Forest fires, a common occurrence around the world, are an important and typical disturbance factor in boreal and hemiboreal forests in Europe. The statistics of forest fires in Latvia over the last century shows occasional extreme fire events. The most severe forest fires occurred in 1963 when 12 013 ha of forest area was burned. In the period 1992 – 2014, the area of burned forest ranged from 90 ha in 2012 to 8 412 ha in 1992. In Latvia, dry coniferous forest stands are subject to the highest risk; 85% of all forest fires occur in dry Scots pine forests. Scots pine (Pinus sylvestris L.) is the dominant tree species in 915 000 ha of forest area in Latvia occupying ~ 29% of all forest land. The aim of this study was to assess the instantaneous impact of low severity, surface fire occurred in the beginning of August 2014 on the chemical and biological composition of soil organic O layer in a 60-year-old dry-mesic Scots pine (Pinus sylvestris L.) forest stand classified as Vaccinio vitis – idaeo – Pinetum community in Rucava, Peši site, SW Latvia.

   Our results indicate that the composition and decay level of the soil organic O layer in drymesic Scots pine forests is mostly dependent on the abundance of herbs and especially grasses in the ground vegetation layer. Analysis of the soil organic O layer showed that the instantaneous impact of low severity surface fire on soil organic O layer is not only quantitative, but also qualitative. Fire-affected part of the 60-year-old dry-mesic Scots pine forest stand had reduced soil organic O layer thickness by 24%. The mean differences between organic C and total N stored in the soil organic O layer in the burned and unburned part of the forest stand were 16.5 t C ha^-1 and 0.3 t N ha^-1 or 26.5% of the organic C pool and 15.7% of the total N pool in the soil organic O layer was lost due to the low severity surface fire. Properties of the mineral topsoil (E horizon) below the soil organic O layer were not affected by surface fire.

Keywords: dry-mesic pine (Pinus sylvestris L.) forest, surface fire, soil organic O layer properties, carbon and nitrogen pools.