Drivers of understory plant communities in Sierra Nevada mixed conifer forests with pyrodiversity

Kate Wilkin,Tadashi Moody,Brandon M. Collins,Danny L. Fry,Scott L. Stephens,Lauren Ponisio

doi:10.1186/s42408-021-00111-6

Abstract

BackgroundFire suppression in western North America increased and homogenized overstory cover in conifer forests, which likely affected understory plant communities. We sought to characterize understory plant communities and their drivers using plot-based observations from two contemporary reference sites in the Sierra Nevada, USA. These sites had long-established natural fire programs, which have resulted in restored natural fire regimes. In this study, we investigated how pyrodiversity—the diversity of fire size, severity, season, and frequency—and other environment factors influenced species composition and cover of forest understory plant communities.ResultsUnderstory plant communities were influenced by a combination of environmental, plot-scale recent fire history, and plot-neighborhood pyrodiversity within 50 m. Canopy cover was inversely proportional to understory plant cover, Simpson’s diversity, and evenness. Species richness was strongly influenced by the interaction of plot-based fire experience and plot-neighborhood pyrodiversity within 50 m.ConclusionsPyrodiversity appears to contribute both directly and indirectly to diverse understory plant communities in Sierra Nevada mixed conifer forests. The indirect influence is mediated through variability in tree canopy cover, which is partially related to variation in fire severity, while direct influence is an interaction between local and neighborhood fire activity.

Highlights

Historical western USA dry mixed conifer forests likely supported diverse understory communities that were altered by fire suppression (Tilman and Lehman 2001)
Frequent fire burning with complex spatial patterns of intensity and fuel consumption was a keystone ecological process that recycled nutrients, altered community composition and assembly, and selected for fire-resilient species (Bond and Keeley 2005)
Study site Our reference sites for mixed conifer forests in the Sierra Nevada are within US National Park Service wilderness areas, including Illilouette Creek basin in Yosemite National Park and Sugarloaf Valley in Kings Canyon National Park, hereafter referred to as Illilouette and Sugarloaf, respectively (Fig. 1)

Summary

Introduction

Historical western USA dry mixed conifer forests likely supported diverse understory communities that were altered by fire suppression (Tilman and Lehman 2001). Long-fire-suppressed forests have been homogenized and densified, creating uncharacteristically continuous canopy tree layers (Larson and Churchill 2012; Lydersen et al 2013; Boisramé et al 2016; Lydersen and Collins 2018) These forests have more shade-tolerant trees and denser canopies, which together block light to the forest floor and increase competition for water and nutrients (Kilgore 1973; Minnich et al 1995; Bouldin 1999). We sought to characterize understory plant communities and their drivers using plot-based observations from two contemporary reference sites in the Sierra Nevada, USA. These sites had long-established natural fire programs, which have resulted in restored natural fire regimes. We investigated how pyrodiversity—the diversity of fire size, severity, season, and frequency—and other environment factors influenced species composition and cover of forest understory plant communities

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