The authors found that the 1 April snow water equivalent (SWE) is correlated with the number of lightning-ignited fires and area burned. High levels of persistent snowpack later into the year decreases the possibility of lightning-ignited fires during the fire season. Conversely, low levels lead to an increase in fire activity earlier in the fire season. The type and duration of winter precipitation greatly affects the fuel moisture conditions of the early fire season, and to a lesser extent, the late fire season as fuels are typically dry regardless of the previous winter’s precipitation. However, the relationship between SWE and area burned was weaker as bottom-up controls were stronger predictors of fire behavior on the landscape. However, they found that fire season length is not directly related to fire severity in that a longer fire season does not necessarily result in more severe fire across the landscape, especially in more drought tolerant ecosystems. Convective instability associated with high temperatures may also lead to increased lightning occurrence and thus increased ignition rates. Should spring SWE decrease, landscape flammability may increase and become more variable.

The authors found that fire activity was most common during dry years. Precipitation two years preceding fire years resulted in widespread synchrony across all three sites. They also found a significant shift in the effects of La Niña before and after the 1830s. Prior to the 1830s, La Niña years were significantly associated with fire activity. After 1830, the relationship between fire and La Niña at Peña Nevada was not significant. Extreme ENSO events have been more erratic in this area since the 1970s. Large fire years in 1983 and 1998 occurred during the El Niño phase, but were extremely dry, but the 1992 extreme El Niño was very wet. The authors suggest that ENSO may no longer be a reliable predictor of precipitation and/or the associated fire activity in this region.

Fuel treatments prior to fire did not significantly influence the establishment of non-native species post-fire nor increase the potential for invasion. High severity fire, however, was a strong predictor of non-native species establishment, and invasion was more likely with increasing fire severity, indicated by depth of char in this study.

The author found that historically, the San Francisco Peaks of northern Arizona were dominated by ponderosa pine with an understory of scattered individuals of Douglas-fir, limber pine, and white fire. Currently, however, the species composition has shifted due to fire suppression to dense stands of less fire-tolerant Douglas-fir and white fir.

The authors found that climate-fire relationships varied across the vegetation gradient. Typically fire years occurred during dry years following wet years, except at the lowest elevation sites that were not significantly drier than normal. At the highest elevation, fires occurred during periods that were significantly drier than the 99th percentile.

This article found strong relationships between topographic variables and high severity fire occurrence. Severe fire was more likely to occur on north-facing slopes at high elevations due to the interaction of biomass production and fuel accumulation which in turn influences burn severity. Without the influence of humans on this landscape, climate, topography, and vegetation interact strongly with each other to control burn severity in a region dominated by semi-arid forest, where moisture limits vegetation production.

Consistent with other literature in the Southwest, the authors found that fire activity was associated with anomalously wet years followed by anomalously dry years, typically within the El Niño/La Niña ENSO cycle. However, 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.

The authors found an abrupt cessation of fire after 1868, concurrent with Euro-American settlement, although they also found fire quiescent-periods around 1685 to 1735 and again from 1824 to 1861 related to increased moisture patterns. At the current study site, shade tolerant species, such as white fire and Douglas-fir dominated the understory and were regenerating most abundantly, however, during the historic periods of frequent fire, ponderosa pine dominated the stand until fire ceased in the region. They also found an increase in fire area burned in the mixed-conifer stand when dry years were preceded by one or two wet years resulting in an accumulation of fuels.

The authors suggest that fire severity varied greatly across most of these forests, and that high severity fires were common and burned thousands of hectares at a time. High severity fire typically varied along elevation and moisture gradients, so that areas with increased dominance of Douglas fire typically burned at higher severities while xeric sites dominated by ponderosa pine typically burned at low-severities consistently. However, periods of drought or wind events can override fuel conditions leading to higher fire severities and variation from year to year. Furthermore, under a more variable-severity fire regime, fuel loads tend to be more spatially heterogeneous which is consistent with fuel data from the US Rocky Mountains.

Finally, the authors found that tree regeneration was abundant after, and even enhanced by, high severity fire.

The authors found that climate-fire relationships exist across the western U.S. with approximately 39% (1916-2003) and 64% (1977-2003) of area burned directly related to climate, however the strength of the seasonal or interannual effects of climate varied across the different vegetation types. Southwestern deserts and semiarid desert ecoprovinces tend to be fuel limited. Wetter conditions in the winter prior to the fire season followed by dry condition during the year of the fire resulted in greater area burned. The authors suggest that climate change may lead to one of two paths: first a reduction in fuels due to persistent drought, limiting area burned, or second, wetter conditions leading to an increase in vegetation biomass along with an earlier onset of warmer temperatures that dry these fuels and increase the area burned. The mountainous ecoprovince of Arizona and New Mexico also did not correlate to year-of-fire precipitation, but did have a relationship with precipitation from the winter previous. However, temperature during the year-of-the fire was related to area burned in these areas as well as annual (water year) PDSI.