Long‐term vegetation responses to reintroduction and repeated use of fire in mixed‐conifer forests of the Sierra Nevada

Karen M Webster,Charles B Halpern

doi:10.1890/es10-00018.1

Abstract

Nearly a century of fire suppression has changed fundamental aspects of the structure and functioning of fire‐adapted forests throughout the western U.S. Prescribed fire is increasingly used to restore forest structure and reduce surface fuels with limited consideration of its consequences for biological diversity. In this study, we used more than two decades of data from permanent plots in mixed‐conifer forests of Sequoia and Kings Canyon National Parks, California, to explore changes in plant diversity and abundance following reintroduction and repeated use of fire. Data on stand structure, fuel loading, fire severity and heterogeneity, and the richness and abundance of major growth forms were collected on 51 plots representing one of three treatments: control, first‐entry burn, and second‐entry burn. Understories showed distinct compositional changes over time in first‐ and second‐entry burns. Burned plots supported more than twice as many species as controls 10 yr after treatment; first‐entry plots showed a nearly threefold increase in richness by year 20. Burned plots supported four to five times as many shrub species as controls 5–10 yr after burning. Total plant cover (dominated by perennial forbs and shrubs) increased in first‐entry plots, but did not differ from controls until 20 yr after treatment. Following second‐entry, cover did not change through final sampling (year 10). Nonnative species were rare, occurring in only three plots at low abundance. Higher severity fires led to greater numbers of species and to greater plant cover. Species richness was not correlated with burn heterogeneity. Long‐term observations suggest that reintroduction of fire in previously unmanaged forests can gradually enhance the diversity and abundance of understory species. Repeated burning—necessary to achieve structural and fuel‐reduction objectives—does not appear to have a detrimental effect on plant diversity and may enhance the distributions of species that are adversely affected by fire exclusion. If fire is to play an important role in restoration, however, it will need to be maintained as a frequent and spatially dynamic process on the landscape.

Highlights

Fire plays an important role in many forest ecosystems (Payette et al 1989, Brown and Smith 2000, Bradstock et al 2002, Otterstrom and Schwartz 2006) including mixed-conifer forests of the Sierra Nevada (Vankat and Major 1978, van Wagtendonk and Fites-Kaufman 2006)
We explore the long-term responses of forest understories to reintroduction of fire in previously unmanaged, mixed-conifer forests of Sequoia and Kings Canyon National Parks (SEKI), California
We address the following questions: (1) Does reintroduction of fire affect the composition, diversity, and abundance of understory plants? (2) How do community composition and the diversity and abundance of plant growth forms change with time since burning? (3) Do responses differ after first- and second-entry burns? (4) How do severity and heterogeneity of fire affect patterns of richness and abundance? Do relationships differ for first- and second-entry fires and do they change over time?

Summary

Introduction

Fire plays an important role in many forest ecosystems (Payette et al 1989, Brown and Smith 2000, Bradstock et al 2002, Otterstrom and Schwartz 2006) including mixed-conifer forests of the Sierra Nevada (Vankat and Major 1978, van Wagtendonk and Fites-Kaufman 2006). It has led to unprecedented increases in the density of shade-tolerant conifers and surface fuels and increasing potential for large stand-replacing fire (Vankat and Major 1978, Arno and Brown 1991, Agee 1993, Minnich et al 1995) In response to these changes and to the increasing risk of catastrophic fire, resource managers on federal lands are using fire in combination with mechanical methods (thinning and mastication) to alter forest structure and reduce fuel accumulations (North et al 2007, Schwilk et al 2009, Vaillant et al 2009). Formal tests of this assumption are constrained by a lack of historical data on understory structure, studies of the broader implications of reintroducing fire are critical as managers seek to balance ecological values, societal pressures, and the operational constraints of prescribed burning on federal lands

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Conclusion