What Drives Low-Severity Fire in the Southwestern USA?

Sean Parks,Solomon Dobrowski,Matthew Panunto

doi:10.3390/f9040165

Abstract

Many dry conifer forests in the southwestern USA and elsewhere historically (prior to the late 1800’s) experienced fairly frequent surface fire at intervals ranging from roughly five to 30 years. Due to more than 100 years of successful fire exclusion, however, many of these forests are now denser and more homogenous, and therefore they have a greater probability of experiencing stand-replacing fire compared to prior centuries. Consequently, there is keen interest in restoring such forests to conditions that are conducive to low-severity fire. Yet, there have been no regional assessments in the southwestern USA that have specifically evaluated those factors that promote low-severity fire. Here, we defined low-severity fire using satellite imagery and evaluated the influence of several variables that potentially drive such fire; these variables characterize live fuel, topography, climate (30-year normals), and inter-annual climate variation. We found that live fuel and climate variation (i.e., year-of-fire climate) were the main factors driving low-severity fire; fuel was ~2.4 times more influential than climate variation. Low-severity fire was more likely in settings with lower levels of fuel and in years that were wetter and cooler than average. Surprisingly, the influence of topography and climatic normals was negligible. Our findings elucidate those conditions conducive to low-severity fire and provide valuable information to land managers tasked with restoring forest structures and processes in the southwestern USA and other regions dominated by dry forest types.

Highlights

Wildland fire is an integral component of most dry conifer forest ecosystems in the southwesternUSA and elsewhere [1]
As a result of fire exclusion policies that reduced fire frequency and area burned after the late 19th century [5,6], many dry conifer forests in the southwestern USA are denser and more homogenous compared to the pre-settlement era [7,8]
Excluding studies involving fire refugia, which focus on unburned or low-severity patches within a matrix of moderateto high-severity fire [28,29], little-to-no research has been conducted that focuses on the drivers of low-severity fire in dry conifer forests such as those found in the southwestern USA

Summary

Introduction

Wildland fire is an integral component of most dry conifer forest ecosystems in the southwesternUSA and elsewhere [1]. As a result of fire exclusion policies that reduced fire frequency and area burned after the late 19th century [5,6], many dry conifer forests in the southwestern USA are denser and more homogenous compared to the pre-settlement era [7,8]. There is growing concern that some dry forests are at risk of burning at higher severities (i.e., stand-replacing) than occurred in past centuries [9,10]. Recent research suggests this is the case [11,12,13]. Stand-replacing fire in dry conifer forests has caused substantial concern about enduring conversions to non-forest. For example, that the regeneration of dry conifer species (e.g., ponderosa pine) becomes more limited with increasing fire severity, increasing distance to seed

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