Large-diameter trees dominate snag and surface biomass following reintroduced fire

James A Lutz,Tucker J Furniss,C Alina Cansler,Alistair M S Smith,Mark E Swanson,Sara J Germain,Soren Struckman,Andrew J Larson,Crystal A Kolden,Darren J Mcavoy,Larissa L Yocom

doi:10.1186/s13717-020-00243-8

Abstract

The reintroduction of fire to landscapes where it was once common is considered a priority to restore historical forest dynamics, including reducing tree density and decreasing levels of woody biomass on the forest floor. However, reintroducing fire causes tree mortality that can have unintended ecological outcomes related to woody biomass, with potential impacts to fuel accumulation, carbon sequestration, subsequent fire severity, and forest management. In this study, we examine the interplay between fire and carbon dynamics by asking how reintroduced fire impacts fuel accumulation, carbon sequestration, and subsequent fire severity potential. Beginning pre-fire, and continuing 6 years post-fire, we tracked all live, dead, and fallen trees ≥ 1 cm in diameter and mapped all pieces of deadwood (downed woody debris) originating from tree boles ≥ 10 cm diameter and ≥ 1 m in length in 25.6 ha of an Abies concolor/Pinus lambertiana forest in the central Sierra Nevada, California, USA. We also tracked surface fuels along 2240 m of planar transects pre-fire, immediately post-fire, and 6 years post-fire. Six years after moderate-severity fire, deadwood ≥ 10 cm diameter was 73 Mg ha−1, comprised of 32 Mg ha−1 that persisted through fire and 41 Mg ha−1 of newly fallen wood (compared to 72 Mg ha−1 pre-fire). Woody surface fuel loading was spatially heterogeneous, with mass varying almost four orders of magnitude at the scale of 20 m × 20 m quadrats (minimum, 0.1 Mg ha−1; mean, 73 Mg ha−1; maximum, 497 Mg ha−1). Wood from large-diameter trees (≥ 60 cm diameter) comprised 57% of surface fuel in 2019, but was 75% of snag biomass, indicating high contributions to current and future fuel loading. Reintroduction of fire does not consume all large-diameter fuel and generates high levels of surface fuels ≥ 10 cm diameter within 6 years. Repeated fires are needed to reduce surface fuel loading.

Highlights

Reducing woody biomass fuel loads through reintroduction of fire is seen as an important step to decreasing future fire risk and increasing forest resilience in many ecosystems of western North America (Larson et al 2013, Hessburg et al 2015, Valliant et al 2016)
The principal overstory tree species by basal area are Pinus lambertiana, Abies concolor, and Calocedrus decurrens with small populations of Pseudotsuga menziesii, Abies magnifica, and Pinus ponderosa
Prior to EuroAmerican settlement, fires in the Abies concolor/Pinus lambertiana zone burned at mixed severities, with only small patches of high-severity fire (Scholl and Taylor 2010)

Summary

Introduction

Reducing woody biomass fuel loads through reintroduction of fire is seen as an important step to decreasing future fire risk and increasing forest resilience in many ecosystems of western North America (Larson et al 2013, Hessburg et al 2015, Valliant et al 2016). First-entry fires can create patchy clumps of tree mortality (Kane et al 2019) and higher overall fire severity compared to historical fire due to a more dense and complex fuel matrix (Harris and Taylor 2015, Kolden et al 2017) This can give rise to distinct patterns of tree mortality (Grayson et al 2017), post-fire demography (Varner et al 2007), and large quantities of standing snags or deadwood pools (Miller and Urban 2000, Stephens 2004, Stephens and Moghaddas 2005, Grayson et al 2019, Stenzel et al 2019)

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