Climatic and human influences on fire regimes in ponderosa pine forests inthe Colorado Front Range

In the northern Colorado Front Range, fire suppression during the 20th cent ury is believed to have created a high hazard of catastrophic fire in ponde rosa pine (Pinus ponderosa) forests. Since the early 1990s, resource manage rs have increased the use of prescribed fires to re-create fire regimes and forest structures similar to those of the pre-Euro-American settlement per iod in order both to reduce lire hazard and to improve forest health. To im prove understanding of historical fire regimes, we conducted a study of fir e history along an elevational gradient from similar to 1830 to 2800 m in p onderosa pine forests in the northern Front Range. Fire-scar dates were det ermined from 525 partial cross sections from living and dead trees at 41 sa mple sites. Fire frequencies and fire intervals were analyzed in relation t o changes in human activities and interannual climatic variability as recor ded in instrumental climatic records and tree-ring proxy records. Prior to modern fire suppression, the low elevation, open ponderosa pine fo rests of the northern Front Range were characterized by frequent surface fi res, similar in frequency to many other ponderosa pine ecosystems in the We st. In contrast, in higher elevation forests (above similar to 2400 m) wher e ponderosa pine is mixed with Douglas-fir (Pseudotsuga menziesii) and lodg epole pine (Pinus contorta), the fire regime was characterized by a much lo wer fire frequency and included extensive stand-replacing fires as well as surface fires. In the mid-1800s there was a marked increase in fire occurre nce that can be related both to Euro-American settlement and increased clim atic variability. This episode of increased fire left a legacy of dense, ev en-aged stands in;higher elevation ponderosa pine forests, whereas increase d stand densities in low elevation forests are attributed mainly to lire ex clusion 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 str ongly associated with years of widespread fire. Years of widespread fire al so tend to be preceded two to four years by wetter than average springs tha t 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 Fl Nino-Southern Oscillation (ENSO) events. The warm (El Nino) p hase of ENSO is associated with greater moisture availability during spring that results in a peak of fire occurrence several years following El Nino events. Conversely, dry springs associated with La Nina events were followe d 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 o ccurred 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 variability in creating extreme fire hazard at a landscape scale.