Effects of Fire on Ectomycorrhizal Fungi in Fennoscandian Boreal Forests

Fire, the primary natural disturbance factor in Fennoscandian boreal forests, is considered to have exerted major selection pressure on most boreal forest organisms. However, recent studies show that few ectomycorrhizal (EM) fungi appear to have evolved post-fi re adaptations, no succession of EM fungi following fi re is apparent after low intensity fi res, and only two EM fungal taxa exclusively fruit at post-fi re conditions. In this paper I review the present knowledge of effects of forest fi re on EM fungal communities in Fennscandian boreal forests, put into perspective by a comparison from other parts of the world. Characteristically, these boreal forests consist of less than a handful of tree species, e.g. Scots pine and Norway spruce, while the below ground communities of EM fungi is impressively species rich with presently more than 700 known taxa. Commonly, forest fi res in Fennoscandia have been of low intensity, with a considerable portion of the trees surviving and the organic humus layer partly escaping combustion. Hence, EM fungi appear to largely have evolved under conditions characterised by a more or less continuous presence of their hosts. In fact, the composition of EM fungi within a forest appear be more variable due to spatial variation than due to wildfi re. However, in areas with high intensity burns and high tree mortality, most EM fungi may locally be killed. Thus, the legacy of EM fungi following wildfi re depends on the survival of trees, which determine the potential for mycorrhizal growth, and the combustion and heating of the organic soil, which directly correlate to mortality of mycorrhizas. The questions if and to what degree fi res may be of signifi cance for yet unidentifi ed spatiotemporal dynamics of EM fungal populations and communities are discussed. Recent experiments indicate a few EM fungi are favoured by high intensity burn conditions whereas others disappear. The consequences of wildfi res in temperate conifer forests differ considerably from those in boreal forests. Wildfi res in temperate conifer forests are typically high intensity stand-replacing fi res that cause a total combustion of organic layers. Subsequently, prefi re EM fungal communities are largely eradicated and a succession of post-fi re EM fungi is initiated.