The Ecological Importance of Mixed-Severity Fires

Nature's Phoenix

Dominick A. DellaSala, Chad Hanson, Chad T. Hanson

• Publisher: Elsevier
• Year: 2015
• Topic: Biological Sciences
• eBook ISBN:
  9780128027608
• Subtopic: Biology
• Index: Biological Sciences

The Ecological Importance of High-Severity Fires, presents information on the current paradigm shift in the way people think about wildfire and ecosystems. While much of the current forest management in fire-adapted ecosystems, especially forests, is focused on fire prevention and suppression, little has been reported on the ecological role of fire, and nothing has been presented on the importance of high-severity fire with regards to the maintenance of native biodiversity and fire-dependent ecosystems and species. This text fills that void, providing a comprehensive reference for documenting and synthesizing fire's ecological role. - Offers the first reference written on mixed- and high-severity fires and their relevance for biodiversity - Contains a broad synthesis of the ecology of mixed- and high-severity fires covering such topics as vegetation, birds, mammals, insects, aquatics, and management actions - Explores the conservation vs. public controversy issues around megafires in a rapidly warming world

Section 1

Biodiversity of Mixed- and High-Severity Fires

Chapter 1

Setting the Stage for Mixed- and High-Severity Fire

Chad T. Hanson¹; Rosemary L. Sherriff²; Richard L. Hutto³; Dominick A. DellaSala⁴; Thomas T. Veblen⁵; William L. Baker^{6 1} John Muir Project of Earth Island Institute, Berkeley, CA, USA
² Department of Geography, Humboldt State University, Arcata, CA
³ Division of Biological Sciences, University of Montana, Missoula, MT, USA
⁴ Geos Institute, Ashland, OR, USA
⁵ Department of Geography, University of Colorado-Boulder, Boulder, CO, USA
⁶ Program in Ecology and Department of Geography, University of Wyoming, Laramie, WY, USA

Abstract

Drier montane forest ecosystems and some shrub habitats are assumed to have historically been maintained by lower-severity fires that created open and park-like structures in western North America. High-severity fires—especially larger patches—also often are assumed to be unnatural and ecologically damaging. These assumptions drive current land management policies where fires burn in higher severities. However, evidence indicates that montane forests, including ponderosa pine and mixed-conifer forests, were historically far more variable in tree densities and fire regimes and were maintained by a mixed-severity fire regime. Though the unique complex early seral forests created by higher-severity fires are important for a wide variety of biota, they may now be relatively rare because of fire suppression, land use alterations, and postfire logging. Management designed to encourage primarily homogeneous, low-severity fire in areas that historically had mixed-severity fire is a poor substitute for the ecosystem benefits created by these fires.

Keywords

Biodiversity

Ecological resilience

Forests

High-severity fire

Mixed-severity fire

1.1 Earlier Hypotheses and Current Research

In the late 19th century and early 20th century, fire—especially patches of high severity wherein most or all of the dominant vegetation is killed—was generally considered to be a categorically destructive force. Clements (1936) hypothesized that the mature/old state of vegetation would result in a stable “climax” condition and described natural disturbance forces such as fire as a threat to this state, characterizing mature forest that experienced high-severity fire as a “disclimax” state. One early report opined that there is no excuse or justification for allowing fires to continue to occur at all in chaparral and forest ecosystems (Kinney, 1900). After a series of large fires in North America in 1910, land managers established a policy goal of the complete elimination of fire from all North American forests (a “one size fits all” policy) through unsuccessful attempts to achieve 100% fire suppression (Pyne, 1982; Egan, 2010). Through the mid-20th century, and in recent decades, views have shifted to broadly acknowledge the importance of low- and low/moderate-severity fire. In this chapter we focus on drier montane forests of western North America as a case study of how diverse, competing, and rather complex sets of evidence are converging on a new story that embraces not just low-severity fire but also mixed- and high-severity fire in these ecosystems. Thus this chapter exemplifies how mixed- and high-severity fire is being better understood and appreciated as scientific evidence accumulates.

A commonly articulated hypothesis is that dry forests at low elevations in western North America were historically open and park-like, and heavily dominated by low-severity and low/moderate-severity fire (Weaver, 1943; Cooper, 1962; Covington, 2000; Agee and Skinner, 2005; Stephens and Ruth, 2005). Under this hypothesis, high-severity fire patches were rare, or at least were believed to be small to moderate in size, and larger patches (generally hundreds of hectares or larger) that burn today often are considered to be unnatural and ecologically harmful. While this model fits reasonably well in some low-elevation, xeric forest systems (Perry et al., 2011; Williams and Baker, 2012a, 2013), it has been extrapolated far beyond where it seems to apply best. That higher fire severities occurred historically, albeit at a wide variety of spatial and temporal scales, in most or all fire-dependent vegetation types of western North America is becoming increasingly clear (Veblen and Lorenz, 1986; Mast et al., 1998; Taylor and Skinner, 1998; Brown et al., 1999; Kaufmann et al., 2000; Heyerdahl et al., 2001, 2012; Wright and Agee, 2004; Sherriff and Veblen, 2006, 2007; Baker et al., 2007; Hessburg et al., 2007; Klenner et al., 2008; Amoroso et al., 2011; Perry et al., 2011; Schoennagel et al., 2011; Williams and Baker, 2012a; Marcoux et al., 2013; Odion et al., 2014; Hanson and Odion, 2015a).

A key extension of the concept of historical forests characterized by open structure coupled with a low- or low/moderate-severity fire regime is that current areas of dense forest structure—and larger, higher-severity fire patches in such areas—are the result of unnatural fuel accumulation from decades of fire suppression policies, leading to higher-severity fire effects outside the natural range of variability. The most fire-suppressed forests (i.e., those that have gone without fire for periods that exceed their “average” natural fire cycles) are, therefore, expected to experience unnaturally high proportions of higher-severity fire if they burn (Covington and Moore, 1994; Covington, 2000; Agee, 2002; Agee and Skinner, 2005; Stephens and Ruth, 2005; Roos and Swetnam, 2012; Williams, 2012; Stephens et al., 2013; Steel et al., 2015).

We recognize that the historical low-severity fire regime described above has not been applied to all forest types in western North America (e.g., Romme and Despain, 1989; Agee, 1993). The idea has, however, been widely applied in principle to most forest types, and widespread acceptance of the low- and low/moderate-severity fire regime has been the primary basis driving fire management policy in an overwhelmingly large proportion of montane forests in the western United States. Thus many management plans explicitly adopt a low-severity fire regime model without rigorously examining evidence of its applicability to the management of the ecosystem type under consideration. A key research need has been to determine the particular ecosystem types to which the low-severity fire regime applies. Scientists recently rigorously investigated the hypothesis that forests are burning in a largely unnatural fashion and found that historical forest structure and fire regimes were far more variable than previously believed, and that ecosystem responses to large, intense fires often differ from past assumptions (Figure 1.1; see also Chapters 2–5). We discuss these notions in greater depth throughout this book.

Figure 1.1 Natural regeneration of native vegetation—including conifers, deciduous trees, and shrubs—in large high-severity fire patches. Top: Star Fire of 2001 (photo by Chad Hanson, 2013); bottom: Storrie Fire of 2000 (photo by Chad Hanson, 2007) (see also Chapter 2).

Do Open and Park-Like Structures Provide an Accurate Historical Baseline for Dry Forest Types in Western US Forests?

Using spatially extensive tree ring field data, historical landscape photographs from the late 19th and early 20th centuries, early aerial photography from the 1930s through 1950s, and direct records from late 19th-century land survey...

Table of contents

• Cover image
• Title page
• Table of Contents
• Dedication
• Copyright
• List of Contributors
• Biographies
• Preface
• Acknowledgments
• Section 1: Biodiversity of Mixed- and High-Severity Fires
• Section 2: Global and Regional Perspectives on Mixed- and High-Severity Fires
• Section 3: Managing Mixed- and High-Severity Fires
• Index