Climatic and Human Influences on Fire Regimes in Ponderosa Pine Forests in the Colorado Front Range

In the northern Colorado Front Range, fire suppression during the 20th century is believed to have created a high hazard of catastrophic fire in ponderosa pine (Pinus ponderosa) forests. Since the early 1990s, resource managers have increased the use of prescribed fires to re-create fire regimes and forest structures similar to those of the pre-Euro-American settlement period in order both to reduce fire hazard and to improve forest health. To improve understanding of historical fire regimes, we conducted a study of fire history along an elevational gradient from ∼1830 to 2800 m in ponderosa pine forests in the northern Front Range. Fire-scar dates were determined from 525 partial cross sections from living and dead trees at 41 sample sites. Fire frequencies and fire intervals were analyzed in relation to changes in human activities and interannual climatic variability as recorded in instrumental climatic records and tree-ring proxy records. Prior to modern fire suppression, the low elevation, open ponderosa pine forests of the northern Front Range were characterized by frequent surface fires, similar in frequency to many other ponderosa pine ecosystems in the West. In contrast, in higher elevation forests (above ∼2400 m) where ponderosa pine is mixed with Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), the fire regime was characterized by a much lower fire frequency and included extensive stand-replacing fires as well as surface fires. In the mid-1800s there was a marked increase in fire occurrence that can be related both to Euro-American settlement and increased climatic variability. This episode of increased fire left a legacy of dense, even-aged stands in higher elevation ponderosa pine forests, whereas increased stand densities in low elevation forests are attributed mainly to fire exclusion during the 20th century. Warmer and drier spring–summers, indicated in instrumental climatic records (1873–1995) and in tree-ring proxy records of climate (1600–1983), are strongly associated with years of widespread fire. Years of widespread fire also tend to be preceded two to four years by wetter than average springs that increase the production of fine fuels. Alternation of wet and dry periods over time periods of 2–5 years is conducive to fire spread and is strongly linked to El Niño–Southern Oscillation (ENSO) events. The warm (El Niño) phase of ENSO is associated with greater moisture availability during spring that results in a peak of fire occurrence several years following El Niño events. Conversely, dry springs associated with La Niña events were followed by more widespread fire during the same year. The 1600–1920 fire-scar record indicates that individual years during which high percentages of the 41 sample sites synchronously recorded fire have occurred at least several times per century. The association of these years of widespread fire with very strong ENSO events demonstrates the importance of ENSO-related climatic variabililty in creating extreme fire hazard at a landscape scale.