Boreal soil carbon fluxes one year after a forest wildfire: impacts of burn severity and forest management

In 2018, an extreme drought affected large parts of Europe and led to the worst fire season in over a century in Sweden. We investigated the impacts of the Ljusdal fire, the largest fire complex that year, on soil CO₂ and CH₄ fluxes, nutrient concentrations and microclimate in a Scots pine forest. The measurements were conducted during the first growing season after the fire. In three separate analyses, we compared stands that differed in terms of burn severity (unburnt, low and high burn severity), salvage-logging (logged or unlogged) and stand age (young: 12 years old or mature: ~100 years old at the time of the fire).

 

A mature stand affected by a high severity burn (100% tree mortality) had significantly lower soil respiration compared to a stand affected by a low severity burn (nearly 100% tree survival), but there was no difference in soil respiration between the low burn severity and unburn stands. These results indicate that autotrophic respiration plays a key role in determining post-fire soil respiration. After a high severity burn, salvage logging had no significant effects on forest soils compared to a stand where the dead trees had been left standing, although differences between these two stands are likely to become significant in the future. Stand age had a clear impact on most of the soil properties tested. Despite mean soil temperature being 5 °C warmer at a young site compared to a mature site after a high severity burn, soil respiration was lower at the young site. The young site had been clear-cut and undergone soil scarification and replanting 12 years before the fire, which is likely to have contributed to the lower nutrient availability and thinner soil organic layer there compared to the mature site. Short return intervals between disturbances such as harvesting and wildfire that remove part of the soil organic layer can thus have significant and long-term impacts on nutrient cycling and carbon exchange in the boreal forest. The boreal forest is thus vulnerable to becoming a carbon source, especially in regions where climate change is increasing the frequency of high severity wildfire and commercial timber production is expanding.