Australia: The Land Where Time Began

A biography of the Australian continent 

Mycorrhizal networks and their role in the functioning of forest ecosystems

Evidence of common mycorrhizal networks (CMNs) in forest stands

Roots that belong to the same or different ectomycorrhizal (ECM) tree species interconnect by the extraradical mycelia of ECMs, and therefore form common mycorrhizal networks that allow net transfer of C and nutrients between plants (Simard & Durall, 2004). The importance of ECMs in the dynamics of plant communities has been documented in several review papers (Newman, 1998; Perry et al., 1992; Amaranthus & Perry, 1994). The progress in understanding of structure and functioning of CMNs in the field has, however, been hindered by difficulties that are inherent in the study of mycelial systems in situ in soil without destroying them, and in distinguishing mycelia that are symbiotic from saprophytic ones (Leake et al., 2004).

It was revealed by the use of molecular markers that most ECM fungi colonise several plant hosts simultaneously, which suggests there is a high potential for the formation of CMNs in forest stands (Horton & Bruns, 1998; Kennedy et al., 2003; Richard et al., 2005; Ishida et al., 2007). The fact that some ECM fungal taxa can form simultaneously large genets, supports this, as much as 10 m in width (such as Dahlberg & Stenlid, 1990; Bonello et al., 1998) and therefore extend over an area that can encompass multiple trees that can belong to the same or to different species (Sawyer et al., 2001; Zhou et al., 2001; Bergemann & Miller, 2002; Dunham et al., 2003).

Fungal taxa that belong to Basidiomycetes and Ascomycetes form ectomycorrhizal CMNs. Most of them are generalists, though some ECM taxa are host-specific (Bruns et al., 2002) and non-specific ones are often more abundant that specific ones, in temperate ECM communities at least, (such as Horton & Bruns, 1998; Kennedy et al., 2003; Dickie et al., 2004). Moreover, some Ascomycetes that form ericoid endomycorrhizae with Ericaciae might form another type of CMN, which might be integrated into ECM networks: some species of Ericaciae form both ericoid endomycorrhizae and ECMs (Sellosse et al., 2006). Similarly, myco-heterotrophic and mixo-trophic orchids can share common Basidiomycete symbionts with ECM forest trees.

Implication of CMNs in the transfer of resources between trees

According to Courty et al. interest has recently been aroused by the finding of the significance for functional ecology and the geochemical cycling of the transfer of resources from one tree to another through CMNs. CMNs may indeed form ‘guilds of mutual aid’ among coexisting plants, or redistribute resources along source-sink gradients (Perry et al., 1989; He et al., 2004; Edgerton-Warburton et al., 2007). It is thought that carbon and nutrients translocate through CMNs from source plants to plants that are sinks that differ in some way, such as nutrient status or rate of photosynthesis (Simard et al., 1997b). Analysis using dual (¹³C – ¹⁴C) pulse labelling (Simard et al., 1997a) the net transfer of C through CMN between Pseudotsuga menziesii and Betula papyrifera, noted that the net gain of photosynthetic C  was higher when the receiver tree was in deep shade than full or partial sunlight. The primacy of the hyphal pathway C from the donor to the receiver photosynthetic plants, as well as the fate of the carbon that is transferred, and the physiological relevance of such transfer, have remained arguable (Fitter et al., 1998; Robinson & Fitter, 1999; Wu et al., 2001; Simard et al., 2002; Simard & Durall, 2004).

It has been established, on the other hand, that during their development many orchids switch from saprophytic and pathogenic fungi (such as Rhizoctonia sp.), to ECM fungi that are able to provide large amounts of C over long periods of time (Leake et al., 2004). It has also been noted that this behaviour has also been observed in the achlorophyllous liverwort, Cryptothallus mirabilis, which forms ECM associations with Tulasnella, which is a genus of Basidiomycete that often forms orchid mycorrhizas (Bidartondo et al., 2003). Moreover, it has been shown by recent studies that used stable isotope (¹³C and ¹⁵N), that several green forest orchids displayed ¹³C enrichment intermediate between plants that are fully autotrophic and myco-heterotrophic that receive all the C from their fungal associates (Gebauer & Meyer, 2003; Julou et al., 2005; Tedersdo et al., 2007). These green orchids use a strategy called “mixotrophy” to gain their carbon which combines myco-heterotrophic (Julou et al., 2005). It has been shown (Tedersoo et al., 2007), e.g., that several pyroloid shrubs (Ericaciae) and green forest orchids acquired from 30% to 80% of their C from their associated fungi.

The transfer of water between conspecific or non-conspecific neighbouring plants also involves CMNs, as has been shown recently by the use of isotopic tracers and fluorescent dyes (Edgerton-Warburton et al., 2007). Furthermore, the transfer of water from mature trees that are deep-rooted to seedlings occurred in association with the “hydraulic-lift” (Querejeta et al., 2003). However, hydraulic lift is mainly a physical process and can also be performed by dead roots, and presumably also by dead rhizomorphs (Leffler et al., 2005).

The role of CMNs in the composition and function of forest plant communities

Among the major features that control the coexistence of plant species, and therefore the composition of plant communities, is the competition for resources (such as Ricklefs, 1977; Aarssen, 1983; Tilman, 1982). It has been proposed that sharing of resources is a process that modulates the competition for resources, especially in mycorrhizal plant communities (Perry et al., 1989; Read, 1997). This concept depends largely on CMNs, as the key factor that determines the allocation of resources (Kytoviita et al., 2003). Therefore, CMNs may benefit some plant species more than others, or may mediate the transfer of resources between individual plants, therefore have an influence on the structure and diversity of plant communities (Grime et al., 1987; Smith & Read, 2008; Hart et al., 2003).

According to Courty et al. it is likely, that at least at the local scale, the structure of tree communities is shaped by their being part of a CMN. 2 plants can indeed provide C unequally to a shared fungus or acquire resources unequally that they both support, which implies a net benefit for 1 species to the detriment of the other (Selosse et al., 2006). The differential reinforcement in this context of some plant populations may lead to competitive exclusion and extinction of other plant populations (Chapman & Reiss, 1999; Simard & Durall, 204). A negative feedback in other respects can alter the competitive abilities of the dominant populations and, therefore, promote the diversity of the plant community (Simard and Durall, 2004). Nevertheless, the occurrence and efficiency of CMNs in forest stands appear to be contingent upon, and driven by, temporal variations in composition and abundance of species of fungi which might display significances in the transfer of resources between plants.

Additionally, seedling recruitment can be facilitated by CMNs: by connecting to the existing mycorrhizal networks and receiving C from the adult trees of the overstorey, seedlings that are shaded in the understorey, which allows them to compete efficiently against other tree species in the ECM network, or AM plants that are invading (Molina et al., 1992; Cullings et al., 2000). Furthermore, seedling recruitment is promoted by early incorporation into the existing mycorrhizal networks, as the survival and rates of establishment of seedlings depend on the rate at which they become mycorrhizal (Janos, 1996; Newberry, 2000). For instance, it was noted (Horton et al., 1999) that the survival of Pseudotsuga seedlings increased beneath ECM Arctostaphylos chaparral, compared to the adjacent non-ECM Adenostoma chaparral.

It was observed (Ongeune & Kuyper, 2002) in tropical forests and in temperate forests (Dickie et al., 2002) that seedlings in contact with adult ECM trees show better survival and colonisation by mycorrhizas than seedling isolated from adult plants. Adult trees may therefore function as “nurse trees” for conspecific and nonspecific seedlings, and therefore promote the diversity and coexistence of species in forest stands (Grime et al., 1987; Perry et al., 1989).

The selective pressures behind this phenomenon are not clear, as this cooperation may be detrimental to the adult plants (Newman, 1988). In other respects selection for single plant species might be promoted by ECMs, in particular in the context of high mycorrhizal specificity (Wilkinson, 1998), and, therefore might contribute to maintenance of low-diversity plant communities (McGuire, 2007).

Courty, P.-E., et al. (2010). "The role of ectomycorrhizal communities in forest ecosystem processes: New perspectives and emerging concepts." Soil Biology and Biochemistry 42(5): 679-698.