Historical records of national timber resources document the increasing impacts on forests from Euro-American fuelwood use, lumbering, and land clearing across the country ( Fig. S1 ). Fire use also increased ( Fig. 2 A). Information on regional differences in timber use and damage are not available, but the primary spatial pattern of Euro-American impacts on fire likely followed the westward expansion of the frontier from the Missouri River ca. 1830 to its final close by the early 1900s. The recovery or reforestation following the widespread disturbance of the 1800s can be seen in stand-age data from forest inventories from the western United States ( Fig. S1 ). These data show a modal year of stand origin in the first decade of the 20th century, with half the stands originating between 1870 and 1950 CE.

The western United States is comprised of four regions in the USDA historical dataset: the North and South Pacific and the North and South Rocky Mountains. The original (ca. 1700 CE) stand volume for these four regions together was estimated to be 2.24 × 10 9 board feet (bf, 1,000 bf = 2.36 m 3 ), with about 73% in the north and 27% in the south. The “original” forest in the western United States accounted for about 18% (58.7 million hectares) of the total original U.S. forest area, whereas the remaining stand in 1940 accounted for about 66% of the total forested area in the United States.

Charcoal peak density shows distinct maxima similar to the composite charcoal influx record over the past 1,500 y ( Fig. 2 C and D). The sharpest maxima in peak frequency occur at the beginning of the MCA and LIA, and, as was the case for the other records, during the early 1800s; smaller maxima occur at the ends of the MCA and LIA. The association of peak density maxima with rapid or large warming or cooling events is consistent with that observed during deglaciation ( 64 ), and during the last glacial interval ( 65 ); increased fire at these times would be supported by vegetation changes that increase fuels available for combustion (e.g., due to increased mortality).

Combining the 69 charcoal influx records ( Fig. 2 C; Fig. 3 ) provides an indication of the trends and variability in biomass burning across the western United States during the past 3,000 y. Burning declined slightly over the past 3,000 y, with the lowest levels attained during the LIA, (ca. 1400–1700 CE/550-250 cal y BP) and in the 20th century. Peaks in burning occurred during the MCA ( ca. 950–1250 CE/1000-700 cal y BP) and during the 1800s CE ( Fig. 3 ). There is a large and rapid shift from high burning in the 19th century to low burning in the 20th century that is comparable in magnitude to the decline in fire that occurred during the transition into the LIA.

A temporal summary of the fire-scar data ( Fig. 2 B; Figs. S3 and S4 ) shows that the proportion of sites recording scars increased from about 1400–1800 CE, with a broad maximum between 1800 and 1850. The earliest part of this trend (i.e., prior to ca. 1500 CE) is more uncertain than latter parts, however, because (i) fewer sites were recording fire activity early in the millennium, so the data reflect changes in burning at fewer than 40 locations, and (ii) the early increase in the proportion of sites recording scars may partly reflect an increasing number of trees susceptible to fire scarring at each site ( 63 ). Comparison of analyses using either one or more or two or more scars to indicate fire years across the West as a whole, as well as for the north and south show that trends in fire activity summarized with this method are robust to alternative minimum scarring criteria ( Fig. S4 ).

Charcoal data from the West are more prevalent in the north (where lakes are more common) than in the south ( Fig. 1 C). Charcoal influx rates (CHAR) vary continuously during the past 1,000 y at most sites, although the nature of within-record variability differs from site to site. Some records show low CHAR for the past millennium followed by high CHAR during historic settlement, for example, whereas other records show high variability from decade to decade. In many records there is a tendency toward high CHAR between 1100–1200 CE, between 1800–1900 CE, and in the most recent samples. Low CHAR are common ca. 500 y ago, particularly in the north.

All the fire-scar data and most of the charcoal data come from forested ecosystems ( Fig. 1 A; Fig. S2 ). Fire-scar records (n = 369 sites, > 50,000 individual scars) are more evenly distributed between north and south than the charcoal data, but there are more fire-scar data from low-and midelevation xeric interior forests of the Rocky Mountains than in the higher elevations or more mesic forests, although some data do come from less xeric/midelevation forests; e.g., in Colorado ( 61 ) ( Fig. 1 A). In addition, fires do not always leave scars, especially in forests with high fire frequencies and on young trees, so the fire-scar data likely underestimates true fire frequency ( 62 ). General patterns in the fire-scar data however, should be robust. For example, it is clear that northern sites tend to burn less frequently than southern sites ( Fig. 1 C, Fig. S2 ), and fires were more frequent from ca. 1600 to 1900 CE than after that interval. Specific years when widespread fires occurred are evident when the fire-scar records are not overlapping ( Fig. S2 ). Widespread fires are easier to identify in the northwest in part because there are fewer fires in general, but there also appears to be greater fire synchrony in the north than in the south in general. Widespread fires occur fairly regularly during the high fire period from 1600–1900 CE, but an increase in small fires is also evident from ca. 1850 through the early 1900s (most visible in central and northern records; Fig. S2 ). The most salient feature of the fire-scar data is the widespread, abrupt reduction in fires around 1900 CE.

Climatic and Human Influences on Fire in the West.

Mean annual temperature (MAT) and summer drought (drought-area index, DAI) were summarized in a similar fashion to the charcoal data (see Methods) and also show a general downward trend, at least until the early 1800s (Fig. 2 A–D). The long-term decline in fire is also evident for the 1,500 y prior to the beginning of the joint record at 500 CE (Fig. 3). Superimposed on this trend are several large and generally parallel variations in biomass burning and fire frequency (i.e., “fire activity”) during the past 2,000 y (Fig. 2 C and D). Fire activity was high at 1000, 1400, and 1800 CE, and low at 900, 1600, and 1900 CE. The rise in fire at 1000 CE occurred at the beginning of the MCA, when temperatures (MAT) and drought area (DAI) were both high. Biomass burning remained high for at least two centuries during the MCA (from 750 to 1000 cal y CE), whereas fire frequency declined at 1100 CE. Another increase in fire activity occurred at the beginning of the LIA around 1400 CE, when drought increased rapidly. Biomass burning reached its late Holocene minimum during the LIA, and fire-episode frequency was also low at this time, although it is presently lower. The decline in fire activity during the LIA occurred as drought declined and temperatures reached their 1,500-y minimum (Fig. 2 E and F). Similar trends and centennial-scale variability in climate and fire until the 1800s suggests that baseline levels of fire activity in the West were predominantly controlled by climate.

High fire activity during the MCA has been documented by individual local studies based on both fire-scar and charcoal records [e.g., (54, 66)], and our results indicate that such activity was widespread. Biomass burning was high throughout the MCA and peaked at 1200 CE during a period of severe drought; the level of burning then was similar to that reached about a century ago (during historic settlement). Fire frequency also reached a peak during the MCA at ca. 1000 CE, when both drought and temperatures were particularly high. Fire frequency in the West was higher at this time than at any other time in the past 1,000 y. Warm, dry conditions in the western United States during the MCA resulted from prevailing La Niña-like conditions in the tropical Pacific, which is consistent with both increased drought and high temperatures (Fig. 2 E and F) (10, 67).

Biomass burning and fire frequency were also high during the transition into the LIA, during a prolonged period of severe drought (Fig. 2E); fire then declines to minimum levels at 1500 CE and ca. 1575 CE for fire frequency and biomass burning, respectively (Fig. 2 C and D). The fire-scar record becomes dense enough to analyze during the LIA and indicates very low levels of fire activity then. Evidence from glacial advances in the Sierra Nevada range of California and the Cascade Range of the Pacific Northwest (68) suggest decreases in summer temperature during the LIA of ∼2 °C in Sierra Nevada (69). Native American populations also collapse after approximately 1,500 CE, which would have significantly reduced the impact from human-caused fires where they were important previously (Fig. 2G). The combination of low values for drought, temperature, population, biomass burning, and fire frequency during the LIA suggest that multiple factors, including reduced vegetation productivity from lower temperatures, reduced fire-conducive weather (wetter conditions), and fewer human-caused fires to some extent, combined to reduce fire activity generally during the LIA.

The charcoal influx record over the past 3,000 y (Fig. 3) indicates that variations in biomass burning have been particularly large over the past 1,000 y. The negative excursions in biomass burning during the LIA and in the past century for example, are remarkable in the context of the past 3,000 y. In general however, large shifts in the magnitude and rate of burning have occurred throughout the past. For example, there is an abrupt decrease of charcoal influx around 2,000 y ago comparable to the first step in the decrease between the MCA and LIA, and there is a gradual increase commencing around 1300 CE that is analogous to that leading into the MCA. There are several features of the charcoal records that are not well explained by climate, for example the maximum in peak density around 800 CE, but overall, until the 1800s, increases in temperature and drought are coeval with increases in charcoal influx and peak density.

To further quantify the relationship between biomass burning and climate, we developed a statistical regression model (Generalized Additive Model or GAM; Fig. S3). The regression was fit using centennial changes in biomass burning from temperature and DAI from 500 to 1800 AD (i.e., from the beginning of the joint temperature and drought records to settlement). Climate explains most of the multidecadal to century-scale variations of biomass burning (R2 = 0.85; F = 47.0; p < 0.001). Temperature alone can account for half of the total variance of biomass burning (R2 = 0.53; F = 51.2; p < 0.001), while drought area can explain about one-third of the overall variance (R2 = 0.34; F = 24.4; p < 0.001). The dashed black curve on Fig. 2C shows the fitted (to 1800 CE) and predicted (1800–2000 CE) values from the model (see also Fig. S5). The general features of the influx record are captured, including the upturn in influx at the end of the LIA, and a subsequent peak in biomass burning around 1800 CE. The observed and predicted influx curves diverge after 1800 CE, when the combined effects of landscape fragmentation and fire exclusion reduced biomass burning in the face of post-LIA and 20th century temperature increases. Because the model was fit only to data prior to 1800 CE, we checked whether the predictions over the past 200 y are extrapolations beyond the range of the calibration data (Fig. S6). The values of the predictor (climate) variables fall outside the general envelope of climate values only after 1980 CE, so the divergence between observed and predicted charcoal influx beginning in the 1800s CE is most likely due to nonclimatic controls.

Prior to the 1800s and within the temporal and spatial scales of this study, human activity, expressed as population from the HYDE 3.1 database (Fig. 2G), does not appear to influence either the charcoal influx or peak density variations. Population gradually increased (in contrast to biomass burning, which decreased) until after 1500 CE, when European contact resulted in an abrupt population decline owing to disruptions such as disease and warfare (70). Although the low levels of biomass burning attained throughout the Americas during the LIA are often ascribed to contact (71, 72), the general decline in biomass burning was underway before contact [e.g., (26)], and seems largely accounted for by climate. The divergence between the observed and predicted (by climate) charcoal influx curves after 1800 CE is thus the main expression of human impacts on fire.

During the transition out of the LIA and into the Settlement Era, historical records, fire-scar, and charcoal data (both observed and predicted) track increasing temperatures and drought, showing a multicentury increase in forest fire activity from very low levels during the LIA to very high levels of burning between 1700 and 1900 CE (Fig. 2 A–D).

The close association between observed and predicted biomass burning prior to the late 1800 s suggests that climate changes alone can explain the increase in fire activity between 1600 and 1800 CE. The more variable (25-year smoothed) biomass burning curve (Fig. 2C, thin gray line) however, shows that fire activity increased to very high levels in the 1800s despite an apparently earlier decline in observed and predicted biomass burning (Fig. 2C). The peak in fire activity in the mid to late 1800s is undoubtedly due in part to increased human-caused burning, which reaches its maximum from 1850–1870 CE (Fig. 2A). Settlers arriving in the western United States at this time ignited many fires for clearing forest and brush, lumbering, railroad construction, agriculture, arson, etc. Road building and technological advances were also linked to increased anthropogenic burning (and erosion), such as with the development of steam power and railroads that created sparks leading to large numbers of wildfires until the early 1900s (when the railroads were required to start clearing woodlands within 100 feet of tracks to prevent fires). The introduction of the band saw in 1880 CE, and powerful logging machinery in 1890 CE, for example, also led to changes in harvesting that further altered forests and fuels as well as the locations of intentional and accidental fires. Increased anthropogenic burning in the west from 1850–1900 CE is widely recognized in dendrochronological studies (61), but increased variability in moisture availability associated with ENSO also contributed to increased burning then (74).

Prior to the arrival of large numbers of Euro-Americans in the western United States, the fire-history records show a short-lived decline in fire in the 1810s CE. The annual fire-scar data indicate that this decline in burning was driven by very low fire activity in the years 1816 and 1818 CE; only 13 sites record scars in 1816 and 15 sites in 1818 compared with a century-long average of 36 sites. These results are consistent with the hypothesized effects of widespread cooling following the eruption of Mount Tambora in 1815 CE (75).

Observed and predicted changes in biomass burning begin to diverge in the late 1800s creating a fire deficit that has been growing throughout the 20th century (Fig. 2C). Predicted biomass burning generally rises from the late 1800s CE to present, consistent with increased temperature and drought trends. In contrast, observed biomass burning, as well as fire scars, charcoal-based fire frequencies, and human-caused fires decline rapidly. The minimum in burning during the 20th century is similar to the low fire activity levels that occurred during the LIA. Less than 10% of the original sawtimber stand remained at that point, mostly on the Pacific Coast (48), so while it is plausible that a reduction in forest cover contributed to reduced burning, this seems unlikely because timber extraction and destruction does not necessarily lessen wildfire risk, and in some cases increases it (76).

Multiple factors combined to cause the 20th century fire decline (77), largely due to human activities but also due to ecological processes following the intensive fire activity in the 1800s. Grazing was perhaps the earliest primary cause of fire exclusion in the West. Hundreds of thousands of livestock were introduced to pine forests and grasslands in western states (40, 42) in the late 1800s. The widespread herds reduced grassy fuel loads, compacted soils, and sharply reduced fire frequencies. Road and trail building also created fire breaks that limited the natural spread of fires. Cultural changes were also taking place that may have reduced fire ignitions well before effective fire suppression in the 1940s. By 1900 CE, the western frontier had largely closed and several large catastrophic fires, such as the Peshtigo Fire in Wisconsin in 1871 that killed over one thousand people (78) were helping to change attitudes towards fire and fire policies. In 1891, the Forest Reserve Act was introduced that allowed the President to reserve forests from the public domain (79), and in 1905 the U.S. Forest Service was established with a primary mission of suppressing all fires that occurred on reserved lands. Responsibility for fire management was transferred from the Army to the National Park Service when it was created in 1916, and full suppression remained the policy for the next five decades (with greatly increased efficiency in the 1940s) (79). Natural ecosystem changes also likely contributed to decreased fire in the 1900s, however. Increased fire in the late 19th century, for example, resulted in young stands in subalpine forest that were less susceptible to fire in the early 20th century. A major increase in fire-resistant aspen stands due to 19th century fires also likely reduced biomass burning and fire frequencies.

In general, western U.S. forests were fundamentally changed in the 1800 and 1900s from previous centuries. The increased burning of the 1800s and the subsequent widespread exclusion of fire altered stand structure and composition, understory vegetation and fuel loads, and facilitated entry of nonnative species (76). Coupled with timber extraction and land clearance, the consequences for western forests were dramatic.

The fire deficit identified here might appear to contrast with observations of recent increases in western U.S. fire activity (1) and also to the well established fire-climate interactions documented across the region (14). These apparent differences can be reconciled by explicit consideration of the time scale of the variations. We show that mean or baseline levels of biomass burned and fire frequency decreased substantially during the past century compared with previous centuries; the recent increase in “fire activity” (i.e., large-wildfire occurrence) is therefore occurring during a period of unusually low levels of biomass burning. Furthermore, the increase observed since 1980 has a short duration compared with the longer decline in burning from the 19th to 20th centuries, or increases at the beginning of the MCA or following the LIA. Similarly, the associations between large fire occurrence, fire frequency, and climate that are well documented in literature on western U.S. fire regimes (61, 80) are also dependent on scale and fire-regime dimension; interannual and even multidecadal fire synchrony for example, may have been as strong in the past as they are today with no “decoupling” of fire and climate on these time scales. During the past two centuries, however, centennial-scale changes in biomass burned and fire frequency however, are decoupled from climate due to the strong human influences on forests and fires.

Although the changes in fire described here were undoubtedly widespread, our results do not address several important aspects of fire history of the western United States. First, the trends do not reflect subregional patterns of burning or changes in burning in grasslands and shrublands. A good example is the increase in area burned in California during the 20th century. Area burned has expanded consistently during the past 100 y as a result of increasing population growth and drought (81, 82), however this pattern is not reflected in the composite curve due to a lack of paleofire data from that state. Second, differences in interannual to decadal-scale variations in burning such as those due to ENSO are also not reflected in our data. Third, the recent increases in large wildfires across western states also do not appear in the composite tree-ring or charcoal summaries, most likely because their occurrence is too recent to be incorporated into most sediments or fire-scar records. However, increases in fire are evident in individual charcoal records, particularly from the northern forests (Fig. 1).