Fire History and Fire Climatology along a 5º Gradient in Latitude in Colorado and Wyoming, USA

Abstract

We reconstructed fire chronologies covering the past four to six centuries from fire scars recorded in tree-ring series from 18 sites in the central Rocky Mountains of Colorado and Wyoming. Sites are located in forests containing predominately Pinus ponderosa. Median fire-free intervals in fire chronologies are related to latitude, with shorter intervals in southern stands than those in the north. However, strength of this relationship varied through time, with a stronger latitudinal gradient in fire frequency from 1600 to 1800 than from 1700 to 1900. Variability in fire frequency with time may be related to strength of regional climate gradients. Seasonality of fire scars also varied across the latitudinal gradient, from predominately early season fires in the south to late season fires in the north. Superposed epoch analysis of fire years with annual variability in Palmer drought severity indices shows that fire years throughout the gradient were dry, but those in the south were preceded by wet years. This result suggests that fuel amounts may have been limiting in southern forests where fire intervals were shorter and that longer intervals in the north permitted fuel buildup between fires. All chronologies show a general cessation of fire scars beginning in the latter nineteenth century, coincident with widespread Euro-American settlement of the western US.