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Influence of Mycorrhizal Fungi on Temperate and Tropical Plant Community Processes

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Published: 23rd Sep 2019 in Biology

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Assessing and understanding the influence of Mycorrhizal fungi on temperate and tropical plant community processes and composition.

1.0 Introduction

There are a broad range of symbiotic relationships between higher plants and fungi developed over the past 400 million years (Hibbet. 2002). Whilst some relationships are parasitic, for example Ascomycete fungi grow in the tissues of grasses and sedges (Begon et al., 2011), there are numerous examples of symbiotic relationships or mutualisms, between mycorrhizas and plants, which will be discussed throughout this essay. Mycelia typically develop as thinly spread fibres which fill inter-cellular gaps only within stems and leaves. Regular by-products of these symbioses include highly toxic alkaloids which subsequently prevent foraging from grazers (Clay. 1990) and seed eaters (Knock et al., 1993). The mycorrhiza is the location of mutualistic exchange between the root tissue and fungi (Buscot et al., 2000). Mycorrhiza obtain and transport soil nutrients in exchange for carbon (Begon et al., 2011). Some plant species survive despite lacking mycorrhizal fungi in environments with limits factors such as water or nutrient availability (nitrogen being a particularly significant limiting factor) (Veresoglou et al., 2012). However, symbioses are generally considered an ecological necessity for the survival of most plant species (Buscot et al., 2000).

1.1 Ectomycorrhizal and Arbuscular Mycorrhizal Fungi

The three main mycorrhizal groups are; Ectomycorrhiza, Arbuscular mycorrhizas and Ericoid mycorrhizas (Begon et al., 2011). Ectomycorrhiza constitute a global network of between five thousand to six thousand species split between two main groups; Basidiomycete and Ascomycete fungi, both of which develop on tree roots (Buscot et al., 2000). The interface between roots and ectomycorrhizas is typically situated in the leaf litter layer of soils. Ectomycorrhizas establish a protective mantle which encircles the root, whereby hyphae proliferate into the litter layer by spreading and elongating (Begon et al., 2011). From there, hyphae uptake water and nutrients whilst fruiting and releasing substantial; concentrations of wind-borne spores (Wallander et al., 2011). Mycelium also protrude inward from the mantle; connecting the cells to the host and establishing a sizeable surface area for sufficient photo-assimilate exchange. Fungi typically induce morphogenic changes within the host plant’s roots; causing them to become truncated and pointed, whereas organically poor soil layers tend to exhibit longer host roots (Hynson et al., 2012). Arbuscular mycorrhizas account for approximately two thirds of all root fungi for tropical and non-woody species (Begon et al., 2011).

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Nearly all-natural plant communities contain Arbuscular Mycorrhizal Fungi (AMF), which typically benefit host plants (Van der Heijden et al., 1998). Most AM fungi are not species specific and can colonise any plants inhabiting a given soil type (Begon et al., 2011). High AMF colonization is indicative of healthy and therefore sufficient symbiosis, whereby the fungus improves the root’s nutrient uptake capability in exchange for plant sugars (Smith et al., 1998). Importantly, AMF establish in both roots and soils and similar to ECM, their hyphae also elongate and spread into the litter layer; up-taking water and nutrients in which the host plant can utilise and benefit (Begon et al., 2011). The fibres essentially act as structural stabilisers for the soil; also networking the symbiotic effects between plants.

There are examples of studies which have investigated the possibility that the species composition of AMF communities subsequently influences the framework of plant communities. For example Van der Heijden et al., 1998 designed a pot experiment utilising; Festuca ovina Hieracium pilosella, and Bromus erectus; inoculating each plant with either four of each AMF species, a mixture of the four or a control (not inoculated). For ecological continuity, the AMF species were sourced from the same calcareous grassland from which the three species of plants were derived. Surprisingly, they found that the dependency of particular plants varied markedly, due to differing responses in growth according to particular AMF species. Resultantly, they stated that plant mycorrhizal dependency is a moveable value that should not be used definitively. The variation in dependency was influenced by both the selected species of plant and the combinations of AMF. Differential effects on plant growth were distinctly apparent and AMF species which occurred in the form natural AMF communities could possibly regulate the structure of the plant community. This experimental design has significant implications for future studies and subsequent conservation management; regarding how plant community structure and composition is specifically affected by AMF.

Significant amounts of photo-assimilates can be conveyed to the mycorrhiza in temperate forests (Pepin & Korner. 2002), where hyphae develop “underground highways”, for carbon and nutrient exchange both with and between plants (Hoberg et al., 2008). ECM fungi are extremely efficient at sourcing limited supplies of both phosphorus and nitrogen from the litter layer and the diversity of plant species niches is likely to be influenced by the high fungal species diversity (Buscot et al., 2000). Carbon which is exchanged from plant to fungus is found mostly in the form of basic hexose sugars; fructose and glucose (Korner et al., 2005). The fungi’s consumption of these sugars can constitute up to thirty percent of the plant’s yearly photosynthesis production. Plants however, are mostly nitrogen limited because nitrogen mineralisation (the conversion of organic to inorganic nitrogen) is generally restricted on the forest floor and ammonia is the most commonly available form of inorganic nitrogen (Eleftheriadis et al., 2018). The various plants which are interconnected through mutual mycorrhizal networks are therefore termed as “guilds” (Simard et al., 1997). The particular assembly of fungal species is likely to directly effect the plant community composition and subsequently the said ecosystem’s yield (Van der Heijden et al., 1998; Van der Heijden et al., 1998 (ii)), with significant improvements in plant fitness by increasing their ability to efficiently uptake nitrogen and phosphorus (Johnson et al., 2010). Therefore, mutualisms are essential for plants in ensuring nitrogen transfer from fungus to plant through enzymic degradation; whereby fungi obtain ammonium in preference to other forms of inorganic nitrogen and bypass ammonium reduction zones by inducing substantial hyphal proliferation (Hynson et al., 2012). Although such mutualisms often appear to be evenly balanced, sudden environmental changes often emphasise the more exploitative element of these relationships. For example, ECM growth is directly correlated to the production of its partner plant’s hexose sugars. When direct nitrate availability is high (natural or anthropogenic circumstances), the plant’s metabolism is diverted to amino acid synthesis in preference to hexose production. Subsequently, the ECM degenerates (Began et al., 2006). Therefore, it is very apparent that plants only support as much ECM fungus as they require.

A recent temperate forest study by Klein et al., 2016, indicates that carbon transfer via mycorrhiza has a significant part to play in facilitation between different tree species; concluding that interspecific, bidirectional transfer is most likely assisted by common ectomycorrhiza networks (typically abundant in healthy temperate forest ecosystems). This form of transfer accounted for forty percent of the fine root carbon (approximately 280 kilograms per hectare in a given year of transmission between trees, equivalent to four percent of the forest net carbon intake). Also significant about this study is the fact that the mycorrhizal connections between trees are shown to persist between different species. Therefore, the fungi are essentially allowing interspecific facilitation between different tree species such as Larch, Norway Spruce, Scots Pine and Beech. Klein et al., 2016 therefore emphasised that ECM fungi probably directly alter the course of their constituent plant community structure and composition, by facilitating bidirectional carbon exchange between different species. Ericoid mycorrhizas coexist with the dominant heathland species of the temperate, boreal and Australian heathlands (Beg0n et al., 2011). However, it is Ectomycorrhizas and Arbuscular mycorrhizas which form the main focus of this essay

Spatial ecology can be vital in bettering our comprehension of the complicated relationships among fungi and plants, which ultimately influences the community in terms of its processes and species composition (Tilman & Kareiva, 1997). Pickles et al., 2010 implemented spatial analysis to assess the arrangement of each species in an ECM fungal community. They established that the distribution for ECM was significantly different between Scots Pine stands that were clear felled and ones that had been left to grow. Spatially, the mycorrhizal arrangement was heavily affected by the underlying pattern of fine roots and root density was also improved by the occurrence of certain fungi species (Pickles et al., 2010). The removal of trees subsequently led to the loss of the beneficial fungi which required living pine roots as a substrate (Hoberg et al., 2002; Guo et al., 2008). In addition, given the reasonably short lifespans of particular mycorrhizas (Downes et al., 1992), it is hardly surprising that clear felled areas lost the beneficial fungi and newly planted trees lacked higher root density as a repercussion of reduced fungal diversity (caused by the initial felling) (Pickles et al., 2010). The second part of this essay will discuss the various management strategies available to conservation groups, charities and NGOs across the globe, who are working to restore temperate, boreal and tropical swamp forests and prairies.

 

1.2 Aims and Objectives

The aim of this essay is to explore the mutualistic exchanges between Arbuscular and Ectomycorrhizal fungi and their constituent plants (which often express very unclear cost/benefit relationships); and then specifically investigate how learnt knowledge of these interactions can be applied to the conservation, restoration and management of certain temperate, boreal and tropical habitats.

 

2.0 An introduction to habitat management with the utilisation of fungi: A focus on inoculation

The significance of mycorrhizal associations in habitat revival is fundamental for optimising regeneration (Quoreshi. 2008). Yet, the application of mycorrhizal biological techniques is still largely considered unconventional in many areas across the globe (Dunabeitia et al., 2004). Mycorrhizal fungi inoculated plants are currently the most commonly used “biological tools” for assisting the regeneration of degraded landscapes (Quoreshi. 2008). The disruption of microbial processes in soil systems is an integral element of soil disturbance, and its restoration is an essential approach for habitat recovery (Bois et al., 2005). Drastic disturbance changes the processes and configuration of both mycorrhizal fungi and their host plants significantly (Allen et al., 2005). Some anthropogenic activities such as mining can severely reduce the soil’s moisture, biological processes, bulk density, pH and nutrient load (Bois et al., 2005). In such circumstances it is important to increase the amount of mycorrhizal inoculation in regeneration projects to restore the land’s productivity to levels comparable to it’s optimum yield, prior to disruption (Dunabeitia et al., 2004). Otherwise, subsequent regeneration could be held up and reduced (Bois et al., 2005). In terms of forest restoration, it is vital for trees to develop beneficial fungal mutualisms for sufficient nutrient uptake, especially during the seedling and sapling phases (Dunabeitia et al., 2004).

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Restoration methods frequently incorporate phytobial remediation, which involve the establishment of beneficial fungal networks; through planting nitrogen fixing species, facilitating growth building bacteria and mycorrhizal fungal inoculation (Dunabeitia et al., 2004). In the case of disturbed ecosystems, the planting of native seedlings, inoculated with beneficial mycorrhizal fungi-host mutualisms is the preferred option, because this ensures sufficient protection from disease and efficient nutrient and water consumption (Bois et al., 2005). Both AMF and ECM fungi have are shown to improve the early stages of many hard and softwood species; furthering their survival (Ortega et al., 2004). As more empirical research indicates the implicit benefits of mycorrhizal species to host plants, so the interest in their applications to conservation has grown (Dunabeitia et al., 2004). However, as previously highlighted, some mycorrhizas actually have negative implications for plant growth (Begon et al., 2011). Moreover, it is essential that ecologically appropriate fungi are determined before methods such as inoculation are implemented.

 

2.1 Case Studies

The extent to which AMF inoculum affected the early successional stages of tallgrass prairie restoration, was investigated by Smith et al., 1998, by utilising field plots in a disturbed area of Minnesota. Inoculations were achieved by acquiring native prairie mycorrhizas and placing them below mixes of prairie seed. Control plots were split into two categories: the inclusion of seed and sterile soil and those with only seed. As expected, after 15 months, the roots of inoculated plants exhibited considerably higher percentages of AMF fungi. Surprisingly, inoculation did not influence the total percent cover of plants, however native planted grass cover was much greater inside inoculated plots compared to control groups. They established that increased native grass recruitment positively correlated with enhanced succession; thus inoculation enabled the grasses to outcompete the invasive (but native) plants. Encouraging the development of tallgrass, especially during their early succession within the greater prairie assemblage is vitally important for North America because this specific habitat has become rapidly reduced in spatial area over the last 150 years.

Rates of tropical peat swamp forests (TPSF) deforestation are accelerating at an alarming rate (Graham et al., 2013), and are resultantly causing wildfires, carbon loss from the soil and disordered hydrology. Graham et al., 2013 explored a potential cost-effective strategy to reverse this with the intention to implement this within sustainable management of this natural resource. Dyera polyphylla and Shorea balangeran, their constituent mycorrhiza; Scleroderma columnare (S. balangeran), Gigaspora decipiens (D. polyphylla) and Glomus clarum were utilised because they were considered to be good proxies for the experiment. Comparisons between non-inoculated and inoculated seedlings were presented by transplanting into five forest sections, ranging from untouched to completely degraded forest. Inoculated seedling species in degraded areas facilitated considerably more colonization compared to non-inoculated. Survival rates were high in all forest zones, but surprisingly biomass and growth were unaffected by mycorrhizal treatment. This may be due to the short study period of the experiment, and subsequent future experiments would need to be conducted for longer time periods to determine whether growth and biomass production is increased in response to inoculation. The results still have positive implications for conservation management of this habitat, in spite of the results only definitely proving that colonisation is increased when inoculation is implemented.

Williams et al., 2012 assessed the contrasting survival and growth among Podocarpus cunninghamii (mountain tōtara) seedlings with the inclusion of six differing AMF inoculations; intending to select the most applicable mycorrhizal species for the habitat revival in Mackenzie Basin, New Zealand. Inoculation of P. cunninghamii seedlings was utilised with a spectrum of AMF, ranging from exotic/commercial/ nursery to native species. The native AMF were acquired from the remaining isolated P. cunninghamii forests and post agricultural areas transitioned to grassland. After two growing seasons; survival and growth of plants was compared and assessed. Indigenous AMF treated plants exhibited considerably higher survival rates compared to plants treated with conventional AMF. Native AMF promoted more P. cunninghamii growth compared other inoculations. Similar to the previous examples, this experiment has very important implications for habitat revival, because increased survival and improved growth of native woody species will potentially increase restoration success through reducing management costs and accelerating establishment success.

The final case study is an ongoing trial associated with NGO Trees for Life. The Caledonian forest is the most westerly extent of Eurasia’s boreal forests and is predominated by Scots Pine. Due to centuries of deforestation only 1% of the original 15,000 km2 (3,700,000 acres) forest remains and subsequently many of the well-established mutualistic ectomycorrhizal – tree relationships have been lost (Puplett. 2018). Since 1987 the charity has been working to restore the forest into a 1000 sq. mi target zone in the North West Highlands, but originally supplemented tree seedlings with phosphorus as a main fertiliser to facilitate recruitment. Trees for Life took the decision in 2017 to apply fungal treatment as an alternative treatment to  130, 000 planting holes per year on planting sites on Dundreggan estate (Trees for life. 2018). Some of the mycorrhizal species include Fly Agaric (which is essential for the establishment of Birch), Penny Bun (associates with Sessile Oak) (Puplett. 2018) and Greenfoot Tooth fungus (facilitates Scots Pine) (Featherstone. 2016). Similar to the previous case study, mycorrhizal species were collected from remaining old growth stands in the region. This culminated into a fungal mixture, containing spore-laden granules. This mixture was added as inoculation to 20,000 tree sapling planting holes in the deforested areas of Dundreggan estate in spring 2018 (with the aid of volunteers, as the charity is almost entirely dependent on public participation) (Trees for Life. 2018).

In conclusion, the first part of this essay focusses on the mutualistic mechanisms between Ectomycorrhizal and Arbuscular mycorrhizal fungi and their constituent, host plants. Then the theory behind habitat restoration with the utilisation of mycorrhizas is explored, and finally recent case studies are briefly discussed to give a current context of fungal involvement in habitat management. Both the theory and case studies highlight the specific significance of inoculation as a primary method to implement fungal facilitation in habitat restoration. In the case of this essay, the implications for future conservation management are very significant because all the case studies and theory (except the final case study which is ongoing), indicate that the colonisation process of the target species is accelerated when mutualistic fungal interactions are reintroduced to a system. This is important because this could save time and financial resources for both governmental and NGO organisations involved in restoration programmes across the globe. 

 

Word Count (Excluding Bibliography): 2876

 

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