demonstrating that observed vegetation and fire changes directly correlate with human activity, rather than solely with natural climate-fire relationships.

This multidisciplinary study combines paleoecology, paleoethnobotany, dendrochronology, and archaeology to reconstruct vegetation change, fire activity, drought, and human activity over the past 1200 years at Fish Lake, Utah. Sedimentary charcoal accumulation rates (CHAR) serve as a proxy for past regional fire activity, with higher CHAR values indicating more biomass burned. Sedimentary pollen accumulation rates (grains cm⁻² yr⁻¹) indicate changes in regional vegetation composition. Summed probability distributions (SPDs) of calibrated radiocarbon-dated archaeological sites were used as a proxy for pre-European human activity. A tree-ring reconstruction of drought and a record of El Niño Southern Oscillation (ENSO) variance were included as potentially important climatic drivers of fire activity. The study area, Fish Lake, is strategically located on the boundary between the Great Basin and Colorado Plateau, with mixed-conifer to spruce-fir forests at high elevation (2700 m). Two sediment cores were collected and analyzed for various proxies (e.g., charcoal, pollen). A generalized additive model was used to assess the relationships between fire activity, human activity (represented by the SPD), climate (drought and ENSO), and vegetation (arboreal pollen to non-arboreal pollen ratio). Pairwise Pearson’s correlation tests were also conducted to further analyze the relationships between variables. The resulting time series data were analyzed using a 30-year time window and statistical methods appropriate for the nature of the data. Radiocarbon ages were calibrated using IntCal13 datasets and a chronological model was developed using the BACON software. The study integrates multiple datasets with varying temporal resolutions, carefully considering potential limitations and biases inherent in paleoecological and archaeological data.

The study revealed a strong association between high fire activity (900-1400 CE) and a peak in human activity associated with the expansion of maize agriculture into the Basin-Plateau region. This period, coinciding with the Medieval Climate Anomaly (MCA), a time of increased aridity, showed that fire activity was more strongly correlated with human activity than with drought or ENSO. Pollen and macrofossil analysis from archaeological hearths showed the utilization of various wild resources and the use of quaking aspen as fuel. The generalized additive model indicated that prior to European settlement, human activity was the only significant predictor of fire activity. After 1400 CE, with the abandonment of farming and a decline in population density, fire activity decreased significantly. The post-European settlement period (after ~1900 CE) shows a dramatic increase in fire activity compared to the previous 1200 years, attributed to factors like fuel accumulation due to fire suppression, warming temperatures, and human-caused ignitions. The analysis revealed a positive correlation between charcoal influx and population density, with climate and fuel type having little influence during the farming period. Following the abandonment of farming, climate became the primary driver of fire activity. This implies that Indigenous fire management practices actively mitigated climate's effects on fire regimes during the high period of agricultural activity.

These findings challenge the assumption that high-elevation mixed-conifer fire regimes are solely climate-driven. The results support a growing body of evidence demonstrating humans' capacity to modify forest structure and composition through fire, thereby reducing climate's dominance as a fire regime driver. The high fire activity during the farming period, even during a relatively dry period, suggests that frequent, low-severity fires, similar to those used by other Indigenous groups, may have reduced fuel loads, preventing large-scale wildfires. The shift after 1400 CE highlights the combined effects of population decline and climate change on fire regimes. The contrast between the pre- and post-European settlement periods underscores the significant impact of modern fire suppression on forest structure and fire regimes. The current pattern of wildfire activity is unlike anything observed in the previous millennium and is influenced by factors not previously experienced in the region. Therefore, strategies for addressing current fire activity could benefit from exploring the potential of incorporating elements of pre-European Indigenous fire management. This approach may offer more sustainable solutions for mitigating large wildfires, particularly in light of ongoing climate change.

This study demonstrates the significant influence of Indigenous land use practices on high-elevation fire regimes in the Basin-Plateau region. Frequent, low-severity burning during the farming period actively mitigated the effects of climate variability, particularly drought, on fire activity. The subsequent decline in human activity and the shift to cooler, wetter conditions during the Little Ice Age resulted in a shift toward climate-dominated fire regimes. Understanding these historical fire regimes informs modern fire management strategies by highlighting the potential of incorporating elements of pre-European Indigenous practices to mitigate the risks associated with large-scale wildfires in a changing climate. Further research could explore the spatial extent and ecological implications of Indigenous fire management across broader geographical scales and consider the cultural context of such practices in depth.

While this study provides strong evidence for the influence of Indigenous land use on fire regimes, certain limitations should be considered. The temporal resolution of the multiproxy datasets may obscure short-term climatic variations, potentially impacting the model's ability to fully capture the relationship between climate and fire. Also, the proxy used for human activity (summed probability distribution of radiocarbon dates) provides an estimate of overall human presence, not necessarily the precise intensity or location of fire use within the Fish Lake watershed. Future research could incorporate more detailed proxies for human activities and potentially a more refined temporal resolution of the climate data. It is also important to acknowledge the complexity of modern fire regimes and the multiple interacting factors influencing them.