Fuel succession and fire behavior in the Swedish boreal forest

Successional changes in the potential for fire propagation were analyzed in Vaccinium myrtillus type forests in northern Sweden through fuel sampling and fire simulations in stands with different time since the last fire, and ignition tests in early successional and mature vegetation. Within three decades, fine fuel on the ground reached 0.6 to 0.9 kg.m(-2) and was thereafter fairly constant at this level. In the youngest stands this fuel consisted mainly of litter from the tree canopy, whereas in the oldest stands, pleurocarpous mosses made up the majority of the load. The expansion of pleurocarpous mosses from around year 20 resulted in a shift towards a deeper and less compact fuel bed. Simulations using the Rothermel fire spread model predicted no or minimal fire spread in stands younger than 25 years, and this was corroborated by a number of unsuccessful ignitions on recently burnt sites. We suggest three phases in postfire succession with respect to fire propagation and surface fire intensity: (1) up to about 20 years after fire, no or marginal fire spread; (2) between years 20 and 50 a progressive rise in potential fire intensity; (3) from around year 50 a steady state in potential fire intensity. By setting a limit for the shortest possible fire intervals, this pattern of fuel accumulation might have a significant effect on fire frequencies in the landscape.