A thorough understanding of historic disturbance regimes and their effects on vegetation is necessary for successful wildland management. Fire histories developed from analyzing tree-ring injuries, or fire scars, and tree establishment patterns help measure past variation in fire regimes. When tree-ring-based forest histories are constructed in concert with fire histories, one can examine linkages between fire regimes and vegetation dynamics.
Using tree-ring evidence, researchers developed multi-century fire and forest establishment histories representing broad gradients in elevation and vegetation for 14 sites in Utah and eastern Nevada. Vegetation included pinyon-juniper woodlands, mountain shrublands, and ponderosa pine, dry mixed-conifer, aspen, aspen-conifer and subalpine-conifer (spruce, fir and pine) forest types. Fire regime analysis included frequency, severity and extent.
Results reveal that before 1900, fire frequency and severity varied significantly across short distances and through time. The variable, mixed-severity fire regimes maintained shifting vegetation mosaics as fire-tolerant tree species responded to periods of high-frequency, low-severity fire while fire-sensitive species were restricted to areas of low-frequency (often high-severity) fire. The sharp decline in fire occurrence that began in the mid 1800’s resulted in an increase of fire-sensitive conifers, including an expansion of trees to previously non-forested mountain shrublands. These vegetative changes result in greater fuel loads and fuel continuity, increasing risk for large, high-severity fires.
Managers use information from this study, including site-specific variation in fire frequency, severity and extent as well as historic vegetation composition and structure, to plan and implement fire and fire-surrogate treatments that restore resilient landscapes in Utah and eastern Nevada.