Fire frequency and the vegetative mosaic of a spruce-fir forest in northern Utah

Fire scar and vegetative analysis were used to construct a fire history for the Engelmann spruce/sub-alpine fir (Picea engelmannii/Abies lasiocarpa) vegetation type of the Utah State University (USU) T. W. Daniel Experimental Forest. Three distinct periods of fire frequency were established-presettlement (1700-1855), settlement (1856-1900), and suppression (1910-1990). Mean fire interval (MFI) decreased during the settlement period and greatly increased during the suppression era. The difference was attributed to the influx of ignition sources during the settlement of nearby Cache Valley, located 40 km to the west. Logging an livestock grazing appear to have led to the reduced MFI, which in turn worked as a factor to create the vegetative mosaic now observed on the study area. The increase in MFI during the suppression era permitted the advancement of shade-tolerant species in the understory of the shade-intolerant lodgepole pine (Pinus contorta var. latifolia) and quaking aspen (Populus tremuloides). Continued suppression of disturbance from wildfire will allow the lodgepole pine cover type, which experienced the lowest MFI during the settlement period, to be further invaded by shade-tolerant species, decreasing spatial stand diversity and increasing the risk of more intense fires.