Forest floor charcoal and fire – extent and legacy in Norwegian forests

Fire has the immediate effect that roughly half of carbon and nitrogen is emitted and lost from forest floors, that the remaining ashes fertilize the ground and pools of dead organic matter and stable black carbon is produced. Depending on the intensity of the fire it will potentially have long lasting physical, chemical and biological effects. Fire as a disturbance agent to the forest floor has acted on the forest landscapes in Scandinavia since the last glaciation as a natural phenomenon and as a result of human activities. Fires have likely occurred in all forests in Norway even though sampling and dating of charcoal in selected landscapes indicate a lower frequency along the west coast than in the southeastern forest region and in neighboring Sweden.

Where the availability of synthetic fertilizers in agriculture (ca. 1900) and the significance of timber value and -trade (ca. 1700) mark important shifts in fire occurrence and avoidance, forest fires have been successfully suppressed with documented effects since the 1970’s likely leading to an accumulation of forest floor organic matter.

Using a one-time survey of >8000 registrations of the thickness of the forest floor, its sub-layers, humus form and the occurrence of charcoal in upland forests of the Norwegian National Forest Inventory, we investigate the regional distribution of charcoal occurrence in upland forests indicating earlier fire activity and look for legacies on carbon stocks or forest floor characteristics using available national soil survey data. Forest floors in boreal and cold temperate forests hold 30-60% of total forest soil carbon stocks equivalent in magnitude to that held by the living biomass of trees. Thus, we further estimate the areas and forest floor carbon stocks most likely to gain increased vulnerability to fire under future climate conditions.