47 ANDERS DAHLBERG Department of Forest Mycology and Pathology, Swedish University of Agricultural Science, P.O. Box 7026, 750 07 Uppsala, Sweden Spatial characteristics of Suillus variegatus populations, including the size, distribution and number of genets, were measured in four naturally regenerated stands of Scots pine, Pinus sylvestris in Sweden that were more than 100 years old. In the oldest forest, a continuous tree-layer had been present at least since the last forest fire in 1647. Genets were identified based on somatic incompatibility reactions performed on mycelial cultures from sporocarps. In total, 38 genets were identified from 120 tested sporocarps. The maximal extension of a genet, as reflected by its outermost sporocarps, was 27 m in the oldest stand and ranged between 10 and 17 m in the other stands. On average, genet size was 20 m in the oldest stand and 10 m in the other stands. Other genets were not found within the domains of established genets. The closest detected distance between genets was 15 m, and the average distance was 4 m. The number of genets ranged between 56 and 74 ha - at the studied sites. Possible reasons for the high degree of resemblance in characteristics in old forests between S. variegatus, the dominant bolete in mature forests, and S. bovinus when present, more a characteristic for younger forests and only scarcely occurring in older forests, are discussed. Suillus variegatus (Sw. ex Fr.). Kuntze is one of the most common epigeous ectomycorrhizal (ECM) fungi in Fenno- scandian Scots pine forests, as reflected in the abundance of its sporocarps (Kardell & Eriksson, 1987 ; Hansen & Knudsen, 1992 ; Ohenoja, 1993 ; Blomgren, 1994). It often constitutes 15–25 % (sometimes up to 60 %) of the total epigeous ECM sporocarp biomass production, ranging from 05 to 15 (rarely up to 10) kg .. ha -, in dry, middle-aged to old Scots pine forests of Fennoscandia and Estonia (Rautavaara, 1947 ; Kardell & Eriksson, 1987 ; Hintikka, 1988 ; Kalamees & Silver, 1988 ; Ohenoja, 1993 ; Kalamees & Silver, 1993). Blomgren, 1994 ; It is also present, but less abundant, in younger forests (Hintikka, 1988 ; Kalamees & Silver, 1988 ; Ohenoja, 1993). Similarly, all successional stages of a Scots pine forest may contain sporocarps of S. bovinus (L. :Fr.) Roussel, the common generic relative of S. variegatus (Hintikka, 1988 ; Kalamees & Silver, 1988 ; Ohenoja, 1993). In contrast, however, its sporocarp biomass, both in absolute terms and in relation to the total production of epigeous ectomycorrhizal fungi, diminishes with increasing forest age, i.e. it is more abundant in early successional stages (Hintikka, 1988 ; Dahlberg & Stenlid, 1990, 1994 ; Visser, 1995). In older forests S. variegatus can be considered a core species, with large populations in almost all available localities, while S. bovinus is more of a satellite species with small populations on a few of the available localities (Hanski, 1982). Using somatic incom- patibility, it was shown that in populations of S. bovinus in a Scots pine forest, the number of fungal individuals (genets) decreased over time while genet size increased. Under suitable conditions, there are an average of 60 genets ha - in old forests, with individual mycelia varying in size from 7 to 20 m (Dahlberg & Stenlid, 1990, 1994). Until now, no population structures have been reported for S. variegatus. Studies of somatic incompatibility are based on the ability of a mycelium to recognize and thereafter reject or accept another mycelium depending on the degree of genetic disparity between the two (Rayner, 1991). This response has been used to analyse characteristics such as size and number of genets, which cannot be determined based on the occurrence of sporocarps alone, for a few ectomycorrhizal species (Dahlberg, 1995 ; Dahlberg & Stenlid 1995). In a comparative study of somatic incompatibility reactions and isozyme analyses, individual genets of S. variegatus could be un- ambiguously identified (Sen, 1990). S. variegatus is a heterothallic fungus that generates genetically variable progeny by possessing a bipolar sexual incompatibility (Fries, 1994). Assuming that there is a correlation between sporocarp number and mycorrhizal biomass, the successive replacement of S. bovinus by S. variegatus as forests grow older suggests that the latter is better adapted to later stages of forest succession. Since the energy supply requirements of ECM fungi are met by their hosts’ short-roots, which are continually forming and dying, an ECM fungal genet should have the potential to live as long as its host trees. S. variegatus tends to dominate epigeous ECM sporocarp production for one to several centuries, whereas S. bovinus is generally only abundant for a few decades during the initial stages of succession. Thus, genets of S. variegatus may have the potential to occupy larger domains for a longer time compared with genets of S. bovinus. Both species have well-developed rhizomorphs, which allow established genets to spread vegetatively within a stand Mycol. Res. 101 (1), 47–54 (1997) Printed in Great Britain Population ecology of Suillus variegatus in old Swedish Scots pine forests