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

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 3years after wildfire. We measured C and PyC stocks in the following compartments: standing trees, downed wood, forest floor, and mineral soil (0-5cm), 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 24830gCm(-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. Plain Language Summary Charcoal (i.e., pyrogenic carbon) is an important form of carbon present in forest soil subject to fire events that is characterized by a high resistance to decomposition and thus may remain in soil for hundreds of years before it is transformed into carbon dioxide (an important green house gas). However, charcoal production is so far not considered when assessing the impact of forest fires on green house gas production. It is possible that due to this neglection the impact of forest fires on the carbon cycle is currently overestimated. Here we quantified charcoal production during forest fires in a forest burned by fires of different severity. We found that fire severity does not alter the amount of charcoal created, whereas it influences the distribution in the forest compartments (forest floor versus trees). Our results have important implications for the residence time of charcoal in the terrestrial ecosystems, because charcoal is expected to stay longer in standing trees and coarse woody debris compared to the forest floor, where it is susceptible to rapid losses through erosion.