Rocky Mountain Research Station and University of Arizona scientists dendrochronologically reconstructed disturbance regimes in the Pinaleño Mountains of southeastern Arizona to determine how fire, insect, and tree population dynamics changed during the period of fire exclusion. They found that extensive fires are not a new phenomenon – before about 1880, every 20 years or so more than half the mountain range would burn in one year. Once fire was excluded, tree population abundance increased 1100-1800 percent. Change was fastest and greatest in the mixed-conifer, where different species prevailed. The area occupied by Engelmann spruce quadrupled, and spruce trees grew twice as fast in the mixed-conifer (where the growing season is longer) than in the spruce-fir. Size and extent matter – spruce beetle outbreaks quadrupled in size, doubled in duration, and became increasingly severe. The newly abundant species also allowed fire to easily transition from the surface to the forest canopy, as they tend to retain branches low on the tree boles. Eventually, tree densities, fuel accumulations, and multi-storied stand canopies exceeded firefighting ability to suppress fires, and large high-severity fires occurred in 1996, 2004, and 2017. And in the latter two fires, crown fire spread from the mixed-conifer forest to the spruce-fir, leaving only about 35 acres of spruce-fir unburned.
Species composition and tree abundance changes were greatest in the mixed-conifer forests, and these changes significantly affected bark beetle outbreaks, fire size and severity, and spruce-fir forest vulnerability. The information accrued in this study indicates that restoring natural forest conditions and fire regimes in the mixed-conifer forests will promote forest health and resilience to increasing temperatures, longer fire seasons, and more intense droughts.