Large fire occurrence was highly synchronous across broad spatial scales and significantly correlated to short-term climate anomalies. Specifically, precipitation anomalies 90 days prior to the fire had the strongest influence on large fire occurrence and percent high severity fire more than temperature or relative humidity. The authors suggest that short-term drought events may likely have the strongest influence on western wide fire synchrony.

The authors found that spreading fires in ponderosa pine and dry mixed-conifer forests were associated with prior wet conditions and larger fires affecting mesic forests were more strongly associated with extreme drought conditions the year of fire, historically. Currently, spreading fire only occurs during persistent drought, suggesting that fire suppression has altered fuels so that dry conifer sites are no longer fuel-limited, and fire regimes in these systems behave like more mesic mixed-conifer sites.

The authors did observe that increases in fire occurrence typically occurred during years of drier than average conditions with either one to three moist years preceding the fire year at all of their study sites; however, synchronous fire years were less common than unique fire years at any one study site, suggesting that local conditions may more strongly influence fire activity.

The authors’ results support the intermediate fire-productivity hypothesis across the various ecoregions of the globe. They found that increasing productivity increases fire activity until a moisture threshold is met where moisture levels then increasingly limits flammability and fire activity. Fire that occurs in moist and productive regions, where fuel is not limiting, depends on changes in climate, such as increased temperatures or periods of drought, to burn. Conversely, very arid systems are fuel limited and sensitive to changes in fuel loading and continuity.

Longer projections to the end of the century suggest that fire probabilities will increase in the mid- to high-latitudes and decrease in the tropics. They suggest that although future temperature is expected to rise across the globe, future fire activity seems to be driven by moisture availability in many areas.

The authors found that ponderosa pine forest stands that are closer in proximity to grassland- and shrubland-dominated sites burn at higher frequencies, and furthermore, grassland-adjacent stands are more sensitive to fluctuations in climate. Fire years in grassland ecosystems in the Colorado Front Range are synchronous with year-of-drought and 2-year lagged moisture conditions, which promote increased fuel growth and continuity. Grassland-adjacent forests are more likely to burn due to the increased continuity of fuels when conditions permit fire in grasslands, whereas forest further from grasslands may be fuel limited during drought years.

The authors suggest that although typically previous fires kept the spread of subsequent fires in check most of the time, climate and weather can override fuel limitations and spread into recently burned landscapes.

The study landscape was characterized by patches of ponderosa pine forest divided by a matrix of pinyon–juniper (PJ), sagebrush shrublands, and small grasslands. The authors did not find regional synchrony between the patches of ponderosa pine within the study area, suggesting that bottom-up controls, such as fuels, may have more strongly influenced the historical occurrence of fire in this region than climate. The authors did find, however, as the scale of the analysis increased to larger regional areas, climate-driven fire synchrony increased, and fire was associated with dry condition in the year of the fire and wet conditions two years prior to the fire. The authors suggest that climate may be less influential on fire activity in ecosystems that are fuel limited.

Fire occurrence in mixed-conifer forests generally required more extreme drought to burn compared to ponderosa pine forest ecosystems. However, the connectivity between the forest types may have influenced the fire frequency in mixed conifer as fire moved upslope. Mixed conifer forests are generally not fuel-limited, so the authors suggest this may explain the relationship between fire occurrence in these forest types and the wet-year lags. The authors also found that the last major fire in spruce-fir-dominated forest at the highest elevations occurred during the worst single-year drought in 1685 and was synchronized across distant mountain ranges regionally.