Uncertainty in surface-fire history: the case of ponderosa pine forests inthe western United States

Present understanding of fire ecology in forests subject to surface fires i s based on fire-scar evidence. We present theory and empirical results that suggest that fire-history data have uncertainties and biases when used to estimate the population mean fire interval (FI) or other parameters of the fire regime. First, the population mean FI is difficult to estimate precise ly because of unrecorded fires and can only be shown to lie in a broad rang e. Second, the interval between tree origin and first fire scar estimates a real fire-free interval that warrants inclusion in mean-FI calculations. F inally, inadequate sampling and targeting of multiple-scarred trees and hig h scar densities bias mean FIs toward shorter intervals. In ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) forests of the western United Sta tes, these uncertainties and biases suggest that reported mean FIs of 2-25 years significantly underestimate population mean FIs, which instead may be between 22 and 308 years. We suggest that uncertainty be explicitly stated in fire-history results by bracketing the range of possible population mea n FIs. Research and improved methods may narrow the range, but there is no statistical or other method that can eliminate all uncertainty. Longer mean FIs in ponderosa pine forests suggest that (i) surface fire is still impor tant, but less so in maintaining forest structure, and (ii) some dense patc hes of trees may have occurred in the pre-Euro-American landscape. Creation of low-density forest structure across all parts of ponderosa pine landsca pes, particularly in valuable parks and reserves, is not supported by these results.