Modeling fire interval data from the American Southwest with the Weibull distribution

In this study, the Weibull distribution is tested as a possible model for f ire interval data derived from dendrochronologically-dated fire scars from four sites in the American Southwest. Two- and three-parameter Weibull dist ributions were fit to fire interval data sets, and additional statistical d escriptors based on the Weibull were derived to improve our understanding o f the range of variability in presettlement fire regimes. The three-paramet er models failed to provide improved fits versus the more parsimonious two- parameter models, indicating the Weibull shift parameter may be superfluous for Southwestern fire regimes. The Weibull Modal Interval (MOI) was a supe rior overall measure of central tendency, and appears to identify a common underlying structure in Southwestern fire regimes independent of habitat ty pe and environmental gradients. Unusually short and long fire intervals wer e identified by the lower and upper exceedance intervals (LEI and UEI) and the Maximum Hazard Interval (MHI) based on the Weibull hazard function. Mod el statistics were nearly identical between two pairs of sites that were 26 0 kilometers distant that differed in topography, vegetation, and land-use history. However, differences were observed between sites only 10 kilometer s apart, suggesting the influence of local factors (e.g., topography and su bstrate) over regional influences (e.g., climate). Although the Weibull mod els helped quantify the historical range of variability in presettlement fi re regimes, ecological interpretations of the Weibull parameters proved dif ficult.