Fire history of a pinyon-juniper/ponderosa pine ecosystem in the Intermountain West

Dendrochronology techniques were applied to reconstruct the fire history of a pinyon-juniper/ponderosa pine woodland in the Great Basin. The goals of our study were to describe the frequency and spatial extent of fire on Mount Irish, in southeastern Nevada, and to identify any temporal changes in fire regime over the past few centuries. The topographically complex Mount Irish study area encompasses approximately 1.6 km² and ranges from 2400 to 2560 meters in elevation. The climate of the study area is typical of the southern Great Basin with an average annual rainfall of about 300 millimeters, most of which falls in the winter months with relatively dry summers. Mount Irish is a relatively pristine site with a short history of past EuroAmerican silver mining in the late 1800s. To reconstruct fire history on Mount Irish, we sampled all fire-scarred trees within the study area. We collected a total of 255 fire-scar wedge sample, of which 197 were successfully crossdated. Most of the crossdated samples were from ponderosa pine (87%), and the rest consisted of white fir (11%) and pinyon pine (2%). The fire history reconstruction covers 860 years, from 1146 to 2006 AD, and includes 492 fire scars. Fire severity was low enough at the site to leave evidence of fire scars behind throughout the last 850 years. Pre-settlement fire frequency was also relatively high during the 1780-1840 period when fire was reduced or absent in many other fire history studies of the western United States. A dramatic decrease in fire occurrence was coincident with EuroAmerican settlement of the region. In terms of spatial patterns, fire events were usually patchy but could occur throughout the study area in a single year, which suggests multiple ignitions during a single year. Since older trees were scattered over the landscape evidence for stand replacing fires was scarce.