Fire History of an Old-Growth Ponderosa Pine Stand in the Sheep Range, Desert National Wildlife Refuge, Nevada, USA

Abstract

Southwestern ponderosa pine forests have experienced reduced fire frequency since Euro-American settlement generally because of successful fire suppression policies. We report here on the fire history of a ponderosa pine stand located in the Sheep Range, which is part of the Desert National Wildlife Refuge, in the Mojave Desert. A total of 22 dominant, fire-scarred ponderosa pines were sampled by taking 29 partial cross-sections and 18 wood increment cores. Maximum age of ponderosa pines at the study area exceeded 800 years, and sampled trees were often older than 500 years, so that the site tree-ring chronology covered 522 years (1490-2011). Crossdating revealed both extreme sensitivity and highly synchronous patterns, with the expressed population signal (EPS) exceeding 0.9 in 30-year moving windows throughout the length of the chronology. Fire statistics were calculated for the 1565-2011 period, during which at least 10 of the crossdated trees had been scarred and were recording fire. During the recorder period, there were 16 fires that met the two-tree minimum threshold, yielding a mean fire interval (MFI) of 25 years, a median fire interval (MedFI) of 15 years, and a Weibull median probability interval (WMPI) of 18 years; the point mean fire interval (PMFI) was 69 years. The longest fire-free intervals since 1565 occurred in the past two centuries, with 70 years (1862-1931) followed by another 80 years (1933-2012). The stand-wide 1932 fire is the last event recorded by the sampled trees. Overall there was reduced fire frequency from the late 19th Century to present compared to the previous three centuries. Because there is no record of active fire management in the study area, this finding is consistent with similar results obtained in two remote mountains of the Great Basin Desert, and points to a need for greater spatial coverage in fire history information, even for species that have been actively studied in other environments.