The University of Arizona

Fire History and Climate | CLIMAS

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Fire History and Climate

Fire History and Climate

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Severe and extensive wildfires occurring in the Southwest over the past decade appear to be unprecedented when compared with fire scar records for the region. These records show that fires spread as low-severity surface fires, burning in the understory of relatively open forests. Euro-American settlement near the end of the nineteenth century halted surface fires by introducing livestock grazing, which removed grasses and fine fuels necessary for fires to spread. This was followed by fire suppression policies, which further prevented fires from spreading in the forests. The past century of fire exclusion in ponderosa pine and mixed conifer forests of the Southwest has led to anomalously dense forest conditions in these ecosystems. Today, small trees in the understory act as ‘ladder’ fuels, which connect the forest floor with the canopy of the forest. When fires are started by lightning or abandoned campfires, they now burn uncontrollably through the canopy of the forests.

This study focused on a watershed burned by the Missionary Ridge Fire, which occurred in June 2002, in the San Juan Mountains of southwestern Colorado. Debris flows following the fire exposed a late-Holocene record of fire-related alluvial deposits in the walls of the incised channel. The exposed sediment record contains abundant charcoal within the deposits, providing an ideal opportunity to simultaneously evaluate fire-related sedimentation events and tree-ring records of fire history. The combination of these records yielded information specifically about high-intensity fires.