Key Ecological Function of Charcoal from Wildfire in the Boreal Forest

Abstract

Wildfire is a major disturbance factor in boreal forests where it is important in rejuvenating soil properties and encouraging tree regeneration and growth. However, the mechanisms behind these effects are poorly understood and little is known as to the ecological effects of charcoal produced by wildfire in these ecosystems. In this study we firstly quantified the mass of charcoal in a chronosequence of twelve forest sites in northern Sweden and found charcoal mass in soil to vary from 984 to 2074 kg ha -1 ; these levels appear to be sufficient for charcoal to have important ecological effect through its sorptive abilities. We then investigated the ability of charcoal from 32 forest stands (representing a range of ages from 1 to 350 yr since last fire) to adsorb phenolic compounds produced by the late successional dwarf shrub Empetrum hermaphroditum ; phenolic compounds from this species have previously been shown to have important phytotoxic effects in boreal forests. Charcoal in soil from forests younger than 100 yr was very effective at reducing these effects while older charcoal was not, suggesting that the sorptive ability is likely to be most important in earlier-successional forests. Experimental reheating of deactivated older charcoal from soil showed that temperatures above 450°C could reactivate charcoal. A microcosm experiment also revealed that soil microbes could effectively reactivate young charcoal that had been saturated with phenolics. Finally we investigated the effects of artificially made charcoal on soil microbial properties at six sites. The microbial biomass was consistently enhanced in humus when it was placed adjacent to charcoal particles. Decomposition of plant litter was sometimes also affected by being in the proximity of charcoal but the dircction of these effects was unpredictable. We conclude that charcoal might catalyse important ecological soil processes in early-successional boreal forests, effects that diminish as succession proceeds, and ultimately may have important long-term consequences for stand productivity and ecosystem function, especially in forests under strict fire control.