ECM fungal community on secondarily colonizing timber species
In the sere of vegetation succession in the volcanic desert on Mount Fuji, the establishment of Betula and Larix is very important as an initial stage of forest formation. In a previous study, ECM fungi detected on transplanted current-year seedlings of both tree species were common to those on early-established Salix at this site (Nara & Hogetsu, 2004). Here, I found that ECM fungi on naturally established saplings of Betula and Larix, mostly >10 yr old, were still dominated by ECM fungi common to Salix. This indicates that other ECM fungal species that are associated preferentially with secondary colonizers are not dominant fungi, even a decade after their establishment.
ECM associations of current-year seedlings in this volcanic desert are mainly accomplished via extramatrical mycelia that radiate from established hosts (Nara & Hogetsu, 2004; Nara, 2006). The ECM fungal communities on naturally established saplings of the timber species were quite similar to that on Salix. This indicates that these timber species may have been connected to Salix shrubs by common mycelia of the same ECM fungus, called common mycorrhizal networks (CMNs: Newman, 1988; Simard & Durall, 2004). Because L. amethystina was the most abundant and frequent species on each plant species, CMNs of this fungus would have the highest possibility of occurrence. In a previous study, the effects of CMNs on experimentally connected Salix seedlings varied significantly among ECM fungal species (Nara, 2006). Interestingly, L. amethystina showed no positive effects on Salix seedlings via the CMN, whereas the other 10 ECM fungi improved the N absorption and growth of connected seedlings to varying extents (Nara, 2006). Whereas nutrients absorbed by ECM fungi are shared among different host species in a CMN, there is preferential transport to highly compatible hosts, rather than to less compatible hosts (Finlay, 1989). If L. amethystina is physiologically more compatible with Betula and Larix than Salix, the CMNs of this fungus would be favorable for Betula and Larix.
Host ranges of ECM fungal species during primary succession
Observations of sporocarp–host associations in the field have been used repeatedly to evaluate the ecological specificity and host ranges of a variety of ECM fungi (Molina et al., 1992 and references therein). These studies have accumulated a large amount of information on sporocarp–host specificity; however, an association at one site does not necessarily indicate the same association at other sites because field conditions may differ (Molina et al., 1992). To consider the practical effects of ECM fungi on ecological processes, ECM specificity and compatibility should be examined at each site (Harley & Smith, 1983; Molina et al., 1992). A sporocarp approach is unsuitable in most sites for various reasons, including limited sporocarp production; co-occurrence of multiple host species in close vicinity; or great disparity between sporocarp and underground ECM communities. Instead of the sporocarp approach, a molecular approach is now available to confirm ECM colonization on individual hosts species under natural settings (Gardes & Bruns, 1993; Horton & Bruns, 2001). This enables studies of the specificity of ECM fungi between co-occurring tree species belonging to the same family (Horton & Bruns, 1998; Cullings et al., 2000), and an evaluation of host ranges by comparing ECM fungi on taxonomically distant hosts (Horton et al., 1999; Kennedy et al., 2003; Richard et al., 2005).
Generalists are usually defined as ECM fungi that are compatible with various plant families. I defined generalists as ECM fungi that were compatible with at least two of the three plant families examined (Betulaceae, Betula; Pinaceae, Larix; Salicaceae, Salix). In contrast to this robust definition of generalists, the definition of specialists varies among studies, and the demonstration of specificity is usually difficult. This is partly because specific ECM associations include various levels of host specificity (species, genus or family; Molina et al., 1992). Moreover, it is difficult to determine whether a specific occurrence is caused by real specificity or insufficient sample sizes, because ECM communities are usually composed of many rare species detected in only one or a few samples (Horton & Bruns, 2001; Richard et al., 2005). Molecular phylogenetic studies of some fungal genera have demonstrated that specific ECM associations are common on related taxa within a genus that have evolved from a common ancestor (Kretzer et al., 1996; Grubisha et al., 2002; den Bakker et al., 2004). These studies would be valuable for the definition of specialists.
Although the host ranges of dominant ECM fungi after severe disturbance have not been well studied, specialists (including Rhizopogon species) often dominate the postdisturbance ECM community (Horton & Bruns, 1998; Baar et al., 1999; Bruns et al., 2002). Rhizopogon and phylogenetically related genera such as Suillus show high specificity to the Pinaceae (Molina & Trappe, 1994; Kretzer et al., 1996; Bruns et al., 2002) and adapt well to disturbances by forming dormant spore banks (Baar et al., 1999). The persistence of dormant spores of other ECM genera has not been demonstrated clearly. Thus the dominance of specialists may be restricted to areas where the Pinaceae can dominate following disturbance. Although the effect of specialist dominance on tree succession remains unknown, it should prevent the invasion of incompatible plant species. Therefore, Pinus–Rhizopogon relationships may allow both partners to continue to prosper in repeatedly disturbed sites.
In the volcanic desert on Mount Fuji, however, generalists clearly dominated the ECM community. This is completely different from specialist-dominated ECM communities after disturbance. Because there is no dormant spore bank in primary successional settings, the specialist–generalist patterns of ECM fungi may differ fundamentally between primary and secondary succession. In addition, the host-range patterns may vary with pioneer host species, where conifers are associated with specialists and broad-leaved species are associated with generalists. Although little is known about the effects of generalist dominance on succession, this work shows that the dominance of generalists contributes to ECM associations in secondary colonizing Betula and Larix, and possibly in late-successional tree species such as Fagus, Quercus, Abies and Tsuga.
The presence of a few specialists was also confirmed: Leccinum sp. 1 was found on Betula, and two Suillus spp. were found on Larix. Suillus laricinus, a notable Larix-specific fungus, was also detected from a root system of Betula that was associated with a Larix tree. Under suitable experimental conditions, especially if exogenous sugars are abundant, ECM fungi can colonize ecologically nonhost plants (Finlay, 1989; Molina et al., 1992), although such ecologically incompatible associations are not fully functional (Finlay, 1989). Thus the mycelia of S. laricinus may infect Betula roots because of carbon support from the neighboring Larix. In accordance with other studies, I defined this fungus as a specialist because its occurrence was significantly biased to Larix (P < 0.001, exact χ2 test where S. laricinus ECM root tips in Betula and Salix were pooled and compared with those in Larix), and phylogenetic studies of related taxa support its specificity (Kretzer et al., 1996; Bruns et al., 2002; Grubisha et al., 2002). Consequently, specialists (two Suillus species) represented 25% of the relative abundance of ECM root tips in Larix. Because these specialists were not shared with early-established Salix, Larix could receive exclusive benefits from these fungi. Moreover, specialists may transfer more N to hosts than generalists (Hobbie et al., 2005). Therefore, the relative contribution of specialists to timber establishment may not be proportional to their relative abundance in ECM communities.
Contribution of ECM fungi to tree succession
Under secondary successional settings, remaining host plants sometimes facilitate subsequent recolonization of ECM trees (Perry et al., 1987). This facilitation can be derived from many biotic and abiotic factors (Callaway & Walker, 1997). ECM fungal symbionts may contribute partly to establishment because ECM fungal communities on seedlings near the remaining hosts are different from those on distant seedlings (Horton et al., 1999; Kranabetter, 1999; Dickie et al., 2002a; Ashkannejhad & Horton, 2006). Many remote seedlings, however, are usually colonized by some ECM fungi, irrespective of the early-colonizing hosts in these studies. Thus the facilitated seedling establishment cannot easily be attributed to the effect of ECM fungi unless different ECM communities are experimentally shown to have different effects at each site.
In contrast to these secondary successional sites, ECM colonization itself is nearly impossible for current-year seedlings of host species in the primary successional volcanic desert on Mount Fuji, unless seedlings are accompanied by early-established Salix (Nara & Hogetsu, 2004). Furthermore, facilitated seedling establishment of Salix was solely attributable to ECM colonization in a field inoculation experiment in which all abiotic and biotic conditions were uniform, except for ECM fungi (Nara, 2006). However, the ECM contribution to vegetation succession was uncertain because established Salix did not improve the performance (aside from ECM colonization) of Betula and Larix seedlings in a transplant experiment (Nara & Hogetsu, 2004). Here, I found only 39 and 26 established individuals of Betula and Larix, respectively, in the 21 ha of the study area approx. 300 yr after the last volcanic eruption. Although the establishment of Betula and Larix appears to have accelerated in recent years, the natural establishment of both timber species appears to be too episodic to be studied using experimental approaches.
The natural establishment of Betula and Larix coincided spatially with Salix, without exception. What is the most likely mechanism that explains the observed establishment pattern? On average, secondary colonizers occurred more often in association with large Salix shrubs and in large vegetation patches. Because soil nutrient availability and organic matter content are correlated with patch development in this volcanic desert (Hirose & Tateno, 1984), soil development may be attributable partly to the observed establishment patterns. However, a very small Salix patch (0.03 m2) within a small patch (8.4 m2) was confirmed to have recruited a Larix seedling (1 yr old) where the soil had developed poorly. Thus soil development alone may not be a requisite for the establishment of timber species. Because both Betula and Larix depend obligately on ECM fungi, these timber species must be colonized by some ectomycorrhizal fungi during early establishment. Therefore, compatible and accessible ECM fungi provided by early-established Salix would be an important mechanism that potentially could explain the observed pattern of timber establishment.