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

"Historical range of variability" and "reference conditions" are two concep ts that attempt to characterize ecosystem conditions as they may exist in t he absence of pervasive human impacts. However, to define reference conditi ons from reference landscapes, such as U.S. Forest Service Research Natural Areas, requires a long-term perspective by which to assess whether existin g ecosystem conditions are driven by predominately natural rather than huma n factors. We used fire-scarred trees to reconstruct centuries-long chronol ogies of surface fires in four research natural areas (three established an d 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 a reas were up to approximately 2.5 times longer than any presettlement inter vals. Loss of surface fires most likely is related indirectly to recent lan d and resource use in areas outside of the research natural areas and relat ed directly to fire suppression and livestock grazing in the research natur al areas themselves. Studies that attempt to define reference conditions fo r ponderosa pine ecosystems from existing conditions in these Research Natu ral Areas will need to consider changes that may have occurred in these are as as the result of loss of historical fire patterns. Determination of hist orical fire frequency also should provide useful information for the manage ment or restoration of ecosystem processes and conditions in these or simil ar natural areas.