The authors suggest that climate did not limit tree establishment and regeneration, however, climate did influence fire frequency and thus indirectly influenced age structure pattern in these stands. Within the study sites, age peaks occurred synchronously in the early 19th century, coinciding with a period of increased moisture within the ENSO cycle that reduced fire activity.

The authors found that mixed-conifer ecosystems are drought-limited, not fuel limited; therefore, they do not require prior wet years to build up fuels before burning, but instead will burn when fuel moistures are low.

The authors found that fire was highly synchronous across the region and were associated with climate conditions. Their analysis found strong relationships between dry years, preceded by one or two wetter years, with increased fire activity across the region. Specifically, widespread fire years were synchronous with negative, extreme PDSI events while fire was asynchronous when PDSI was positive. The authors also found stronger relationships with interannual climate variability than with long-term, mulidecadal climate cycles driven by the ENSO.

The authors found that fire was highly asynchronous between the three sites despite exhibiting similar fire regimes, suggesting that at the scale of the study, fine-scale factors were more influential on fire occurrence than regional climate. At long-term scales, ENSO, specifically La Niña events (drought), were correlated with higher fire occurrence at the study sites until approximately 1830.

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.

Both individual and phase combinations of ENSO, PDO, and AMO were strongly associated with moisture variability. As a consequence, periods of regional drought were strongly related to regional synchrony of fire occurrence in upper elevation forests historically. The authors found that all stand-replacing fire years and 18 of 20 synchronous fire years occurred during periods of regional drought that were 2-4 times as dry as normal conditions. No antecedent drought lags were necessary for fire occurrence due to the fact that fuels in high elevation forests are typically dense enough to burn when sufficiently dry conditions allow for successful ignition and spread. The authors suggest that ENSO strongly and consistently affects regional moisture conditions in the Southwest, thereby fire occurrence, including high severity fire occurrence, but that the effects can be tempered by phases of PDO.

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.

The authors found that periods of drought were significantly related to fire occurrence during El Niño oscillations; however, this trend was interrupted in the 1940’s and 1950’s, likely by the introduction to logging and livestock grazing in the area.

Prior to 1892, fire occurrence was significantly associated with drought on both sides of the U.S.-Mexico border. However, post-1892, the fire regime was interrupted at the sites in Arizona, while it continued in Mexico, suggesting that bottom up factors such as livestock grazing and human land use had a stronger influence on fire at finer spatial scales than climate in this case, producing different fire regimes on the same mountain range.

The authors found no significant difference between their models of fire-climate relationship before and after 1600 CE suggesting that the historical range of variation drawn from fire chronologies from c. 1600 to 1900 CE are useful analogs for current and future restoration and management purposes. The authors also found that a dry year preceeded by a wet year 1-3 years earlier was a strong predictor of fire activity throughout the fire record (572 – 1987 CE). An atypically long fire-free period occurred between 1360 to 1455 CE during an unusually wet period. This was followed by high-severity fires during the Medieval Climate Anomaly in the late fourteenth or fifteenth century likely driven by drought conditions and the increase in fuels from the prior century.

The authors suggest that long periods of drought may not be necessary to dry fuels enough to burn in the naturally arid climate of the Great Basin so that fires were actually more common in wet years than dry years when fuels were more abundant.

The authors found strong relationships between synchronized fire activity and summer drought in the year of the fire across all regions.

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.

Fires prior to 1763 were synchronous with region-wide fire years, which coincided with periods of drought. However, after 1763, fires on the peak became asynchronous to widespread-fire years throughout the region. The authors suggest that when this occurs, bottom-up factors may have become more important in controlling the spread of fire. Specifically, Rincon Peak is isolated and surrounded by talus slopes and rock outcrops. However, they also hypothesized that a variation on ENSO, the late 18th-- early 19th century transition period (LEENT) may have occurred at this time, lengthening the fire-free interval. The LEENT period decreased the ENSO signal and limited the wet/dry oscillations and therefore the cycles of high fuel production followed by drought, hence increased flammability, for an extended period. The author’s findings suggest that isolated sky island ponderosa pine forests may be especially susceptible to climate change and drought because of their separation from the larger landscape and the frequent fire regimes that regulate severity in these ecosystems.

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.

Charcoal concentration in the sediment records at both bogs varied considerably over 15,000 years suggesting that climate affected both vegetation and fire regimes. At the both bog sites, a departure in superposed epoch analysis PDSI of -2, indicating very dry conditions, coincided with widespread fire years between the 1500’s and early 1900’s.

Synchronous fire episode frequency occurred between approximately 12,000 and 9,000 years before present and approximately 2,000 to 1,000 years before present. The authors suggest that the first period was associated with increased biomass and lightning ignitions due to climate change while the second was due to persistent drought with intermittent wetter periods providing conditions for increased fire activity.

The authors found very strong relationships between broad-scale climate anomalies and fire activity in subalpine forest ecosystems. Fires generally occurred during periods of prolonged and/or extreme drought during the cool (negative) phases of ENSO, cool (negative) phase of PDO and warm (positive) phase of AMO.

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 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 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.

The authors found very strong relationships between broad-scale climate anomalies and synchronous fire activity in subalpine forest ecosystems. Periods of synchronous fire activity generally occurred during periods of prolonged and/or extreme drought during the La Niña phase of ENSO, cool (negative) phase of PDO and warm (positive) phase of AMO. The opposite phases of these indicators are associated with a protracted fire-free period from 1790 to 1850.

Charcoal records showed that regional synchroneity of fire frequency occurred historically due to fluctuations in climate where moister climate conditions resulted in fuel build up followed by drier conditions which resulted in regional fire years.

The authors found that fire frequency was not consistent during the Holocene, but that fire activity was driven by short-term climatic cycles. Wetter periods were associated with higher fire occurrence due to the increase in available vegetation, whereas dry periods were associated with less dense vegetation and therefore, less fire activity.

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 summarized findings on high-severity fire regimes in subalpine forests and found that climatic variation is the predominant influence on fire frequency and severity in this ecosystem type and suggest that fuel reduction treatments would move stand structure away from its historical range of variability. For low severity fire regimes in low-elevation ponderosa pine forest, the authors found that fire frequency and severity were driven by the spatial and temporal variation of fine fuels more so than climate.

The authors found that large fires have occurred in these piñon stands historically, and they estimated a fire return interval of approximately 400 years between large, stand-replacing fires. Since 1995, however, the park has burned more than three times the total area burned in the previous century. But, generally, these fires were lower in severity than historical fires. The authors assert that the current fire regime is not necessarily outside of the historical range of variability, however, the amount of fire over the past decades may be unprecedented. Furthermore, based on superposed epoch analysis the authors found that large fire years were associated with lower than normal precipitation events in the preceding winter/spring to burning. Therefore, current fire activity may be a reaction to current drought conditions after an abnormally wet period in the 1970’s and 1980’s, which allowed for a build-up of canopy fuels.

The authors found that fire was frequent with median intervals of less than 15 years in the Jeffrey pine-mixed conifer forests. However, a period of reduced fire occurrence from the 1790s to 1830s coincided with a similar fire-free period across the Southwest and southern South America. The authors and others found a reduction in the amplitude of ENSO during this time that likely contributed to this reduction in fire occurrence. However, an increase in livestock grazing intensity and a reduction of fire use by native peoples occurred at the same time and likely also had strong influences on the fire regime during this period.

The authors found synchrony between fires during dry years as measured by the Palmer Drought Stress Index, with severe drought common in regional fire years.

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.

The authors found synchrony of fire occurrence across the region when dry years were proceeded by wet years. Strong El Niño events associated with especially wet years suppressed fire regionally, but also resulted in a pulse of fine fuel growth which became the primary fuels for surface fire during drought years. The authors found that at local scales, however, topography and elevation had a stronger influence on fire frequency and pattern than climate.

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.

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.

Historically, fire was highly synchronous across the region and appeared to be linked to climate conditions.

The authors found strong relationships between area burned and highly positive values of the Southern Oscillation index (SOI), which is associated with reduced rainfall and severe winter-spring drought in the Southwest. They also found regional synchrony of large fires during the high-SO phase, which is associated with limited spring precipitation resulting in reduced tree growth. This suggests that seasonal climate, and not just local weather conditions, affect fire activity at a regional scale.