%0 Articles %T Climate and forest fires in Finland – influence of lightning-caused ignitions and fuel moisture %A Larjavaara, Markku %D 2005 %J Dissertationes Forestales %V 2005 %N 5 %R doi:10.14214/df.5 %U http://dissertationesforestales.fi/article/1788 %X The objectives of this research were to examine how lightning characteristics influence ignition probability of a lightning stroke, to study the distribution of lightning-ignited forest fires and to examine fuel moisture variation. This research utilized the comprehensive forest fire records collected by the Finnish Ministry of the Interior. Individual fires were systematically linked with individual lightning strokes to compute probabilities that a stroke has ignited a fire. A method of grouping lightning flashes into thunderstorms was developed in order to compare the ignition probability of a stroke and the characteristics of the thunderstorm to which the stroke belongs. To examine fuel moisture variation, the spatial and temporal variation in ignition probability was studied based on meteorological data, ignition experiments and Finnish forest fire risk index model. The results supported the hypothesis that a stroke in a small thunderstorm is more likely to result in ignition than a stroke in a large thunderstorm. However, the results contradicted theories predicting that positive strokes or flashes of high multiplicity would more likely result in ignition than negative strokes or flashes of low multiplicity. Positive and negative strokes were equally likely to ignite a fire and that the ignition probability of a stroke decreases with increasing multiplicity of a flash. Fuels were at their driest in late May and June in southern Finland and in late June in northern Finland. Lightning-caused ignitions were most frequent in July. Fuels were significantly dryer and lightning strokes are more frequent in southern Finland than in northern Finland. As a result, the density of lightning caused forest fires was twenty times higher in southern Finland than in northern Finland. Assuming that the natural fire cycle in southern Finland is at least 100 years, the results suggests that in northern Finland the natural fire cycle is several thousand years. If natural disturbance dynamics are mimicked in forest management and restoration, this south-north difference in natural fire regime characteristics, and consequently in forest disturbance dynamics in general, should be taken into account.