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What is the evidence that that the structure of a plant community is determined by the diversity of the mycorrhizal fungi in the soil?

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Introduction

What is the evidence that that the structure of a plant community is determined by the diversity of the mycorrhizal fungi in the soil? Plants are the cornerstone of the global ecosystem. Being primary producers they convert energy from the sun into the creation of carbon compounds that serve as the fuel for life. However this cornerstone has an unappreciated yet vital set of foundations, mycorrhizal fungi that aid in the collection of nutrients needed for assimilation in return for some of the plants organic compounds. In this paper I shall describe the pathway science has taken as it tries to understand more about the complex interactions between plants and fungi, from the initial questions raised by Grime (1987) regarding the necessity of mycorrhizal fungi, to the landmark studies by Simard (1997) and Heijden (1998) and finally onto the effects man is having on mycorrhizal biodiversity and its implications for the future Lilleskov (2001). Mycorrhizal fungi are fungi that form associations with plant roots, either externally, ectomycorrhizae, or internally endomycorrhizae. Ectomycorrhizae ensheaf the plant root (creating the mantle) and send out projections called mycelia which scavenge the soil at a level inaccessible to plant roots (>3�m). This opens up a new world of resources for the symbiont to exploit, which would usually be limiting factors such as Nitrogen (N) and Phosphorous (P). They are abundant in nearly all land plant families (~80%); up to 90% of most trees "feeding" roots have such an association (Read 1997). Ectomycorrhizae assist plants in a variety of ways, through solubilising N and P from litter as well as P from rock, translocation of N and P over long distances, store P and polyphosphate and N as amino acids and release nutrient to the plant across a specialised interface, the hartig net. ...read more.

Middle

If mycorrhizal colonisation results in an equalisation of resource availability it would be expected to reduce the dominance of aggressive species and promote co-existence and greater biodiversity. Problems with the correct identification of fungal species has led to confusion and mismatching of data in previous experiments (Helgason 1998) has been alleviated thanks to molecular PCR identification techniques and has yielded some interesting results about plant mycorrhizal diversity. In arable sites 92% of obtained fungal sequences represented Glomus mossae or closely related taxa whereas sequences obtained from were much more diverse. However in both wood and field identical sequences from different plant species were found, suggesting the wide range of associations noted in culture may also be realised in nature. Therefore the low diversity of fungi in arable fields could not be due to monoculture per se but could reflect other aspects of the agronomic process such as ploughing, fertilising or the addition of fungicide. The evidence was now in place for the existence of a common mycelial network of ectomycorrhizal fungi creating guild in plant ecosystems. Although the transfer of nutrients down resource gradients had been confirmed and that AMF presence increases biodiversity (through subordinate donation of assimilates Grime 1987) and productivity, the effects that fungal (particularly AMF) communities had on plant community structure had been paid little attention. Heijdens' paper in Ecology provided evidence for the hypothesis that AMF community structure could potentially influence plant community structure. Based on Grimes' work Heijden hypothesised that although AMF species have a wide range of possible symbiont species if a differential response according to the species occurred the AMF community could directly affect the plant community. ...read more.

Conclusion

Resulting in a more efficient exploitation of soil P and resources available to the system. The effects of increasing AMF diversity have been shown here to clearly have an effect on plant community structure and ecosystem productivity. The present situation is described by Sen (2000) in their paper, stating that there is an increasing (and long overdue) appreciation for the central role played by mycorrhizal symbiosis in plant communities, a large progression from the thinking about plant communities in the 80's where they were not usually considered. Regarding current methodology, the analysis of intact mycorrhizal systems has greatly improved in the laboratory by the use of two dimensional microcosms developed by Read and co-workers. They provide an ideal solution for determining mycorrhizal driven cycling in soils. Studies in the field still apply and provide the field with large quantities of data and applications for this data have become very important when considering conserving biodiversity. In almost all of the papers cited here the conclusions have called for mycorrhizal communities to play a more important role when considering appropriate methods for conserving species. One worrying paper produced by Lilleskov (2001) states that ectomycorrhizal communities are losing diversity due to the increased use of fertilisers in the soil. Although plants welcome the additional Nitrogen, it upsets the balance of the symbiosis and hence the balance of the ecosystem. Possible proximal reasons for this could be the increased carbon cost of assimilation of the Nitrogen for the fungi or the fact that since Nitrogen is no longer the limiting factor functional shifts can be seen towards species that specialise in other resource limiting factors. This clearly demonstrates the importance of plant-fungal interactions and if one of the members of this symbiosis is perturbed then the effects will echo thought the global ecosystem. ...read more.

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