Fire severity alters the distribution of pyrogenic carbon stocks across ecosystem pools in a Californian mixed‐conifer forest

Abstract

Pyrogenic carbon (PyC) is hypothesized to play an important role in the carbon (C) cycle due to its resistance to decomposition; however, much uncertainty still exists regarding the stocks of PyC that persist on‐site after the initial erosion in postfire forests. Therefore, understanding how fire characteristics influence PyC stocks is vital, particularly in the context of California forests for which an increase of high‐severity fires is predicted over the next decades. We measured forest C and persistent PyC stocks in areas burned by low‐to‐moderate and high‐severity fire, as well as in adjacent unburned areas in a California mixed‐conifer forest, 2 to 3 years after wildfire. We measured C and PyC stocks in the following compartments: standing trees, downed wood, forest floor, and mineral soil (0–5 cm), and we identified PyC using the weak nitric acid digestion method. We found that the total stock of PyC did not differ among fire severity classes (overall mean 248 ± 30 g C m−2); however, fire severity influenced the distribution of PyC in the individual compartments. Areas burned by high‐severity fire had 2.5 times more PyC stocked in the coarse woody debris (p < 0.05), 3.3 times more PyC stocked in standing trees (p < 0.05), and a lower PyC stock in the forest floor (−22%, p < 0.05) compared to low‐to‐moderate fire severity areas. These results have important implications for the permanence time of PyC, which is putatively higher in standing trees and coarse woody debris compared to the forest floor, where it is susceptible to rapid losses through erosion.

Effects of Fire Severity and Topography on Soil Black Carbon Accumulation in Boreal Forest of Northeast China

“…This result supports our second hypothesis that the soil BC content is high in forests burned by moderate-severity fires. Our result supports the finding that the pyrogenic carbon in soils was higher after moderate forest fire disturbances in California, USA [29]. This is because fuel is exposed to high temperatures during high-severity fires, resulting in more complete combustion than in moderate-severity fires and thus reduced production of BC [23,53].…”

“…Forest fire severity coupled with topographic conditions may impact the formation and content of BC in soils [24,29]. We found soil BC stocks on flat and shady slopes was higher than those on sunny slopes.…”

“…Many factors may influence soil BC [28] and forest fire severity is a major contributing factor to soil BC retention. For example, a previous study showed that less BC stocked in the forest floor was burned by high-severity fires than by low-to-moderate severity fires [29]. In contrast, a previous study reported that BC tended to increase in areas subjected to low-severity burning [30].…”

Fire severity, time since fire, and site-level characteristics influence streamwater chemistry at baseflow conditions in catchments of the Sierra Nevada, California, USA

“…As such, an increase in fire severity can lead to decreased C and N concentrations and stocks in surface soils (Homann et al 2011), decreasing the amount of C and N that is vertically transported to lower soil depths and ultimately exported to streams. In studies conducted two to three years following a fire in the Sierra Nevada, high-severity fires were reported to decrease total C stocks in organic horizons relative to unburned treatments (Maestrini et al 2017, Adkins et al 2019. During high-severity fires, increases in soil heating can also decrease mineral soil C concentrations (Almendros et al 2003, Araya et al 2017) and lead to low molecular weight DOM (Norwood et al 2013, Santos et al 2016 transported from soils to streams.…”

Atmospheric and Surface Climate Associated With 1986–2013 Wildfires in North America

“…Wildfire is a major source of ecosystem disturbance (Burton, 2005;Isaak et al, 2010;Rieman et al, 2010), and the impact of wildfire on society is wide-ranging (Dombeck et al, 2004: Gill et al, 2013Kochi et al, 2010;McKenzie et al, 2014). Wildfire also plays a role in the dynamics of Earth's carbon budget (Balshi et al, 2007;Bird et al, 2015;Bowman et al, 2013;Mack et al, 2011;Maestrini et al, 2017). Humans have influenced fire regimes for millennia by modifying landscapes, suppressing fires, and serving as a source of ignition.…”