Patterns of Ecosystem Structure and Wildfire Carbon Combustion Across Six Ecoregions of the North American Boreal Forest

Evan S Kane,Xanthe J Walker,Laura Bourgeau-Chavez,Nicola J Day,Sander Veraverbeke,Catherine M Dieleman,Brendan M Rogers,Jennifer L Baltzer,Merritt R Turetsky,Jill F Johnstone,Michelle C Mack

doi:10.3389/ffgc.2020.00087

Abstract

Increases in fire frequency, extent, and severity are expected to strongly impact the structure and function of boreal forest ecosystems. An important function of the boreal forest is its ability to sequester and store carbon (C). Increasing disturbance from wildfires, emitting large amounts of C to the atmosphere, may create a positive feedback to climate warming. Variation in ecosystem structure and function throughout the boreal forest are important for predicting the effects of climate warming and changing fire regimes on C dynamics. In this study, we compiled data on soil characteristics, stand structure, pre-fire C pools, C loss from fire, and the potential drivers of these C metrics from 527 sites distributed across six ecoregions of North America’s western boreal forests. We assessed structural and functional differences between these fire-prone ecoregions using data from 417 recently burned sites (2004-2015) and estimated ecoregion-specific relationships between soil characteristics and depth from 167 of these sites plus an additional 110 sites (27 burned, 83 unburned). We found that northern boreal ecoregions were generally older, stored and emitted proportionally more belowground than aboveground C and exhibited lower rates of C accumulation over time than southern ecoregions. We present ecoregion specific estimates of depth-wise soil characteristics that are important for predicting C combustion from fire. As climate continues to warm and disturbance from wildfires increases, the C dynamics of these fire-prone ecoregions are likely to change with significant implications for the global C cycle and its feedbacks to climate change.

Highlights

The boreal forest is one of the largest biomes on earth, covering almost 1.9 billion hectares and encompassing ∼30% of the global forested area (Brandt et al, 2013)
In assessing the pre-fire structure of these fire-prone ecosystems (Q1), we found that stand age, proportion of black spruce, stand basal area, aboveground biomass, aboveground C pools, soil organic layer (SOL) depths, belowground C pools, total C pools, and the proportion of total C stored belowground differed between ecoregions (Figure 1, Table 1 and Supplementary Table S2)
Pre-fire aboveground C increased slightly in the Taiga Plains and Taiga Shield, but we found no significant relationships between aboveground C pools and time of stand establishment in the Alaskan (Alaska Boreal Interior and Boreal Cordillera) or southern boreal (Boreal Plains and Softwood Shield) ecoregions (Figure 3A and Table 2)

Summary

Introduction

The boreal forest is one of the largest biomes on earth, covering almost 1.9 billion hectares and encompassing ∼30% of the global forested area (Brandt et al, 2013). The most significant and critical function of the boreal forest is its ability to sequester and store carbon (C), as it contains approximately one third of terrestrial C stocks (Pan et al, 2011). This globally important biome is becoming increasingly vulnerable to change as the climate continues to warm. Increasing fire extent, frequency, and severity could alter boreal forest C storage (Bond-Lamberty et al, 2007; Walker et al, 2019): from the historical net accumulation of C from the atmosphere over multiple fire cycles, to a net loss, which in turn would cause a positive feedback to global climate warming (Oris et al, 2013; Li et al, 2017). Understanding the structure and function of these fire-prone ecoregions is needed to quantify the role of fire in the global C cycle and its feedbacks to climate change

Methods

Results

Conclusion