The authors related the increase in sheep herding and livestock grazing, beginning in approximately 1830, to a significant reduction in fire frequency across the Chuska Mountains. The current forest stand is dense with even-aged cohorts that established in the early 1900s, similar to other stand in the Southwest. The authors suggest, however, fire suppression and land use change alone were not responsible for the drastic changes to the forest structure. Instead, anthropogenic disturbance concurrent with favorable climate conditions for ponderosa pine regeneration likely both influenced the major structural alteration of these forest stands.

Historically, fire was highly synchronous across the region and appeared to be linked to climate conditions. The cessation of fire beginning around 1883 in these forests has led to an abundance of resprouting oak, a species typically highly susceptible to fire. In contrast to presettlement conditions, currently there is abundant fuel able to support high?intensity fire behavior, including torching through live fuel ladders and crown fire, in hot, dry, windy weather. However, herbaceous fuel loading is probably greatly reduced in the contemporary forest.

The authors' results support the varying constraints hypothesis that wildfire occurrence requires key biophysical factors to coincide, however, the relationship between fuel moisture and fuel availability varies along fairly predictable spatial patterns. In areas where fuel is abundant, fuel moisture availability has the strongest constraint on fire activity, whereas in fuel-limited systems, fire is constrained more strongly by fuel availability. In these ecosystems, lagged fuel moisture is more representative of subsequent potential fire activity.

The authors found that during the period of deglaciation, there was an increase in the level of burning and fire occurrence, although fire activity varied across the continent likely due to spatially complex climate controls and their consequent effects on vegetation changes.

The authors’ reconstructions of fire and climate within El Malpais National Monument show a cessation of fire occurrence around 1790. Prior to 1790, fire was frequency and synchronous with periods of drought. After 1790, an increase in annual precipitation decreased fire frequency and shifted the fire season from midsummer to late spring.

Reduced winter moisture either in the form of reduced snowpack or cool season rain likely increase extreme fire activity earlier in the spring (May and June) through impacts to fuel conditions. Longer freeze-free seasons favor cold-intolerant annual grasses, including cheatgrass (Bromus tectorum) red brome (Bromus rubens) and buffelgrass (Pennisetum ciliare) allowing them to germinate during warmer temperatures and expand their range creating highly flammable ecosystems.

The authors found that recent declines in cool season precipitation appear to have advanced the timing of the fire season across the Sonoran, Mojave deserts in the Southwest as well as the Colorado and the Great Basin desert. CGM models in this study project winter precipitation to continue to decrease across the warm deserts in the study area and an increase in the annual mean temperature by 2.5-3°C by the middle of the 21st century. This future climate change is likely to increase the potential for megafires across the Southwest by increasing the frequency of weather conditions conducive to extreme fire activity, especially an increase in temperature and decrease in corresponding precipitation and humidity indicators. Along with the predicted changes in temperature and precipitation, periods of extreme fire danger are expected to increase by one to three weeks across these areas leading to the occurrence of active fire seasons every fifth year to every other year. Furthermore, in these ecosystems, climate change may result in a fire-invasive feedback loop in deserts where increases in the amount and continuity of annual grasses lead to more extensive and frequent wildfire, thereby increasing the potential for invasive species to proliferate while decreasing the ability of slower-regenerating, native shrub species.

The authors found that ENSO has a strong interannual influence on PDSI and, hence, fire activity across the western U.S., whereas PDO captures the variation in fire across decadal scales in the Pacific Northwest. AMO influences the strength of both ENSO and PDO across multi-decadal scales across the continent. In the Southwest, prior wet years, often associated with warm ENSO events, prime fuels for synchronous fire activity during the subsequent fire season.

The author proposes that climate change may affect fire activity via alteration of fuel conditions, fuel loading, or changes in ignitions. Changes in moisture regimes and length of the fire season could alter fuel conditions, increasing their flammability.

The authors found evidence of large patches (>100 acres) of stand-replacing fire in upper elevation mixed-conifer forests prior to European settlement in the region via aspen and conifer recruitment pulses, corresponding fire scar and mortality dates, and lack of surviving trees prior to large fire dates. This suggests that recent large patches of high severity fire are within the historical range of variability for upper elevation forests. However, in aspen stands, fire suppression over the past 130 years has likely affected the age and stand structure of these stands. Aspen stands shifted from frequently burned stands with dense and multi-aged cohorts of trees toward a monoculture of open, same-aged trees.