Charcoal compounds are known for being resistant to decomposition therefore representing an important carbon sink. As a result, ignoring the formation of charcoal during a wildfire may result in overestimation of the impact on the carbon cycle. Additionally, charcoal plays an important role in nutrient dynamics and may contain water pollutants in the form of polycyclic aromatic hydrocarbons (PAH). We present here a charcoal budget of a post-fire northern Californian mixed-conifer forest (located in the Lassen and Plumas National Forests) and tested the use of a fire severity index as a predictor for charcoal production.
We measured charcoal in downed and standing trees, the forest floor layer, and the upper mineral soil. The investigated area underwent a wildfire in 2012, and soil and forest floor samples and tree data were collected respectively two and three years later. Fire severity over the burn area was mapped using the Relative delta Normalized Burn Ratio (RdNBR) calculated through the use of multispectral satellite data (Landsat 7). The volume of charcoal in standing trees was estimated by measuring scorch height and depth of the charcoal layer in the tree bark, and for the downed wood we adopted a modified version of the planar intercept method. Charcoal in forest floor and topsoil was estimated using the weak nitric acid digestion method.
We found a statistically significant correlation between fire severity and the charcoal concentration in the forest floor. However, we did not observe statistical differences among fire severity levels, for the charcoal stock of forest floor and mineral soil. We estimated that between 1 and 6% (first and third quartile) of the tree volume of standing trees is charred after a fire. While we observed that fire severity affected scorch height, we did not observe any effect on the charring depth in the bark. Results on the distribution of charcoal among the four compartments (downed and standing trees, forest floor, and mineral soil) in different fire severity classes will be presented.
To our knowledge, our study is the first to identify relationships between fire severity and charcoal concentrations in a burned forest. This study advances the accuracy of charcoal estimates in post-fire forests, and also shows the possibility of using widely applied indices of fire severity to predict charcoal formation in Californian mixed-coniferous forests. This provides an opportunity to to account for charcoal in current wildfire carbon emission models.