Abstract: To assess the relative importance of climate and humans in burning over recent millennia, we reconstruct fire history from three independent sources, specifically, historical, fire-scar, and charcoal data. We then compare the results to data on climate and population. We also construct a statistical model to determine the extent to which climate alone can predict biomass burning. The historical data were obtained from the United States Department of Agriculture Forest Service estimates of the extent, use, and destruction of original saw timber (i.e., trees older than 50 y in 1630 CE) from 1630 to 1940 CE ( 2 ). Fire-scar data ( n  = 359) were obtained from the International Multiproxy Paleofire Database, which provides annually resolved data on fires from around 1400 CE to present (primarily from dry interior ponderosa pine forests). We estimate temporal trends in fire by using the proportion of site records with scars for each year. Sedimentary charcoal accumulation rates from 69 records are used to identify decadal-to centennial-scale trends in biomass burning over the past three millennia, and a subset ( n  = 41) of records are further analyzed to reconstruct fire-episode frequency over this interval. Temperature, drought, and population changes are used to understand the changes in forest fire activity, while temperature and drought are used in a statistical regression model (Generalized Additive Model or GAM) to explain the variability in biomass burning during the past 1,500 y. Anthropogenic impacts on fire occur against the backdrop of climate variability. There have been marked human influences on western wildfires since Euro-American settlement, including increased ignitions (e.g., from forest clearance, agriculture, logging, and railroads), and fire exclusion (e.g., from landscape fragmentation, grazing, and suppression). Other significant impacts on vegetation and fire occurred indirectly, such as changes in plant succession pathways (e.g., when shrublands previously maintained by frequent fire converted to forest after fires were regularly excluded) and the introduction of nonnative species. Indigenous fire use prior to settlement varied in intensity, extent, and persistence, and probably varied with season, migration, and cultural and technological developments. Climate is generally considered to be the primary control of contemporary fire regimes in the western United States ( 1 ). Climate influences fire primarily through variations in temperature and precipitation (and thus available moisture). Increasing temperature leads to more fires; however, this is only true if sufficient vegetation is present to support the spread of fire. Precipitation is also linked to fire—there must be sufficient moisture to support continuous vegetation on the landscape, but not so much that fuels never become dry enough to burn. Forest fires in the western United States have been increasing in extent for several decades, prompting much research into the causes and consequences of such changes. The overall level of fire activity in a given place is governed by processes relating to climate, people, and vegetation that operate over decades and centuries; yet, most fire research is based on much shorter time scales. Given that future climate change is expected to drive fire activity well above its historical range of variability, a long-term perspective provides essential context to current changes. We use sedimentary charcoal accumulation rates to construct baseline levels of burning for the past 3,000 y in the American West; we then compare this record to independent fire-history data obtained from historical records and fire scars. We also create a statistical model, based only on independent temperature and drought reconstructions, that predicts 85% of the variability in biomass burnt (thought to reflect area burnt) prior to the 1800s, before human and ecological influences became dominant. Large shifts in biomass burning since the 1800s are not unprecedented, but their causes and effects differ greatly from climate-driven shifts in the past. Fire regimes are currently in disequilibrium with the climate, due to the opposing forces of fire exclusion practices (e.g., grazing and fire suppression) and global warming; consequently, a large “fire deficit” exists. The 20th Century Fire Deficit in the Western United States. Observed and predicted changes in biomass burning diverge in the late 1800s, despite increasing temperature and drought (Fig. 4 A ). Observed biomass burning, fire scars, charcoal-based fire frequencies, and human-caused fires decline to levels similar to the levels during the LIA. In contrast, predicted biomass burning rises from 1880 CE to present, which is consistent with increased temperature and drought. This pattern indicates that nonclimatic factors became the dominant control of fires around 1880 CE. The decline in fires during the 20th century may be explained by multiple factors. In the late 1800s, widespread domestic livestock grazing reduced grassy fuel loads, compacted soils, and greatly reduced fire frequencies. By 1900 CE, the western frontier had largely closed, and intentionally set fires probably declined due to changing attitudes and policies towards fire. In addition, landscape fragmentation from trail and road building limited the spread of fire. Furthermore, after the 1940s, fire suppression became highly effective, preventing the spread of many forest fires. However, ecological factors also played a role, as the number of young stands and aspen stands, which are resistant to burning, increased after logging and previous extensive burning. Consequently, a fire deficit now exists and has been growing throughout the 20th century, pushing fire regimes into disequilibrium with climate. Hence, while current levels of large-scale biomass burning ( 1 ) remain within the realm of natural variability during the past 1,000 y, if levels of burning were to come into equilibrium with climate, they would exceed the natural range of variability experienced in at least the last 3,000 y. Three independent fire-history reconstructions for the western United States show that there have been large changes in wildfires since the 1800s. In earlier periods, changes of this scale were driven by climate; in the past 200 y, human behavior has played a much larger role. Fire suppression practices have greatly reduced fire, whereas global warming has increased the probability of fire. A widening gap, or fire deficit, therefore exists between actual levels of burning and expected levels of burning given current climate conditions. Recent increases in catastrophic wildfires in the West are an indication of this deficit, and suggest that current fire suppression practices are unsustainable. Fires are projected to increase even further in coming decades, and may require reevaluation of fire management policies and potential investment of additional resources. Climate Controls Fire Until the Beginning of Euro-American Settlement of the West. A statistical model, calibrated using data from 500 to 1800 CE, predicts multidecadal-to-centennial changes in biomass burning from temperature and drought area indices. Climate explains most of the multidecadal-to century-scale variations of biomass burning. Temperature alone accounts for half of the total variance of biomass burning, while drought area explains about 1/3 of overall variance. Downward trends were recorded for temperature, drought, biomass burning, fire frequency (see full text), and predicted biomass burning during the past two millennia until the Settlement Era ( Fig. P1 A – E ). In contrast, the human population gradually increased until ca. 500 y ago, when Native American populations began to collapse after European contact ( Fig. P1 F ). The population began to recover around 350 y ago, and then rose dramatically since settlement. Biomass burning was high when climate changed rapidly at the beginning of the Medieval Climate Anomaly (MCA), which was around 1000 CE when both temperature and drought were also high. Fire activity was high again at the onset of the Little Ice Age (LIA), around 1400 CE, when drought was high. However, as the LIA progressed, biomass burning declined substantially, which coincided with large declines in temperature, drought, and population ( Fig. P1 A , D and E ). Fig. P1. ( A ) Smoothed and standardized 25-year (gray) and 100-year (red) trend line through standardized biomass burning records ( n  = 69) along with predicted biomass burning based on a GAM (black dashed line) fit to the 100-year charcoal values. ( B ) Smoothed proportions of dendrochronological sites recording fire scars. ( C ) Estimated historical sawtimber affected by lightning- and human-caused fires in the western United States ( 2 ) ( D ) Smoothed gridded temperature anomalies for the western United States ( 3 ). ( E ) Smoothed Palmer Drought Severity Index for the western United States ( 4 ). ( F ) Population estimates for the western United States ( 5 ). All smoothed curves are plotted with 95% bootstrap confidence intervals. The transition from the LIA into the Settlement Era is marked by a sharp increase in burning, as evidenced by historical records, fire-scar, and (observed and predicted) charcoal data. The increase in fire activity, which reached its maximum ca. 1850–1870 CE (Fig. 4 C ), is consistent with increased ignitions from land clearance, logging, agriculture, and railroads during settlement, and also with increasing temperature and drought, which reached a maximum between 1700 and 1900 CE ( Fig. P1 A – E ). Thus, climate and humans acted synergistically to increase fire.

Abstract: Fire is a primary mode of natural disturbance in the forests of the Pacific Northwest. Increased fuel loads following fire suppression and the occurrence of several large and severe fires have led to the perception that in many areas there is a greatly increased risk of high-severity fire compared with presettlement forests. To reconstruct the variability of the fire regime in the Siskiyou Mountains, Oregon, we analyzed a 10-m, 2,000-y sediment core for charcoal, pollen, and sedimentological data. The record reveals a highly episodic pattern of fire in which 77% of the 68 charcoal peaks before Euro-American settlement cluster within nine distinct periods marked by a 15-y mean interval. The 11 largest charcoal peaks are significantly related to decadal-scale drought periods and are followed by pulses of minerogenic sediment suggestive of rapid sediment delivery. After logging in the 1950s, sediment load was increased fourfold compared with that from the most severe presettlement fire. Less severe fires, marked by smaller charcoal peaks and no sediment pulses, are not correlated significantly with drought periods. Pollen indicators of closed forests are consistent with fire-free periods of sufficient length to maintain dense forest and indicate a fire-triggered switch to more open conditions during the Medieval Climatic Anomaly. Our results indicate that over millennia fire was more episodic than revealed by nearby shorter tree-ring records and that recent severe fires have precedents during earlier drought episodes but also that sediment loads resulting from logging and road building have no precedent in earlier fire events.