Fire history of ponderosa pine-dominated forests in the Colorado Front Range: Elevation, topography and land-use

Much of the current forest management in the western United States is based on the assumption that fire suppression has unnaturally increased the density and crown fire hazard in many forest types. This model is well documented by studies in the Southwestern ponderosa pine forests (Pinus ponderosa ), however, there is debate as to the geographic extent of the model. Studies in Boulder County of the Colorado Front Range have identified a significant influence of both stand-replacing and surface fires in most of the ponderosa pine-dominated forests of the montane zone and a limited influence of fire suppression on forest structure. However, the applicability of the findings from the Boulder County studies to other areas of montane forest in Colorado needs to be tested. The primary goal of this study was to determine spatio-temporal patterns of fire frequency for the Jefferson County montane zone (sites in central and northern Jefferson County and the eastern part of Gilpin County), south of Boulder County. The results were contrasted with those of Boulder County to determine any differences in spatio-temporal patterns of fire frequencies between the regions that may reflect inherent differences in the physical and biotic environments or differences in land-use practices.;Sites in Jefferson County show a general increased length of fire intervals with increasing elevation, as did Boulder County, although there is substantial variation among sites due to site-specific characteristics. In addition to elevation, the adjacency of sites to grasslands had a considerable effect on fire frequency. Fire frequency in Jefferson County was higher at sample sites near grasslands resulting in high spatial variability of fire frequencies, particularly at high elevations.;The overall temporal pattern of fire in Jefferson County was similar to the pattern for Boulder County. Jefferson County had an increase in fire frequency during the mid-19th century which coincided with a period when climate was more favorable to the ignition and spread of fire. Although fire occurrence increased before major Euro-American settlement, increased ignitions during this time period likely contributed to the sustained period of increased fire occurrence in the 19th century. The lower montane zone showed a substantial decrease in fire frequency in the late 19th century that can be explained by grazing influences. Fire frequency decreased in the 20th century likely as a response to fire suppression, however, fire was not completely excluded from the region. The decrease in fire occurrence in the 20th century was more profound in Boulder County than Jefferson County which continued to experience widespread fires until the late 20th century. Although fire frequency in Jefferson County was reduced during the 20th century, it was not necessarily less frequent or widespread than it was during the 18th century. Further analysis of stand structure must be completed to assess fire severity, however, 20th century fire intervals are not greatly if at all outside the historical range of variability of the past several centuries for much of the montane zone.;Despite the greater intensity of late 19th to early 20 th century mining in Boulder County, the hypothesis that fire frequency would be greater in Boulder than in Jefferson County could not be supported. In fact, Jefferson County had significantly more frequent fires in the upper montane zone (2397--2730 m) before fire suppression (1780--1920; p = 0.011). Potential explanations for the greater fire frequencies in the upper montane zone of Jefferson County include greater proximity of sample sites to grasslands, location of sample sites on less steep topography conducive to greater spread of low-severity fires, and/or less active fire suppression due to a higher proportion of private to federal land in Jefferson County during the first two-thirds of the 20th century. Despite some modest differences in fire history between the montane zones of the two counties, the overall spatial pattern associated with elevation and the temporal pattern associated with climatic variation are similar.