The historic range of variability of ponderosa pine in the northern Colorado Front Range: Past fire types and fire effects

Current forest ecosystem management it the western U.S. is largely based on the perception that prior to the late 19th century relatively frequent surface fires maintained open, park-like forests but the past century of fire exclusion has resulted in substantial increases in woody fuel density. Although this model is well supported by data on particular forest types (e.g., some Southwest Pinus ponderosa ecosystems), it has been indiscriminately applied to other geographic areas and across forest types. The goal of this study was to examine fire history in combination with intensive tree age data to determine past fire types and fire effects across the elevational range of ponderosa pine in the northern Colorado Front Range. This study relates variation in historic fire regimes to variation topography, climate variability and human activities. Fire history trends show a decrease in fire frequency in the 20th century from the prior two centuries, especially in the lower montane zone. Frequent surface fires maintained open stands of ponderosa pine at low elevation, and the past century of fire exclusion has resulted in much denser ponderosa pine stands along with changes in potential fire hazard. However, these results indicate only a small percentage of the ponderosa pine zone historically had a relatively high frequency of fire return intervals (MFI < 30 years) At increasingly mesic sites (e.g., higher elevation, north-facing slopes), less frequent stand-replacing fires were characteristic of the pre-settlement forests. Higher elevation stands experienced coincident stand-replacing fires in the late 1800s promoting periodic pulses of establishment initiating the present dense forest patches. More than 80% of the ponderosa pine study area is reconstructed to have had moderate (MFI > 30 years) to low fire frequency (MFI > 40 years). Drought condition are strongly associated with fire occurrence across fire frequency types; however, moisture conditions in years prior to fire occurrence are most important for the high fire frequency sites. The most important management implication of this study is both simple and robust: only a small fraction of the ponderosa pine zone of the northern Front Range fits the widespread notion that the historic fire regime was characterized mainly by frequent surface fires that maintained open, savanna-like stands.