Ectomycorrhizal Communities, their Role in Forest Ecosystem Processes: New Perspectives      and Emerging Concepts

Forest trees are benefited in a number of ways by their fungal symbionts forming ectomycorrhizal networks, as well as their bacteria, though the enhancement of soil nutrient mobilisation and uptake is the most important. As the tree allocates carbohydrate to the fungus through the root interface, the movement of nutrients is reciprocal, which makes the relationship a mutualistic association. It has been suggested by many field observations that a number of key ecosystem functions have been contributed to by mycorrhizal fungi such as carbon cycling, mobilising of nutrients from soil organic matter, mobilisation of soil minerals and linking trees through common mycorrhizal networks. It has been very difficult until recently to study trees and their fungal associates in forest ecosystems and most of the work that has been done on ectomycorrhizal functioning has been carried out under laboratory or nursery conditions. In this review Courty et al. discuss the possibility of working at another scale, in forest settings. Many new techniques are emerging that make possible the in situ study of the functional diversity of mycorrhizal communities. The integrative development of research on the functional ecology of ectomycorrhizas, as well as their associated bacteria,      with the potential implications of such research for the management of the effects of climate change on forests, should be helped by this approach. 

According to Courty et al. the colonisation of the land by plants would not have been possible without fungi. Phototrophs faced considerable difficulties when they moved from the aquatic to an aerial habitat, among which were limitations of the availability of water and the scarcity of soluble minerals, especially phosphorus. These difficulties were overcome by the photosynthesising organisms forming mutualistic associations with fungi, mycorrhiza. Complementary adaptations are displayed by the 2 partners to enable them to live on land: the fungal mycelium adapted well to the 3-D exploration of the substrate and some species have a potential for weathering which may allow access to mineral elements that are not soluble (Lapeyrie et al., 1991; Hoffland et al., 2004. Phototrophs are adapted well for the gathering of photons and gas exchange, which allows the exploitation of atmospheric resources. Moreover, they display conducting structures that link aerial and soil-based parts.

Among the various types of mycorrhizal symbioses at the present time, the primitive fungi those that are non-septate that form arbuscular mycorrhizas (AMs) are largely dominant and are involved with about 80-90% of phototrophs (Mosse, 1973; Mosse et al., 1981). The arbuscular mycorrhizal symbiosis is so widespread that there has been a suggestion that it is ancestral to the Plantae (Pirozinski & Malloch, 1975; Selosse & Le Tacon, 1998; Heckman et al., 2001; Wang & Qui, 2006).

A different symbiosis arose in some groups of Gymnosperms and Angiosperms more recently: the association with septate ectomycorrhizal fungi (ECM fungi). The fungus forms a sheath around the roots in such associations and penetrates into the cortex but remains intercellular, forming the Hartig net. The primitive arbuscular associations (AMs) have been partially replaced by Basidiomycota and Ascomycota in many trees and in some species of shrub. The rise of the ectomycorrhizal fungi is difficult to date (Alexander, 2006), but Courty et al. suggested that it may be speculated that it originated between 220 and 150 Ma (Selosse & Le Tacon, 1998; Bruns & Shefferson, 2004; Alexander, 2006). The replacement of arbuscular mycorrhizas by ectomycorrhizas favoured the colonisation of land areas where organic matter accumulates (such as in temporal and boreal zones). The saprophytic abilities of septate fungi give access to organic nitrogen as well as phosphorus which are then passed on to the host.

The ectomycorrhizal symbiosis forms a significant component of forest ecosystems at present in boreal, temporal and Mediterranean climate zones. Ectomycorrhizal fungi in these regions are found associated with trees belonging to the families Pinaceae, Abietaceae, Fagaceae, Tiliaceae, Betulaceae and Myrtaceae. Both ectomycorrhizas and arbuscular mycorrhizas are formed on the Salicaceae and some of the Rosaceae, as well as the genus Alnus (Betulaceae). Ectomycorrhizal fungi could also be present in the tropics, however, where some families of trees, such Dipterocarpaceae, are associated exclusively with ectomycorrhizal fungi. In temperate regions the main groups of fungi that form ectomycorrhizas have been listed by Trappe (1977). Each mycorrhizal tree species may form symbiotic associations with several hundred fungal species. Some ectomycorrhizal fungi are associated with all of the hosts that are capable of forming ectomycorrhizal associations, but others are restricted to a single genus or even species of tree. As a consequence of this, a mixed forest stand is richer in species of ectomycorrhizal fungal species than a pure stand (Le Tacon et al., 1984; Bruns, 1995; Massicotte et al., 1999; Dahlberg et al., 2001; Richard et al., 2005).

Ectomycorrhizal fungi are essential to the health and growth of forest trees in nature. Forest trees can benefit in a number of ways, though the enhancing of uptake of nutrients from soil, particularly of elements that have a low mobility in soil such as phosphorus, and micronutrients (Smith & read, 2008), and also for nitrogen (Martin, 1985; Chalot & Bran, 1998). The tree reciprocates this by the allocation of carbohydrates to the fungus through the root interface, which makes the association a mutualistic association. A large and diverse community of microorganisms, fungi and bacteria, that can inhibit or stimulate each other in hosted by the ectomycorrhizosphere, which forms a very specific interface between the soil and the trees. Mycorrhizal development is constantly promoted by some of the bacteria in the ectomycorrhizosphere, which led to the concept of ‘mycorrhization’ helper bacteria (MHBs) (Garbaye, 1994; Frey-Klett & Garbaye, 2005). Ectomycorrhizal fungi and bacteria also contribute jointly to weathering and processes of solubilisation (Calvaruso et al., 2006; Uroz et al., 2009).

Studies of the functional structure of ectomycorrhizal communities, and their precise role in ecosystem processes and biogeochemical processing have been limited to date by the high richness of species of ectomycorrhizal       communities, the high functional diversity of ectomycorrhizal fungi, and the lack of appropriate methods of investigation. New techniques have, however, been developed that allow exploration in situ of the functional diversity of ectomycorrhizal communities. The key to understanding the contribution of ectomycorrhizal communities to ecological processes of interest for the sustainable management of forests, soils and landscape conservation is this functional diversity. The possible role of associated bacteria in functions of ectomycorrhizas has also been studied recently and this has revealed some interesting interactions.

The aim of this study was to review the most recent findings in this field, such as for studying ectomycorrhizal communities in situ and their roles in:

1.    Cycling,

2.    Mobilisation of nutrients from soil organic matter,

3.    Mobilisation from soil minerals,

4.    Common mycorrhizal networks and their role in the functioning of forest plant communities, and

5.    The function of ectomycorrhizal communities after climate change.

This new knowledge will be put in perspective within the concepts of functional ecology as well as their implications in sustainable forest management.

In this study the focus on ecosystem processes in terms of the nutrition of trees and the cycling of nutrients does not extend to include the non-nutritional functions of mycorrhizas, such as the exploitation of soil water and solutes, the use of water by trees (Brownlee et al., 1983; Wartinger et al., 1994; Unestam & Sun, 1995; Smith & Read, 2008), the production of fungal auxins and their effect on the development of trees (Slankis, 1973; Gay et al., 1984; Karabaghli-Degron et al., 1988; Barker & Tagu, 2000) or the protection of roots against soil borne pathogens (Marx, 2009; Sen, 2001).

Conclusions

It has been shown by this review that ECMs formed by symbiotic fungi with the fine roots of forest trees, as well as their associated bacteria contribute significantly to a number of important ecosystem functions, especially to the cycling of nutrients and fluxes of C. In the performance of 2 types of functions in which their fungal partner is complementary to the photosynthetic plant, their role is particularly crucial: degrading organic matter and weathering minerals, and coupling the autotrophic and heterotrophic C cycles in order to mobilise nutrients. The latter is specific to the symbiotic nature of ECMs, with the fungi that are associated with the fine roots sometimes acquiring C from the soil through the breakdown of organic matter by enzymes as well as the photosynthates produced by the tree.

A striking characteristic of all ECM communities is, however, their functional and taxonomic diversity. The reason for the maintenance of such a high taxonomic diversity, is probably that the soil is a very heterogeneous and continuously changing habitat comprised on many small niches, the arrangement of which is consistently modified (soil has been described as a ‘4-D medium’ because of its instability). Competitive interactions in a given spatiotemporal ecological niche therefore never lasts for long enough to drastically exclude partners, which leads to a co-occurrence of many ECM types in a volume of soil of a dimension scale that is higher than that of the micro-niches (Bruns, 1995).

In terms of the stability of ecosystems main functional consequence of this diversity is positive. The functional complementarity of the symbionts is the key to the flexibility and resilience of forests that are facing conditions that are adverse and changing, because each single tree in the forest stand is associated with the many different fungi that form the ECM community. The research challenge is, therefore, is to describe and understand how the functional diversity of ECM  communities is shaped by the local environmental conditions, its response to natural and anthropogenic disturbances, and how the overall functioning and stability of the ecosystem is affected by it. According to Courty et al. there are more working hypotheses than actual mechanisms: while some pathways are documented reasonably well (7,10,11), those involving the functional structure of the ECM community (5,6,8,9) will necessitate further research.

Unravelling of the complex of interactions that shape the functional structure and the activity profile of the community of symbiotic fungi that results, among other approaches that are relevant to plant eco-physiology and soil science, is necessitated to gain understanding and modelling of the functional role of the ECM symbiosis at the ecosystem level. In order to take up this challenge new techniques are available or are being developed rapidly, at least as far as the mechanical aspects of the ECM ecology are concerned. However, expanding these findings to quantitative data about fluxes in ecosystems, which is the only way that applied objectives, such as the sustainable management of forests or conservation of ecosystems, can be attained implies that developing field experiments in a range of local conditions and the modelling of large datasets is necessary.

Finally, it is suggested by Courty et al. that such a challenge might be an interesting opportunity to address a basic question in the ecology of communities from a novel point of view, that of the structure-function relationship. When using plant or animal communities in situ, the main difficulty with such studies, is to find a relevant quantitative indicator of the overall performance of the community that can be operational in complex ecosystems. This is easier with microbial communities (measuring chemical reactions or fluxes in small systems), but then the difficulty is the very large species richness of the natural communities. Contrasting with this, indirect synthetic indicators of the functioning of ECM can be measured more easily as a result of the symbiotic nature of the populations that are being studied: mineral nutrition of trees (analysis of leaves), photosynthetic performance (measurements of gas exchange) and growth (as is assessed from realtime stem diameter or length of the shoot monitoring, annual ring measurements, etc.). The mineral nutrition measurement is probably particularly relevant as an indicator in the respect, due to the important role fungi have in mobilising nutrients from soil and making them available to the tree. It is therefore believed by Courty et al. that the present developments in the functional ecology of ECM community will contribute to the general progress of ecology. Table 2 summarises the main directions in which it seems to be necessary to aim in order to reach meet challenges.

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.