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October 17, 2024

After over a century of human-driven fire suppression in the American Southwest, forest fires in recent decades are growing in extent and severity. Historically, dry conifer forests burned on a regular basis, raising the question of whether current wildfire regimes are a return to the norm. To study this, researchers used tree rings to explore 300 years of forest fire history across the Southwest.

A fire-scarred southwestern white pine in the Jemez Mountains, New Mexico.
A fire-scarred southwestern white pine in the Jemez Mountains, New Mexico. The tree survived and recorded multiple low-severity fires, the last of which burned over 140 years ago in 1880.  

How do you study forest fire history?

After a forest fire, surviving trees will continue to grow new layers of wood to heal fire injuries. As tree rings provide information about the growth history and age of trees, scientists can use tree-ring fire scars to obtain information about the year, season, severity, frequency, size, and fire-climate relationships of fires that occurred centuries to millennia prior to modern records. 

In this study, researchers quantified modern fire severity at hundreds of tree-ring fire-scar sites across the Southwest. They specifically chose sites which had historically burned frequently with low-severity fire for centuries prior to human-driven fire exclusion. 

 

Wildfires have become less frequent but more severe

Researchers found that, even with the increase in fire in recent decades, dry conifer forests are burning at less than 20% of historical levels. In addition, they found these forests are now burning at greater fire severity, often killing forests that survived many fires in the past. 

Moreover, suppressed wildfires tended to burn more severely than prescribed burns and wildfires managed for resource benefit. These findings inform the restoration of low-severity fire to dry conifer forests to increase resilience to increasing high-severity fire.

A cross-section of a fire-scarred ponderosa pine from the Sante Fe Watershed in New Mexico.
A cross-section of a fire-scarred ponderosa pine from the Santa Fe Watershed, New Mexico. Some trees record past fires as fire scars, which are preserved in the tree rings. Fire scars can be used to date the year, and sometimes the season, of past fires.

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