Fuel conditions and fire behavior characteristics of managed Picea abies and Pinus sylvestris forests in Finland

Abstract

The objectives of this study were to quantify and qualify the impact of structural stand characteristics on ignition potential, surface fuel moisture, and fire behavior in Pinus sylvestris L. and Picea abies (L.) Karst stands and to test the applicability of the Canadian Fire Weather Index System and the Finnish Fire Risk index for the modeling of stand type –specific fire danger in Finland. Additionally, the study analyzes the seasonal patterns of fire activity and the relationship between observed fire activity and fire weather indices at different stages of growing season. Field experiments on ignition potential, fuel moisture, and fire behavior were carried out in Pinus sylvestris or Picea abies dominated stands ranging 0–60 years in age during fire seasons 2001 and 2002. The field observations were analyzed in relation to stand structure and the outputs of the fire weather index systems. The relationship between fire activity and fire weather indices was studied based on national fire statistics 1996–2003, effective temperature sum, the Canadian Fire Weather Index System, and the Finnish Fire Risk Index. Clear differences were found in the development of fire danger between open and closed stands and between closed Picea abies and Pinus sylvestris stands. Point fire ignition potential was highest in Pinus clear-cuts and nearly non-existing in closed Picea stands. Moss-dominated surface fuels were driest in clear-cut and sapling stage stands and presented the highest moisture content under closed Picea canopy. Pinus sylvestris stands carried fire under a wide range of fire weather conditions within which Picea abies stands consistently failed to sustain surface fire. In the national fire statistics, the daily number of reported ignitions presented three seasonal peaks, whereas the daily area burned had its most substantial peak during early summer and a smaller one very late in the season. The Finnish Fire Risk Index and The Canadian Fire Weather Index were mostly capable of explaining ignition potential and fuel moisture but unable to explain fire behavior in the experimental fires. The fire weather indices correlated with fire activity fairly well during the mid-part growing season; before and after the most active period of growing season fire activity was to a larger extent disconnected from fire danger levels indicated by the indices. In conclusion, the development of fire danger under the same general weather conditions varied significantly between the fuel types defined by dominant tree species and stand age. Furthermore, the stage of the growing season influenced fire danger and the ability of the fire weather indices to assess the burning conditions. The modification of the general fire weather indices into stand-specific fire danger assessment tools would facilitate fire use and fire suppression operations.