Experimental fire behaviour in managed Pinus sylvestris and Picea abies stands of Finland

Fire behaviour characteristics were studied in managed Pinus sylvestris L. and Picea abies L. Karst stands in a series of field burning experiments. Stand characteristics, surface fuel moisture content, mid-flame wind speed, rates of spread, flame heights, and torching were recorded. The Canadian Forest Fire Weather Index System (FWI System) and Finnish Fire Risk Index (FFI) were used to evaluate burning conditions and analyse the observed fire behaviour. Mid-flame wind speed was a good predictor (R-2 = 0.96 for exponential curve) of the fire spread rates. Torching formed the strongest correlation with the height of the dead branch limit. An increase in predicted fire weather hazard from FWI 4 to FWI 20 ( FWI= the FWI code of the FWI System) increased burn coverage remarkably in 15-45-year old Pinus stands and to a lesser extent in Pinus and Picea clear-cuts, but did not affect 40-60-year-old Picea stands. The FFI was unable to predict burn coverage or any other fire behaviour characteristics.