How geomorphic context governs the influence of wildfire on floodplain organic carbon in fire‐prone environments of the western United States

Abstract

We draw on published studies of floodplain organic carbon storage, wildfire-related effects on floodplains in temperate and high latitudes, and case studies to propose a conceptual model of the effects of wildfire on floodplain organic carbon storage in relation to climate and valley geometry. Soil organic carbon typically constitutes the largest carbon stock in floodplains in fireprone regions, although downed wood can contain significant organic carbon. We focus on the influence of wildfire on soil organic carbon and downed wood as opposed to standing vegetation to emphasize the geomorphic influences resulting from wildfire on floodplain organic carbon stocks. The net effect of wildfire varies depending on site-specific characteristics including climate and valley geometry. Wildfire is likely to reduce carbon stock in steep, confined valley segments because increased water and sediment yields following fire create net floodplain erosion. The net effect of fire in partly confined valleys depends on site-specific interactions among floodplain aggradation and erosion, and, in high-latitude regions, permafrost degradation. In unconfined valleys in temperate latitudes, wildfire is likely to slightly increase floodplain organic carbon stock as a result of floodplain aggradation and wood deposition. In unconfined valleys in high latitudes underlain by permafrost, wildfire is likely in the short-term to significantly decrease floodplain organic carbon via permafrost degradation and reduce organic-layer thickness. Permafrost degradation reduces floodplain erosional resistance, leading to enhanced stream bank erosion and greater carbon fluxes into channels. The implications of warming climate and increased wildfires for floodplain organic carbon stock thus vary. Increasing wildfire extent, frequency, and severity may result in significant redistribution of organic carbon from floodplains to the atmosphere via combustion in all environments examined here, as well as redistribution from upper to lower portions of watersheds in the temperate zone and from floodplains to the oceans via riverine transport in the high-latitudes.

An Integrative Conceptualization of Floodplain Storage

“…It is important to note that the higher values summarized in Marron (1992) represent river corridors with increased sediment inputs as a result of mining or land use changes. These river corridors might be expected to store a greater proportion of sediment inputs on floodplains, as also demonstrated for river corridors responding to natural disturbances such as massive landslides (Clague et al, 2003) or widespread debris flows (Sutfin & Wohl, 2019), volcanic eruptions (Major et al, 2018), jökulhlaups (Oswald & Wohl, 2008), and wildfire (Wohl et al, 2020). Schenk et al (2013) proposed that floodplain storage be expressed as gross and net floodplain trapping factors.…”

Wildfire and earth surface processes

“…Since the persistence of soil carbon is influenced by factors such as landscape position and burial depth, understanding the lateral redistribution of carbon following fire is important for determining its ultimate fate. The transport of organic and pyrogenic carbon (PyC) was the focus of two studies within the special issue (Galanter et al 2018; Wohl et al 2020). Galanter et al (2018) developed a conceptual model for PyC.…”

“…Over short timescales, they showed that PyC is concentrated in the soil O horizons and on hillslopes, but over decadal timescales, PyC is routed and retained in soil A horizons and low‐gradient landscape positions. Wohl et al (2020) examined how organic carbon in valley floodplains changed after fire depending on (1) the valley confinement and (2) the presence of permafrost. Generally, they found that organic carbon tends to decrease in confined valleys and increase in unconfined valleys.…”

“…Their analysis suggests that fine sediment was preferentially transported out of the catchment, with implications for downstream water quality, and that the transition from debris flow to hyperconcentrated flow corresponded with a change in the scaling relationship between channel slope and contributing area.In addition to transporting sediment, the intensified hydrologic and geomorphic responses induced by fire can lead to redistribution of organic and pyrogenic carbon across the landscape. Since the persistence of soil carbon is influenced by factors such as landscape position and burial depth, understanding the lateral redistribution of carbon following fire is important for determining its ultimate fate.The transport of organic and pyrogenic carbon (PyC) was the focus of two studies within the special issue(Galanter et al 2018;Wohl et al 2020). Galanter et al (2018) developed a conceptual model for PyC.Over short timescales, they showed that PyC is concentrated in the soil O horizons and on hillslopes, but over decadal timescales, PyC isF I G U R E 1Burned versus unburned hillslope in the San Gabriel Complex Fire on 14 September 2016 (Los Angeles County, California, USA).…”

Geomorphology and climate interact to control organic carbon stock and age in mountain river valley bottoms

“…Rivers and wetlands may store more organic carbon (OC) in soil and downed wood than surrounding parts of the terrestrial landscape (Aufdenkampe et al, 2011; Sutfin et al, 2016; Wohl et al, 2017; Wohl et al, 2020). This storage varies in both spatial distribution of OC concentration (Battin et al, 2008; Scott & Wohl, 2018b; Sutfin et al, 2016; Sutfin & Wohl, 2017; Wohl et al, 2017) and residence time (Marwick et al, 2015; Omengo et al, 2016; Barnes et al, 2018).…”