CHAPTER 1 General Introduction
5. Mycorrhizal fungi in ecological interactions
5.1 Mycorrhizal fungi in plant interactions
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Current research is addressing the question whether mycorrhizal fungi influence the outcome of plant competitive interactions. This is particularly important, not only to understand the interactions of plants in natural ecosystems, but also the effects of global change, such as the dispersal of invasive aliens on ecosystem structure and function (Dawson et al. 2012). The mycorrhizal status has a great impact on plant competition. Experiments with AM plants showed that usually plant size decreases without mycorrhizal association. This is based on the potential inability of mycorrhiza forming plants to effective use soil resources in the absence of mycorrhizal colonization (Facelli et al. 1999, van der Heijden et al. 2003). In mycorrhizal association the level of interplant competition increases considerably with enhanced use of available soil volume.
Also mycorrhizal types EM or AM might differently modify plant interactions. Aerts (2002) suggested a theoretical model of plant competition for two nutrients between plant species with different mycorrhizal types, based on Tilman´s model (Tilman 1982).
Figure 5: A hypothetical model to predict the effect of mycorrhizal colonization on plant coexistence in temperate forests based on Tilman´s R* model. The species that can grow on the lowest resource concentration (R*) is competitively superior to the other species. In a non-mycorrhizal (NM) situation the plant species associated with AM fungi (A) out-competes the plant species associated with EM fungi (B), because of its higher uptake capacity for both nitrogen (N) and phosphorus (P). In the mycorrhizal situation a co-existence is possible because of the increased capacity of the plant species with EM to take up N and a higher capacity of the plant associated with AM fungi to take up P. Adapted from (Aerts 2002).
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The model is based on the assumption that nutrient utilization of two resources can lead to niche differentiation or out-competition between species. The species, which can reduce the resource to the lowest level and maintain growth, wins the competition. Co-existence is possible, when the growth of the species is differentially limited by the resources. Figure 5 demonstrates the suggested role of mycorrhizal type on plant interactions. In the absence of mycorrhizal colonisation, plant species associated with AM fungi is predicted to have a competitive advantage over plant species associated with EM fungi due to their presumably faster N and P uptake (Schulz et al. 2011, Stadler et al. 1993). The mycorrhizal colonization changes the situation. The suggested higher uptake capacity of EM for N, and AM for P leds to an increased P status of plant associated with AM fungi and increased N status of plant associated with EM fungi. According to Tilman´s model, both species can co-exist under these conditions.
The shift between co-existence and competition however varies with the total amount of the nutrient acquisition. Moreover, a number of influencing factors, such as plant species identity and species assemblages of root colonizing fungi have a great influence on plant performance (van der Heijden et al. 2003, van der Heijden et al. 1998). In an experiment with the AM forming plant species Hieracium pilosella, Bromus erectus, and Festuca ovina and four AM fungi, van der Heiden et al. (1998) demonstrated that plant species differ in their dependency on AM. This was reflected by the differing growth response of plant species on mycorrhizal colonisation, as well as by different effects of both AM species identity and species assemblages on several plant growth variables. Mycorrhizal diversity might also acts as an insurance to sustain plant productivity under changing environmental conditions. In a greenhouse experiment (Wagg et al. 2011) demonstrated that under nutrient limited con-ditions high number of AM mycorrhizal species relaxed the interspecific competition by reducing the growth suppression of the competitively weaker plant species. In nutrient-rich systems, the mixture of four AM fungal species was equally beneficial for the plant productivity as the most beneficial mycorrhizal fungal species in low nutrient system (Wagg et al. 2011).
5.2 Mycorrhizal networks
Both AM and EM fungi form simultaneous associations with trees of one or more taxa (Bent et al. 2011). These mycorrhizal networks (MN) are able to transport nutrients and carbon between tree individuals, and create facilitative effects of nutrient and water partitioning. This
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might be particularly important to relax the aboveground competition between mature plants and seedlings (Teste & Simard 2008). In a review of 60 cases, in which seedlings and larger plants were grown together, van der Heijden and Horton (2009) demonstrated that MN promoted seedling growth in 48% of the cases, whereas in 27% cases the effect of MN was neutral and in 25% cases negative (van der Heijden & Horton 2009). Generally, plants with EM benefitted from the MN, while the effects of AM association varied (van der Heijden &
Horton 2009). The type of mycorrhizal association might be particularly important, thus MN can strongly affect the growth and survival of plant species excluded from the prevailing MN (Booth 2004) and finally enhance the dominance of plants with one mycorrhizal type over another (McGuire 2007).
5.3 Trophic interactions with soil fauna
Mycorrhiza serve as an important channel of plant mediated carbon to soil food web (Pollierer et al. 2007). The use of 13CO2 gas labelling has currently confirmed C from recent photoassimilates as the most important C source of soil animals. Besides living or dead roots and root exudates, EM hyphae presumably contribute in a considerable manner to the nutrition of soil animals (Landeweert et al. 2001).
Spore findings of EM in guts of arthropod fungivores (mites, springtails, millipedes, beetles, fly larvae) and predators (centipedes) suggest that diverse soil animals feed on mycorrhiza and serve the spore dispersal of belowground fruiting species (Lilleskov & Bruns 2005).
Feeding experiments with axenic fungal cultures have shown that soil animal species feed selectively on distinct fungal species (Hiol et al. 1994, Scheu & Simmerling 2004). However, due to differences in EM metabolism in the symbiotic stage and the large variety of EM species in natural communities (Lang & Polle 2011), feeding choice experiments can hardly reflect animal behaviour under natural conditions. Currently, no firm proof for the mycorrhizal structures as primary diet of certain soil animals exists (Högberg et al. 2010, Pollierer et al. 2007). Furthermore other kinds of interaction, such as interactions between mycorrhizal and saprophytic fungi (Cairney & Meharg 2002, Mougel et al. 2006) or soil bacteria (Frey-Klett et al. 2007) occur. However, they are not considered in this thesis, since the research here focused on interactions with soil fauna.
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