Spatial distribution of lightning-ignited forest fires in Finland

We examined the spatial and temporal distribution of forest fires ignited by lightning in Finland. The study was based on all reported lightning-ignited forest fires in Finland for the years from 1985 to 1992 and from 1996 to 2001. There was a strong decreasing gradient in the density of reported lightning-ignited forest fires from south to north, ranging from ca. 0.1 fires/(year 100 km2) on the southern coast to less than 0.01 fires/(year 100 km2) in the northern Finland. The mean date of reported ignitions was 10 July. A method was developed to estimate the density of unreported lightning-ignited forest fires. Their proportion varied from 70% of all lightning-ignited forest fires in the most sparsely populated municipalities to 63% in the most densely populated rural municipality. The results about the density of reported and estimation of unreported lightning-ignited forest fires indicate that forests in southern Finland would naturally have notably shorter fire intervals than forests in the northern parts of the country. This variability in natural fire regime characteristics suggests that, different forest management and silvicultural approaches should be applied for the maintenance of natural forest structures, processes and biodiversity in different parts of Finland.