Historical surface fire frequency in ponderosa pine stands in research natural areas, central Rocky Mountains and Black Hills, USA

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"Historical range of variability" and "reference conditions" are two concepts that attempt to characterize ecosystem conditions as they may exist in the absence of pervasive human impacts. However, to define reference conditions from reference landscapes, such as U.S. Forest Service Research Natural Areas, requires a long-term perspective by which to assess whether existing ecosystem conditions are driven by predominately natural rather than human factors. We used fire-scarred trees to reconstruct centuries-long chronologies of surface fires in four research natural areas (three established and one proposed) that contain ponderosa pine (Pinus ponderosa Laws.) forests in South Dakota, Wyoming, and Colorado. Fire frequency was variable among research natural areas, but recent fire-free periods in three of the four areas were up to approximately 2.5 times longer than any presettlement intervals. Loss of surface fires most likely is related indirectly to recent land and resource use in areas outside of the research natural areas and related directly to fire suppression and livestock grazing in the research natural areas themselves. Studies that attempt to define reference conditions for ponderosa pine ecosystems from existing conditions in these Research Natural Areas will need to consider changes that may have occurred in these areas as the result of loss of historical fire patterns. Determination of historical fire frequency also should provide useful information for the management or restoration of ecosystem processes and conditions in these or similar natural areas.