Sporocarp occurrence and ectomycorrhizal biomass of S. pictus
Suillus pictus was the ECM species forming the highest number of sporocarps in the study plot (25% of all ECM sporocarps in 2002). Kikuchi & Futai (2003) also reported that S. pictus was the most dominant species, both in the sporocarp and the ectomycorrhizal community in a Korean pine (Pinus koraiensis) plantation in the same experimental forest station. Among the ECM species, only S. pictus is specific to five-needled pine species (Murata, 1976; Wu et al., 2000); all the other species, such as, Strobilomyces confusus, Lactarius chrysorrheus and Tylopilus castaneiceps, can associate with broad-leaved trees and/or two-needled pines (Imazeki & Hongo, 1989; Palfner, 1998).
Five-needled pines including Japanese white pine are not indigenous to this area and most of the ectomycorrhizal fungi seem to have come from the nearby forests of Quercus spp. and P. densiflora (two-needled pine species). S. pictus, which is specific to five-needled pine species, might be competitive with other fungi that have a broad host range, becoming dominant in the study plot. Colonization of Japanese white pine trees by S. pictus might have occurred in the nursery and persisted even after planting in the field. In this experimental station, some other five-needled pine species such as P. wallichiana, P. koraiensis and P. strobus had been planted. Inoculum of S. pictus might have come from the plantation site of these trees and colonized Japanese white pine seedlings in the nursery. Selosse et al. (1999) confirmed the persistence of nursery inoculated Laccaria bicolor for > 10 yr after plantation in the field.
Sporocarps of S. pictus occurred in only < 20% of the subplots and showed aggregated distribution. The spatial distributions of S. pictus sporocarps and other species did not overlap and S. pictus seemed to prefer places with low litter accumulation compared with other dominant species such as S. confusus and L. chrysorrheus for fructification. The average thickness of the litter layer in places where sporocarps of S. pictus occurred (1.5 cm) was significantly thinner than in places with occurrence of other dominant ECM sporocarps (2.0–2.4 cm, t-test, P = 0.05, unpublished data). Kikuchi & Futai (2003) also reported that S. pictus sporocarps mostly occurred in the disturbed area with low litter accumulation. However, there were many places with low litter accumulation without sporocarp occurrence of S. pictus in the present study plot and spatial distribution of sporocarps of S. pictus seems to be determined by factors other than litter accumulation, spatial distribution of mycorrhizas and competition between other ECM species.
The biomass of fine roots and total mycorrhizas were 107.3 ± 19.1, and 38.6 ± 7.2 g d. wt m−2 (mean ± SE, average of 100 and 300 cm2 samples), respectively, in this plantation. These values are within the range of biomass in other reports (Vogt et al., 1983; Dahlberg & Stenlid, 1994; Kikuchi & Futai, 2003). As for the distribution of mycorrhizas, there were no significant differences among the biomass of mycorrhizas and fine roots in plot 2 and four areas in plot 1 estimated using different sample sizes, which indicated rather uniform distribution of mycorrhizas and fine roots on a large scale in the study plot. This is probably because Japanese white pines of the same age were planted uniformly and densely in the corresponding plot. Similar to the sporocarp community, S. pictus was dominant in the mycorrhizal community in the study plot, and amounted to 24–43% of total mycorrhizas. This species was also dominant in a P. koraiensis stand and occupied 75% of total mycorrhizas (Kikuchi & Futai, 2003). Most of the mycorrhizas of S. pictus were distributed in the upper soil layer as shown in Kikuchi & Futai (2003).
Suillus pictus mycorrhizas were found at almost every sampling point and it was revealed that S. pictus mycorrhizas were distributed widely, irrespective of sporocarps occurrence in the study plot. Gardes & Brun (1996) also demonstrated that the spatial distribution of sporocarps of S. pungens did not always reflect that of mycorrhizas in a Californian bishop pine forest.
Relationship between genet distribution of sporocarps and ectomycorrhizas of S. pictus
Temporal distribution of S. pictus sporocarps differed among genets. Sporocarps of type D occurred mostly in the latter part of October, while those of the other three genets occurred mostly in late September. Sporocarps of S. pictus occurred at around 20°C (the mean temperature in May, June, September and October in 2002 was 18.4°C, 22.6°C, 23.2°C and 17.0°C, respectively). Phenology of sporocarp occurrence differs between species and temperature affects it greatly in field (Wilkins & Harris, 1946). There may be differences in the response to temperature among genets, even within a species. Although sporocarps of types A and C occurred both in early summer and autumn, while types B and D produced sporocarps only during the autumn, this difference may have simply resulted from the fact that types A and C produced numerous sporocarps, thus increasing the chance of production in early summer.
Spatial distribution as well as temporal distribution of S. pictus sporocarps differed among genets. Genet area sizes of sporocarps did not correlate with those of ectomycorrhizas or with the number of sporocarps of each genet. Spatial distributions of genets of S. pictus ectomycorrhizas were wider than those of sporocarps, which suggested that the production and spatial distribution of S. pictus sporocarps did not always reflect the biomass and distribution of mycorrhizas of S. pictus either at species level or at genet level. Sporocarps of each genet type occurred in or around the patches of corresponding genet of mycorrhizas in general. Several sporocarps of type A occurred somewhat away from its mycorrhizas, as in rows no. 3, 7 and 8 (Fig. 4). There may probably be small patches of type A mycorrhizas in the vicinity of these sporocarps that were not detected by our sampling method.
Concerning those species that form fairy rings of sporocarps, the spatial distribution of sporocarps seems to overlap that of mycorrhizas (Ogawa, 1975; Last et al., 1983; Dighton & Mason, 1985). Guidot et al. (2001) employed the PCR-RFLP method using species-specific primer and demonstrated that the spatial distribution of genets of Hebeloma cylindorsporum sporocarps represented well that of ectomycorrhizas in the Pinus pinaster stands on both occasions that H. cylindrosporum formed fairy rings, and formed only small patches of a few sporocarps. Zhou et al. (2001b) also revealed that the genet distribution of sporocarps reflected that of corresponding ectomycorrhizas for S. grevillei using a species-specific SSR marker. However, they also reported that the number of sporocarps was not always consistent with the size of the subterranean part of the genet, and sporocarps were not always centered over the subterranean parts. Therefore, genet of ECM fungi should be estimated on mycorrhizas for the analysis of genet structure and propagative manner of ECM fungi, as shown in the present study.
Concerning the genet distribution of mycorrhizas on a small scale, Zhou et al. (2001b) reported that the mycorrhizal genet distribution at the upper soil layer did not always reflect that at the lower soil layer. In the present study, the same genet types were detected in the top soil layer and in 5–10 cm soil for all the six sampling points examined and we considered that genets of S. pictus scarcely intermingle with each other on a small scale in this plot. As 49–79% of S. pictus mycorrhizas were distributed at a soil depth of 0–10 cm, the genet distribution of mycorrhizas revealed in the present study is considered to reflect the actual genet distribution fairly accurately. Genet expansions of mycorrhizas were almost the same among the four genet types, while genet area sizes of mycorrhizas differed greatly among genets. Small genet area sizes observed for genet types such as types A and D, which seemed to be present at a low density, may be underestimated in the present study as we examined only one ectomycorrhiza per intersection, or a progressive replacement and fragmentation of some genets (such as type A, which could have been initially uniformly distributed) by others such as type B may have occurred. If the latter was true, the competition between genets within a species is rather intense in the study plot.
Only four genets of large genet expansion were found in this plot and no small genets were found. Genet density of S. pictus in this stand was estimated as 83.3 ha−1, which is far smaller than that of S. bovinus (667 ha−1) in a 20-yr-old Scots pine plantation (Dahlberg & Stenlid, 1994). Based on the genet distribution of sporocarps, it has been assumed that species forming many small genets mainly propagated by colonization from spores and species that form a few large genets propagated mainly by mycelial extension (Dahlberg & Stenlid, 1990, 1994; Dahlberg, 1997; Anderson et al., 1998; Bonello et al., 1998; Gherbi et al., 1999; Zhou et al., 1999). Dahlberg & Stenlid (1995) has shown that Suillus spp. established by colonization from spores at the early stage formed many small genets in a young-aged stand and, expanded by mycelia, formed a few large genets as the host tree population became mature. Zhou et al. (1999) demonstrated that S. grevillei was propagated by colonization from spores even in a mature Larix stand and Redecker et al. (2001) also revealed the importance of colonization from spores for Amanita francheti, Lactarius xanthoglactus and Russula cremonicolor in mature forests.
By contrast to these results, S. pictus seems to have propagated mainly by mycelial extension even though the age of the tees is not great (28 yr) and the trees are, on average, small (mean d.b.h. = 9.8 cm). Guidot et al. (2002) demonstrated that H. cylindrosporum formed larger genets and mainly propagated by mycelial extension under conditions with low competition between other fungi. It formed many small genets, mainly propagated by colonization of spores under conditions with high competition between other fungi even in the mature forests. In the present study plot, the competition between S. pictus and other fungi seems to be not so intense and could propagate mainly by mycelial extension. Kretzer et al. (2003) showed that Rhizopogon vesiculosus propagate mainly by mycelial extension while R. vinicolor formed small genet only. Spore colonization seemed to be important for this species in mature Douglas fir stands. Even two sister species of Rhizopogon propagated differently in the same stand. The importance of the capacity to survive and expand for a long period in the soil as a vegetative mycelium and the capacity to colonize from spores seems to be different between species and under different environmental conditions.
The estimation of mycelial growth rate of ECM fungi under natural conditions varies greatly among ECM species. The value of 0.1–0.2 m yr−1 has been estimated (Ogawa, 1975; Last et al., 1983; Dahlberg & Stenlid, 1990; Bastide et al., 1994) based on the outward growth of fairly ring or the genet expansion of sporocarps, while Gryta et al. (2000) reported that the two genet of H. cylindrosporum extended at a rate of c. 0.45–0.6 m yr−1. Bonello et al. (1998) estimated mycelial growth rate of S. pungens to be 0.5 m yr−1 and Selosse et al. (1999) estimated 1.1 m yr−1 for Laccaria bicolor.
If S. pictus grows at a similar rate to S. pungens (0.5 m yr−1) for 25 yr (after plantation), genet expansion will be 25 m, which might explain some of the present genet expansion (25.3–30.0 m). But this assumption requires that all the four genet had colonized immediately after plantation. It is more likely that S. pictus belonging to each genet colonized the Japanese white pines in the nursery, and persisted even after planting in the field, as already discussed. Another possibility is that the growth rate of S. pictus hyphae and rhizomorphs may be far faster than presumed as in the case of L. bicolor (Selosse et al., 1999). Both possibilities can explain the fact that the expansion of the genet was almost the same among four genets whose area sizes varied greatly. The differences in area size among each genet might be a result of intraspecific competition and/or colonization rate of each genet on the nursery seedlings at planting.
The ratio of biomass of sporocarps to that of mycorrhizas ranged from 4.4 to 548.3 and varied greatly among genet types (Table 1b). For instance, type C, which had the second largest estimated biomass of ectomycorrhizas, produced the largest number of sporocarps, while type B produced the smallest number of sporocarps in spite of its largest estimated biomass of ectomycorrhizas. The ratio of biomass of sporocarps to that of mycorrhizas of S. pictus in a Korean pine stand was calculated as 19.1 (Kikuchi & Futai, 2003). They established their plot along the forest edge where S. pictus sporocarps were frequently found and the distribution of sporocarps and mycorrhizas of S. pictus overlapped better than our study and they regarded all the mycorrhizas of S. pictus as involved in the production of sporocarps. However, many of the mycorrhizas included in the calculation in the present study might not have played a role in the production of sporocarps, especially types B and D. Each genet may have a different strategy for investing in the sporocarp production, or soil conditions may have determined sporocarp production mainly and most of the areas occupied by type B might be inappropriate for fructification.
Our present study revealed the relationship between the genet distribution of sporocarps and ectomycorrhizas on a large scale (480 m2) for the first time. The spatial distributions of genets estimated on mycorrhizas were, in the main, far wider than those of sporocarps. We also revealed that the ratio of biomass of sporocarps to that of ectomycorrhizas varied among genets in the same stand. We conclude that vegetative growth of mycelia played an important role in the propagation of S. pictus in this plot. At this site, spatial distribution of sporocarps in the previous 2 yr was almost the same as in 2002. Therefore, the tendency in the production of sporocarp of each genet seems to be relatively stable at least for the period of 3 yr. Long-term monitoring of genet distribution is required to elucidate the dynamics of genets of S. pictus in the soil.