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Wildfires are likely to have a major influence on below-ground patterns and processes in forests but these effects and their consequences to forest succession are generally poorly known. Ectomycorrhizal macrofungi (ECM) is a key below-ground ecological group, mainly because of their functional relationships to trees. During severe fire disturbances, below-ground mycelia of ECM may be negatively affected. This study analyses long-term effects of a wildfire on ECM assemblages.

Using data from 12-years post-fire succession of ECM, we investigated the long-term fire effects on ECM, focusing on the effects of spatial variability in fire severity. The wildfire occurred in 1992 in eastern Finland, and covered 143 ha. We established sampling plots to four types of sites within the burned area, based on the observed fire severity, and also on unburned control sites. ECM assemblages were surveyed every autumn during the following 12 years period. A total of 133 ECM species and 14 469 ECM sporocarps were sampled.

Fire severity was closely associated with the post-fire ECM richness and assemblage patterns. The highest overall ECM species richness was in the low severity surface fire area, 87 species in total, followed by the intermediate ground fire area, 72 species; they both included post-disturbance ECM pioneer species but also species that are usually associated with old-growth forests. The high-severity crown fire area comprised only 15 ECM species and the high-severity treeless ground fire area 23 species. Assemblage composition (based on NMDS) was different in the two most severely burned areas, in comparison to less severely burned areas and controls.

We conclude that fire affects ECM assemblages and that the severity of a wildfire is connected to post-fire patterns of ECM richness and composition. The results suggest a functional link between wildfire and post-fire tree growth, mediated through the effects of fire on ECM. These connections may be important in maintaining small-scale, within-stand spatial heterogeneity in the natural post-disturbance forests.