Quantifying Changes in Total and Pyrogenic Carbon Stocks Across Fire Severity Gradients Using Active Wildfire Incidents

Abstract

Positive feedbacks between wildfire emissions and climate are expected to increase in strength in the future; however, fires not only release carbon (C) from terrestrial to atmospheric pools, they also produce pyrogenic C (PyC) which contributes to longer-term C stability. Our objective was to quantify wildfire impacts on total C and PyC stocks in California mixed-conifer forest, and to investigate patterns in C and PyC stocks and changes across gradients of fire severity, using metrics derived from remote sensing and field observations. Our unique study accessed active wildfires to establish and measure plots within days before and after fire, prior to substantial erosion. We measured pre-and post-fire aboveground forest structure and woody fuels to calculate aboveground biomass, C and PyC, and collected forest floor and 0-5 cm mineral soil samples. Immediate tree mortality increased with severity, but overstory C loss was minimal and limited primarily to foliage. Fire released 85% of understory and herbaceous C (comprising <1.0% of total ecosystem C). The greatest C losses occurred from downed wood and forest floor pools (19.3 ± 5.1 Mg ha −1 and 25.9 ± 3.2 Mg ha −1 , respectively). Tree bark and downed wood contributed the greatest PyC gains (1.5 ± 0.3 Mg ha −1 and 1.9 ± 0.8 Mg ha −1 , respectively), and PyC in tree bark showed non-significant positive trends with increasing severity. Overall PyC losses of 1.9 ± 0.3 Mg ha −1 and 0.5 ± 0

High‐severity wildfire leads to multi‐decadal impacts on soil biogeochemistry in mixed‐conifer forests

“…However, there were, on average, 20% decreases in soil C regardless of burn severity directly after fire across five mixed‐conifer sites in California when the site was assessed pre/post‐fire (Miesel et al. ). This difference highlights the nuanced congruence between fire severity and mineral SOC loss, the importance of time since fire, and potential differences between experimental and retrospective studies.…”

“…, Miesel et al. ). However, comparisons among fires in different years are difficult due to changing methods in soil burn severity mapping.…”

“…) in contrast to 20% loss during high‐severity fire (Miesel et al. ). The long‐term 45–69% decreases in mineral SOC, relative to the unburned site, found in our study are uncommon; however, areas burning at high severity are proportionally increasing over time (Miller et al.…”

Global Pyrogenic Carbon Production During Recent Decades Has Created the Potential for a Large, Long‐Term Sink of Atmospheric CO2

“…Their results indicated a PyC/CO 2 ratio of 1.1–5.1%. Miesel et al () quantified the PyC production and carbon losses by five wildfires in the California mixed‐conifer forest. They reported a PyC/CO 2 ratio of 4.1%.…”

Prescribed fire science: the case for a refined research agenda

“…The fundamentally experimental nature of prescribed burning offers opportunity for developing mechanistic understanding of fire. Whereas linking pre-fire conditions, fire behavior, and fire effects is difficult and a rarity in wildfire studies (Lydersen et al 2014;Miesel et al 2018), the potential to do so exists in almost every prescribed fire (Table 1). Prescribed fire experiments link fire behavior to fire effects in real time, providing the mechanism for addressing questions of fire's role in shaping current and future ecosystems.…”