The authors found that fires within the ponderosa pine stands did not spread to the adjacent pinyon-juniper woodland. Historically, fire in pinyon-juniper was typically severe and stand-replacing, but limited in size with fire return intervals of 300 to 400 years. This has resulted in numerous small patches of same-aged pinyon stands.

The authors found a positive feedback between the initial severity of a fire and the severity of the reburn fire. High severity reburns usually occurred after a high severity burn. This pattern was stronger in the mesic, high-elevation forests of the Gila National Forest where the frequency of fire is longer. Conversely, low severity fires were typically followed by subsequent low severity fires, thereby maintaining the low-severity fire regime characteristics typical of these ecosystems.

The authors found that that regional synchronous fire years in montane ecosystems coincided with severe drought years. No antecedent climate conditions were found to have any significant effects on fire activity.

The authors found that mechanical thinning with or without prescribed fire was effective at reducing wildfire hazard in pinyon-juniper woodlands and reduced stand density and canopy fuel loads. Using low severity surface fire alone did not affect overall fuels significantly, although it did reduce the tree densities of juniper trees more so than the control treatment.

The authors found that large, mixed-severity fires can result in greater heterogeneity of spatial patterns of fire. High-severity patch size varied considerably across both fires in the study. Regeneration also varied considerably within high-severity patches with most seedlings establishing and surviving after 5-10 years post-fire.

Despite subsequent management and other disturbance, including fire, on these study sites, the authors found that the spatial pattern of fire severity was still significantly tied to ponderosa pine regeneration patterns and still impacted the sites decades after the fire. Regenerating seedling density of ponderosa pine decreased with increasing distance to seed source in the high severity patches of fire. Though long-distance dispersal did occur, the rate of recovery was slower with increasing distance to seed source in the larger high-severity patches.

General climate trends after the fires did not relate clearly to forest regeneration for either site, and despite periods of wet years and drought years, forest regeneration occurred at both sites. Therefore, the authors stated that future large fires are not necessarily catastrophic to ecosystems and that process-based restoration may provide resiliency in the forests in the face of changing fire regimes.

Thinning and burning significantly reduced the overall density (>2.5 cm) of ponderosa pine stems by 66%, although this did not meet the objective. Large snags and logs >50cm were retained within objectives. Canopy fuel loads were substantially reduced, allowing for the reintroduction of surface fires.

From 1984 to 2006, fire and bark beetle outbreaks have caused approximately 14-18% of the mortality in forested areas of the Southwest, and the area burned has increased substantially with a greater proportion burning at high severity since 1984. Recent elevated temperatures are thought to have led to both the increase in bark beetle outbreak and high severity fire.

The authors found that regional fire years were highly associated with drought with no prior lag years of moisture required. They suggest that because fire return intervals were generally longer in the Black Hills than in ponderosa pine forests of the Southwest, so that fuels had adequate time to build up between fire years. Fire years in the Black Hills were also related to La Niña years (summer-dry conditions) and cool phases of PDO, which magnifies the effects of the La Niña drought. Finally, episodes of synchronous tree recruitment and growth were highly associated with periods of increased moisture. These pluvial periods limited fire frequency and likely contributed to the increased establishment and survivorship of seedlings. The authors propose that variations in fire frequency and timing were more important in shaping forest structure than variations in fire severity.

The authors linked a large period of megadrought in the 1580’s to high mortality of ponderosa pine as no trees predated this event, and seedlings nearest the time of the event grew especially slowly. Due to the lack of surviving trees, there is no evidence of fire-caused mortality during this event.

Pulses of regeneration occurred during several periods of increased moisture; however, the authors suggest it was the absence of fire that allowed may trees to survive into adulthood and not due to episodic mortality caused by fire or other disturbance. Climate change likely affected fuel conditions that were less favorable for burning during these fire?quiescent periods allowing fuels to build up until drier years when synchronous regional fires occurred. This pattern has been commonly attributed to the ENSO cycle with wet El Niño years followed by dry La Niña years.

The authors found that fire activity was strongly related to interannual drought conditions typically associated with cycles of ENSO. Increased fire activity tends to occur during the La Niña cycle, in years of reduced moisture availability and were often followed 2-4 years of increased moisture availability, associated with El Niño years.