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- Materials and methods
Molina et al. (1992) identified a range of host specificity among ECM fungi and concluded that, overall, host specificity was moderate to low as required if they are to form networks between plants. This conclusion was based largely on laboratory and glasshouse studies and on observations of fruiting behaviour (Grand 1968; Molina & Trappe 1982a; Molina & Trappe 1982b). Massicotte et al. (1999), for example, found that conifer and hardwood species shared many of the same ECM morphotypes when grown together in pots. However, the ecological significance of these results is unclear because of the wider range of niches potentially present in natural soil environments (Harley & Smith 1983).
Recent field studies indicate that ECM host specificity between canopy trees is low (Horton & Bruns 1998; Cullings et al. 2000). There also appears to be low ECM host specificity between understorey and canopy plants (Visser 1995; Jonsson et al. 1999). Visser (1995) documented that many of the ECM fungi associated with Pinus banksiana, the canopy dominant tree, were also present on Arctostaphylos uva-ursi, a common understorey plant in mature P. banksiana woodlands. Jonsson et al. (1999) found the dominant fungal species in Pinus sylvestris forests made up 92% and 73% of the P. sylvestris seedling and mature P. sylvestris tree mycorrhizas, respectively. Although both studies examined host specificity between understorey and canopy plants, it is still not clear whether understorey species with the potential to develop into canopy species can be linked into ECM networks with a different canopy species. Interspecific connections between the forest understorey and canopy may be important for maintaining forest communities following disturbance (Perry et al. 1992) and for long-term forest structure (Newberry et al. 2000).
Determining the level of host specificity in ECM assemblages is an essential first step towards investigating the ecological role of ECM networks. If there is a high potential for ECM networks, further research can focus on how actual ECM connections influence interspecific interactions between hosts. In this study, we sampled the ECM assemblages associated with a Pseudotsuga menziesii canopy and Lithocarpus densiflora understorey in three adjacent stands. Our objectives were (i) to document the ECM assemblages in mixed evergreen forest stands and examine their structure, diversity, and similarity, and (ii) to determine the potential for common ECM networks between understorey and canopy tree species.
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- Materials and methods
Although the total number of single-host ECM taxa was higher than multiple-host ECM taxa across the three stands, multiple-host ECM taxa were significantly more abundant. Additionally, 78% of the multiple-host ECM taxa were found at least once on both hosts in the same core, which suggests that the potential for the Lithocarpus understorey to be connected by mycorrhizal networks to the Pseudotsuga canopy is high. These potential connections appeared to be very common, although the shared ECM taxa varied between cores.
Our results agree with other studies that have reported that multiple-host taxa have greater abundance than single-host taxa (Horton & Bruns 1998; Horton et al. 1999; Cullings et al. 2000). We cannot, however, be certain that the ECM taxa that occurred on both hosts within the same core belonged to the same genotype, as the sizes of genets vary dramatically between species and locations (Dahlberg & Stenlid 1994; Gherbi et al. 1999; Gryta et al. 2000; Fiore-Donno & Martin 2001; Guidot et al. 2001; Redecker et al. 2001). However, in most cases, genets extend through spaces larger than a 10-cm diameter core, suggesting that, within each core, we sampled the same fungal genotype. However, there may be some overlap in the distribution of genets that could result in more than one genotype per core (Jany et al. 2002). Demonstrating that plants are actually connected by the ECM networks would require identification of the same fungal genotypes on both hosts within a core, which could be accomplished using taxon specific primers (Egger 1995). The number of hosts per ECM taxa may also have been underestimated due to the restricted area sampled. If additional samples were examined, some additional taxa, particularly those that we found only once, would probably be encountered on both hosts. This means that we are likely to have underestimated the abundance of multiple-host ECM taxa.
We found significantly higher ECM taxa richness on Pseudotsuga than Lithocarpus, which may be due to a number of factors. The Pseudotsuga individuals examined were likely to be older than those of Lithocarpus and therefore have much larger and more developed root systems that could potentially harbour a greater number of ECM taxa. Because Pseudotsuga individuals were in the forest canopy, they have more fixed carbon than understorey Lithocarpus individuals and may allocate more below-ground, resulting in a more diverse ECM community. Alternatively, this difference may reflect intrinsic differences in the number of mycorrhizal associations that can be formed by these two hosts. Pseudotsuga is reported to associate with over 2000 ECM species across its geographical range (Trappe 1977), but little is known about the diversity of mycorrhizal associations for Lithocarpus. However, in a glasshouse study, Massicotte et al. (1999) found 14 morphotypes on Pseudotsuga seedlings and 10 on Lithocarpus seedlings, with all 10 of the morphotypes on Lithocarpus also found on Pseudotsuga. Finally, the differences in ECM taxa richness may be the result of temporal variation in some factor that differs between hosts (e.g. fine root production), which could influence the number of ECM taxa associated with each host at the time of sampling.
The relative abundances of the multiple-host ECM taxa varied between hosts. Although significant host preferences were not detected when cores were combined (Fig. 3), on an individual core basis, the relative abundance of a given ECM taxon often varied considerably between the two hosts. Other studies have shown that ECM fungi may exhibit some degree of host preference (Molina et al. 1997; Cullings et al. 2001). While ECM fungi may interact with multiple hosts, they may not interact equally due to differential carbon input (Cullings et al. 2001), colonization susceptibility (Molina et al. 1997), and local soil environmental conditions (Gehring et al. 1998). It is possible that differential ECM abundances may affect the significance of ECM networks, as suggested by Finlay (1989), who found that differential ECM abundances affected the amounts of resources transferred between linked hosts.
We do not know if any materials are being transferred between the Pseudotsuga canopy and Lithocarpus understorey. Hogberg et al. (1999) found that up to 90% of the carbon in below-ground ECM networks was supplied by the canopy trees in a mixed species Swedish forest, but they did not determine if carbon moved into understorey plants. Simard et al. (1997) found that significant amounts of carbon were transferred from Betula papyrifera seedlings growing in full sun to experimentally shaded Pseudotsuga seedlings, resulting in a significant net carbon gain, which suggests that carbon may move preferentially to plants in shaded environments. Yet even if Lithocarpus individuals are not receiving carbon directly from the Pseudotsuga canopy, they may benefit from mycorrhizal networks because individuals that tap into mycorrhizal networks will have access to larger nutrient pools (Newman 1988). In addition, their ECM associates will be receiving the majority of their carbon from the canopy individuals (Hogberg et al. 1999), which will decrease the carbon cost of the symbiosis for understorey individuals. This lowered cost and enhanced nutrient availability would be very likely to aid establishment and persistence in light-limited forest understories. Recent evidence of such facilitation was put forth by Horton et al. (1999), who found that seedlings of Pseudotsuga menziesii shared many ECM fungi with mature Arctostaphylos spp. and showed that Pseudotsuga seedlings successfully established only in areas with Arctostaphylos individuals.
Similarly high spatial heterogeneity has been observed in other studies and appears to be a general pattern of ECM assemblages (Dahlberg 2001; Horton & Bruns 2001). Despite the patchy distributions of individual taxa, we found that multiple-host ECM taxa were present in the majority of the cores. The widespread occurrence of multiple-host taxa would allow Lithocarpus individuals to tap into ECM networks throughout the forest understorey, but the composition of these networks varies due to the spatial heterogeneity of the community. Additionally, temporal variation in fungal species abundances could influence the occurrence and composition of ECM networks. Because there may be significant differences in the transfer of resources depending on the specific ECM taxa shared between hosts (Finlay 1989), studies that directly address the autecology of different ECM taxa will significantly enhance our understanding of functional aspects of mycorrhizal networks.