Antecedentes legales

There is abundant evidence for mycorrhizal fungi benefiting the perform- ance of their host plants. This is due to improved plant uptake of water and P (Smith and Read 1997) and also increased protection against attack by pathogens (Newsham et al. 1995) and above-ground herbivorous insects (Gange and West 1994), leading to increased fecundity (Newsham et al. 1994). However, while mycorrhizal fungi associate with the majority of plants within any community, they confer different degrees of benefit on certain plant species, thereby directly influencing the structure of plant communities. One of the first studies to illustrate this experimentally was by Grime et al. (1987) who assembled diverse grassland communities in microcosms and allowed them to develop in the presence and absence of AM fungi. The presence of mycorrhizal fungi led to a shift in plant com- munity composition, reducing the dominance of Festuca ovina in favour of several subordinate herb species that benefited most from AM fungal infec- tion. The net effect of this was a significant increase in plant species diver- sity due to a relaxation of plant competitive interactions.

Positive effects of AM fungi on subordinate plant species have also been shown to occur in the field. For example, Gange et al. (1993) applied a

selective biocide to an early successional grassland community in southern England, which led to a suppression of AM fungal infection and an associ- ated increase in the abundance of competitively dominant grasses at the expense of subordinate herb species, thereby reducing plant species rich- ness. Several mechanisms have been proposed to explain increases in plant diversity resulting from AM fungal infection. Allen (1991) suggested that mycorrhizal fungi might increase plant diversity due to spatial heterogene- ity of fungal infectivity in field soil, allowing mycotrophic (i.e. species that depend on mycorrhizal associations) and non-mycotrophic species to coexist in patches of high and low inoculum. Alternatively, Grime et al. (1987) suggested that AM fungi increase plant diversity due to interplant transfers of carbon and nutrients via hyphal links, which lead to more even distri- bution of resources within the plant community, reducing the ability of cer- tain species to monopolize resources. The net effect of such nutrient distribution would be a more equitable competitive interaction between plant species, promoting species coexistence and greater plant diversity. This view is also consistent with findings of Van der Heijden (2004) that AM fungi promoted seedling establishment in perennial grassland by integ- rating emerging seedlings into extensive hyphal networks and by supply- ing nutrients, especially P, to the seedlings. AM fungi therefore act as a symbiotic support system that promotes seedling establishment and reduces recruitment limitation in grassland (Van der Heijden 2004). Not all studies show that AM fungi promote plant diversity. Hartnett and Wilson (1999), for example, found the opposite to be the case in tallgrass prairie, where suppression of AM fungal infection using the fungicide Benomyl led to a large increase in plant species diversity. This was due to a reduced abundance of dominant, obligately mycotrophic C4 tall grasses and consequent competitive release of subordinate facultative mycotrophs. This suggests that active AM fungal associations reduce floristic diversity in tallgrass prairie due to the promotion of competitive dominance of mycotrophic species. Similarly, it has been proposed that in tropical rain- forests ectomycorrhizal associations encourage dominance of certain tree species, at the expense of arbuscular mycorrhizal trees that are less able to acquire nutrients and tolerate pathogen attack, thereby reducing species coexistence (Connell and Lowman 1989). These studies suggest another mechanism for mycorrhizal effects on plant communities, whereby differ- ences in host plant responses to colonization by mycorrhizal fungi result in changes in plant diversity if the dominant competitors are more strongly or weakly mycotrophic than their neighbours.

In all the above studies, effects of mycorrhizal fungi on plant diversity have been attributed to selective and beneficial effects on certain species, either relaxing or enhancing competitive dominance. However, recent evidence indicates that the costs and benefits of maintaining symbiosis with AM fungi vary greatly between plant species, in that individual plant responses

can range from positive (mutualism) to neutral (commensalism), and even to negative (parasitism), suggesting that the symbiosis is better defined as a continuum from parasitism to mutualism (Johnson et al. 1997; Klironomos 2003). This suggests that in addition to differences in mycorrhizal depend- ency (Harnett and Wilson 1999), interspecies differences in the direction and intensity of response to mycorrhizal infection, along the mutualism– parasitism continuum, also act as structuring forces in plant communities. This idea has been tested in grassland communities (Klironomos 2003), but has not yet been examined in other types of ecosystem.

Variations in mycorrhizal fungal diversity may also strongly affect plant diversity. It is well known that mycorrhizal fungi are highly diverse (e.g. Daniell et al. 2001) and different species have been shown to induce differ- ent growth responses in plants (Van der Heijden et al. 1998a). Therefore, it would be reasonable to assume that variations in the diversity of mycor- rhizal fungi would affect the diversity of the plant community. This idea was tested by Van der Heijden et al. (1998b) by manipulating the number of native AM species (1–14 species) in experimental grassland units con- taining 15 plant species. They found that plant diversity and productivity were positively related to AM fungal diversity (Fig. 4.11(a),(b)). The mech- anism for this was thought to be related to a reported increase in AM hyphal length in the more diverse treatments, enabling more efficient exploitation and partitioning of soil P reserves, thereby relaxing plant com- petition and increasing plant productivity (Fig. 4.11(c)). It has been pro- posed by Van der Putten (2005), however, that these effects may only be observed in relatively nutrient-poor environments. In fertile soils, effects of AM fungal diversity may be neutral or even negative, when related to nutri- ent uptake only. This has been shown for ectomycorrhizal fungi by Jonsson et al. (2001). These authors found that in a low fertility substrate, ectomy- corrhizal diversity enhanced biomass production of Betula pendula, but no effects were found in fertile substrate, while for Pinus sylvestris the effects of ectomycorrhizal species richness were negative even in highly infertile substrate. In a similar way, ericoid mycorrhizal fungi might even reduce plant species diversity in nutrient-poor soils due to monopolisation of the available nutrients by their host species (Northup et al. 1995).