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Historic fire regime forensics: Deciphering drivers and variability from tree rings

Date: August 18, 2016

Tree-ring-based fire histories from 10 eastern Great Basin sites reveal historic drivers of fire occurrence while multi-century fire and tree recruitment chronologies from two eastern Great Basin watersheds demonstrate drivers of variability


A blackened triangular “cat face” at the base of this ponderosa pine is classical evidence that multiple, non-lethal fires have been recorded as injuries in the annual growth rings (photo by Stan Kitchen).
A blackened triangular “cat face” at the base of this ponderosa pine is classical evidence that multiple, non-lethal fires have been recorded as injuries in the annual growth rings (photo by Stan Kitchen).

Proper management of naturally forested landscapes requires an understanding of the temporal and spatial patterns in which key disturbance processes are manifest and their effects on species composition and structure. Linked fire and forest histories constructed from tree-ring evidence provide valuable information about drivers of fire occurrence and about the variability and interactions of fire regimes and vegetation on heterogeneous landscapes.

Wildfire is a keystone process affecting the composition, structure, and health of western North American forests. An understanding of factors that controlled the frequency and severity of wildfire in historic times may be useful in restoring resiliency to these forests. In a recent study, tree-ring evidence was used to reconstruct multi-century histories of fire frequency and seasonality for 10 eastern Great Basin sites. These histories were then compared to regional reconstructions of climate variability.

Across the region, fires were most common during dry years that were preceded by one or more years of above average precipitation. Fires were least common after extended periods of drought, suggesting that fires were historically fuel limited. These fire probabilities were related to the occurrence of multi-year climate cycles known to vary with variability in Pacific Ocean surface temperatures (El Niño Southern Oscillation and Pacific Decadal Oscillation). Evidence revealed that fires were most common in the early and later part of the fire season. In this region, lightning-caused fires are most common in the mid-season, inferring the importance of Native American ignitions to past fire regimes.

In a separate study, fire and tree recruitment histories were reconstructed for two small watersheds. Fire frequency varied more than 10-fold within each watershed and was closely related to elevation and steepness. Most fires were small, but large fires accounted for a majority of the area burned.

. Patterns in annual growth rings of this ponderosa pine stump
Patterns in annual growth rings of this ponderosa pine stump reveal evidence of at least six non-lethal fires (photo by Stan Kitchen).
The timing of recruitment pulses varied within sites suggesting that fire severity was mixed across space and through time creating an ever-shifting mosaic of vegetation patches intermixed with stands of long-lived, fire resilient trees.

Key Findings

  • Historically, climate was an important synchronizer of fire in the Great Basin Region where fire was most common when multi-year climatic cycles were transitioning from wet to dry phases and least common when transitioning from dry to wet phases.

  • Fire seasonality and frequency patterns demonstrate that Native American ignitions played a significant role in maintaining a high frequency, low severity fire regime in the region.

  • Historically, fire frequency was under strong topographic control and fire severity was mixed and variable through time and space creating a dynamic mosaic of variable-aged, fire-initiated vegetation patches mixed with long-lived, fire-resilient trees.

  • A major change in fire regimes and forest composition began in the 1800s causing shifts in composition and structure at the stand scale and homogenization at the landscape scale, suggesting the need for active management strategies to restore historic vegetative conditions.

Individual fire scars on cut log
Individual fire scars are easy to identify on the exposed surface of this recently cut log. Fingers point to the location of three injuries. By counting the number of rings or years between injuries, an estimate of fire frequency can be calculated that corresponds to the point on the landscape where this tree was growing. (photo by Stan Kitchen)



Principal Investigators: 
External Partners: 
Peter Brown, Co-Principal Investigator, Rocky Mountain Tree Ring Research