Macro-Particle Charcoal C Content following Prescribed Burning in a Mixed-Conifer Forest, Sierra Nevada, California

Abstract

Fire suppression and changing climate have resulted in increased large wildfire frequency and severity in the western United States, causing carbon cycle impacts. Forest thinning and prescribed burning reduce high-severity fire risk, but require removal of biomass and emissions of carbon from burning. During each fire a fraction of the burning vegetation and soil organic matter is converted into charcoal, a relatively stable carbon form. We sought to quantify the effects of pre-fire fuel load and type on charcoal carbon produced by biomass combusted in a prescribed burn under different thinning treatments and to identify more easily measured predictors of charcoal carbon mass in a historically frequent-fire mixed-conifer forest. We hypothesized that charcoal carbon produced from coarse woody debris (CWD) during prescribed burning would be greater than that produced from fine woody debris (FWD). We visually quantified post-treatment charcoal carbon content in the O-horizon and the A-horizon beneath CWD (> 30 cm diameter) and up to 60 cm from CWD that was present prior to treatment. We found no difference in the size of charcoal carbon pools from CWD (treatment means ranged from 0.3–2.0 g m-2 of A-horizon and 0.0–1.7 g m-2 of O-horizon charcoal) and FWD (treatment means ranged from 0.2–1.7 g m-2 of A-horizon and 0.0–1.5 g m-2 of O-horizon charcoal). We also compared treatments and found that the burn-only, understory-thin and burn, and overstory-thin and burn treatments had significantly more charcoal carbon than the control. Charcoal carbon represented 0.29% of total ecosystem carbon. We found that char mass on CWD was an important predictor of charcoal carbon mass, but only explained 18–35% of the variation. Our results help improve our understanding of the effects forest restoration treatments have on ecosystem carbon by providing additional information about charcoal carbon content.

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

“…Because our fuels data were collected for the plot scale, we do not have spatially explicit relationships between downed wood biomass and C or PyC CTR loss from soil at the scale of soil sample locations. However, Weichman et al (2015) found no correlation between PyC stock and distance (up to 60 cm) from charred logs 12 years after a prescribed fire in mixed conifer forest. They suggest that charcoal shed from large-diameter downed wood either takes more than a decade to slough off the bark, or is rapidly redistributed away from the logs.…”

“…For example, a study conducted 4-5 years after fire in an Oregon mixed-conifer forest showed that low-severity areas were a net sink of C even though they had only partially recovered the lost C, whereas high-severity areas were still a C source because of dead wood decomposition (Meigs et al, 2009). Weichman et al (2015) found that 50% of emitted C was recovered 10 years after a prescribed burn in a California mixed-conifer forest, whereas Eskelson et al (2016) reported significant losses of live wood C in low-and moderate-severity areas, but no change in highseverity areas, over 6 years post-fire for 130 Forest Inventory and Analysis plots in the same state. Therefore, in fire-prone forests, increases in fire severity have potential to increase short-term C losses via fire emissions (including emissions from otherwise longer-lived C pools such as dead wood and mineral soil C) as well as decrease longer-term C storage by slowing the rate of postfire forest recovery (Pan et al, 2011).…”

Pyrogenic Carbon Generation From Fire and Forest Restoration Treatments

“…Despite the fact that PyC has widely been recognized as a highly chemically stable form of ecosystem C that is formed in all biomass burning events and is of great importance to global C cycle, to date there has been limited effort to quantify forest fire generated PyC and in particular that affiliated with Rx fire events in forest ecosystems (Alexis et al, 2012;Pingree et al, 2012;Wiechmann et al, 2015). In addition to wildfire events that commonly occur as a result of local climatic conditions at the seasonally dry forest ecosystems of the Rocky Mountain West (Agee, 1996), Rx fires are applied widely as a forest management practice throughout the region to reduce surface fuel loading and restore the historical fire regime (Ryan et al, 2013).…”

Size fractionation as a tool for separating charcoal of different fuel source and recalcitrance in the wildfire ash layer

“…Within this continuum, charcoal comprises coarse particles (mmcm) that retain the physical and chemical properties allowing to identify its biomass source (Bird and Ascough, 2012;Knicker et al, 2008;Scott, 2010). The aromatic structure gives charcoal an inherent increased resistance to biotic and abiotic degradation (Eckmeier et al, 2007;Wiechmann et al, 2015), which is further increased once the charcoal is buried in soil, due to enhanced physical protection, to an extent that charcoal is a common source of paleoenvironmetal and archaeological proxy data (Ascough et al, 2008).…”

“…However, charcoal is a heterogeneous material (Bird and Ascough, 2012;Pyle et al, 2015), whose variability is mostly due to differences in burning conditions (in terms of temperature, duration, flaming or smouldering conditions, oxygen availability et cetera) and source materials (e.g. woody or grass vegetation, leaves or branches, conifers or broadleaves) (Bodí et al, 2014;Knicker et al, 2008;Merino et al, 2015;Santín et al, 2012;Wiechmann et al, 2015) and their interactive effect (Hatton et al, 2016). Therefore, charcoal heterogeneity is particularly high in fire affected natural forest ecosystems, due to an often wide diversity of species, fuel types, structure and density, combined with a high spatial and temporal variability in fire behaviour (temperature, flame residence time, oxygen availability) in a given fire (Pyle et al, 2015;Santín et al, 2016b).…”