Relationships of subalpine forest fires in the Colorado Front Range with interannual and multidecadal-scale climatic variation

Aim An understanding of past relationships between fire occurrence and climate variability will help to elucidate the implications of climate-change scenarios for future patterns of wildfire. In the present study we investigate the relationships between subalpine-zone fire occurrence and climate variability and broad-scale climate patterns in the Pacific and Atlantic Oceans at both interannual and multidecadal time-scales. Location The study area is the subalpine zone of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in the southern sector of the Rocky Mountain National Park, which straddles the continental divide of the northern Colorado Front Range. Methods We compared years of widespread fire from AD 1650 to 1978 for the subalpine zone of southern Rocky Mountain National Park, with climate variables such as measures of drought, and indices such as the El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO). Results Years of extensive subalpine-zone fires are significantly related to climate variability, phases of ENSO, the PDO, and the AMO, as well as to phase combinations of ENSO, the PDO, and the AMO at both interannual and centennial time-scales. Main conclusions Years of extensive fires are related to extreme drought conditions and are significantly related to the La Nina phase of ENSO, the negative (cool) phase of the PDO, and the positive (warm) phase of the AMO. The co-occurrence of the phase combination of La Nina-negative PDO-positive AMO is more important to fire occurrence than the individual influences of the climate patterns. Low-frequency trends in the occurrence of this combination of climate-pattern phases, resulting from trends in the AMO, are the primary climate pattern associated with periods of high fire occurrence (1700-89 and 1851-1919) and a fire-free period (1790-1850). The apparent controlling influence of the AMO on drought and years of large fires in the subalpine forests of the Colorado Front Range probably applies to an extensive area of western North America.