Calibration of charcoal production from trees biomass for soil charcoal analyses in subalpine ecosystems

Abstract

Subalpine ecosystems are mainly affected by surface fires of low to medium intensity. Charcoal production from trees is limited by the duration and the intensity of fires and, thus, by the wood combustibility. Because the combustibility properties of species differ, the functional traits that determine the wood combustibility could explain in part the taxonomic differences of charcoal abundance within the paleo-soil charcoal (pedoanthracological) assemblages. Eight subalpine broadleaf and needleleaf tree species were considered. Wood combustibility was studied by experimental burning on barked-wood samples exposed to constant heat. Two combustibility parameters are estimated (i) the rate of mass loss rate (MLR) and, (ii) the burning rate (BR). While BR describes the proportion of burned biomass per time unit assessing the potential conversion of wood biomass into charcoal, MLR reports the speed of fuel combustion that estimates the potential amount of charcoal produced. A hierarchy of species-specific potential production of charcoal by trees is developed by the use of biplot distribution of MLR and BR. Species were ranked from the worst to the best biomass for the charcoal conversion: Larix decidua, Abies alba, Pinus uncinata, Picea abies, Betula pendula, Salix caprea, Pinus cembra, Sorbus aucuparia. With thin smooth bark and dense wood, broadleaf trees are expected to produce more charcoal than needleleafs. Bark thickness and wood density explain half of combustibility variance. Thus, the production of charcoal tends to vary among species and among individuals of different diameter according to bark thickness. The amount of charcoal is a proxy of both (i) the abundance of understorey-tree (sapling) due to the high combustion susceptibility of small diameter stems (thin and smooth bark), and (ii) the abundance of woody debris on the ground produced by the canopy trees. The results are discussed in light of simulated data from Holocene soil charcoal assemblages for the western subalpine Alps. Among needleleafs, L. decidua is the least productive in terms of charcoal abundance due to its low MLR and BR values, whereas P. cembra and broadleafs should be better represented in the soil charcoal assemblages. This study provides rational elements for the quantitative calibration of soil charcoal through species-specific charcoal mass coefficients.