Distribution of macro charcoal from forest fires in shallow soils of the Northern Alps

Abstract

In our study area in the Northern Limestone Alps (Austria), forest fires are rare; however, they may be stand-replacing and can trigger severe erosion. The aim of our study was to find out if where and to which extent charcoal is stored in the Rendzic Leptosols of the affected slopes and if the charcoal can be used to reconstruct earlier forest fires. We took soil samples from shallow soil pits on >20 slopes affected by forest fires in the last few centuries. On four slopes of different age after fire (2003, 1962, 1946 and 1250 AD), samples were taken from each soil horizon. We pre-screened the samples in the field and investigated 167 of them in the laboratory for macroscopically visible charcoal fragments (>1 mm). We calculated the percentage of charcoal and derived a simplified model of spatial distribution and temporal behaviour. Mean charcoal concentration was 0.452 % (by bulk weight) for known forest fire sites and 0.067 % in their presumably unburnt surrounding. Significantly elevated charcoal concentrations (exceeding 0.15 %) can be used as evidence to delimit historical forest fire sites, even if there is a certain background concentration in the surrounding due to aeolian transport and to earlier fires, and small-scale spatial variability is very high. Charcoal may be found in very different positions within the profiles; frequently, the particles are concentrated on top of the Ah horizon and a post-fire O horizon has developed on top of them. 14C dating of buried charcoal assemblages revealed the existence of earlier events (c. 3 000–500 BP) well before the last known historical forest fires. The vertical distribution of charcoal fragments is influenced by fire characteristics, elapsed time and by the intensity of erosion. Charcoal concentration and mean particle size decrease with age due to erosion and translocation; however, if erosion is moderate, charcoal can be conserved for centuries. A conceptual model of vertical charcoal distribution is presented.