In Addition to Abiotic Factors, Community Composition of Plants Can Be Severely Compromised by

Abstract

In many semi-natural and natural ecosystems, mycorrhizal fungi are the most arable and functionally important group of soil micro-organisms. They are almost wholly dependent on their host plants to supply them with photosynthate in render for which they enable the plant to access greater quantities of nutrients. Thus, in that location is considerable potential for plant communities to regulate the structure and function of mycorrhizal communities. This paper reviews some of the key recent developments that take enabled the influence of establish species richness, composition, and historic period on mycorrhizal communities in boreal forests and temperate grassland to be determined. Information technology discusses the emerging evidence that, in some situations, plant species richness is related to mycorrhizal species richness, in contrast to previous thinking. The paper also includes some preliminary data on the effect of host stand age on root-associated basidiomycete communities. It concludes by highlighting some of the new methodological advances that promise to unravel the linkages between mycorrhizal variety and their function in situ.

Introduction

In recent years, there has been a growing awareness amongst constitute ecologists and soil microbial ecologists that understanding the connectivity between their report organisms is of utmost importance. The interactions between plants and soil micro-organisms are particularly of import because plants represent the primary pathway through which carbon, the element that severely limits microbial growth, enters soil. This is accomplished in a number of ways including litter fall, root turnover, passive exudation of carbon in the rhizosphere in the course of elementary organic compounds, and active transfer of carbon to organisms colonizing plant roots. From the reciprocal viewpoint, micro-organisms have essential roles in the mineralization of nutrients into forms bachelor for institute uptake, as well as a number of other advantages.

One of the main drivers of this joined-up thinking is the importance now placed on agreement global biodiversity. This has occurred not only because of the alarming loss of species that has occurred in the terminal thousand years or and so, but also because the pressures that are at present placed on the terrestrial environment have the potential to cause farther widespread loss of species ( Thomas et al., 2004). From the plant perspective, species richness (here used interchangeably with variety) of ecosystems has received considerable attention in contempo years and has generated an as large amount of controversy. On the one manus, increasing plant species diversity has been shown to pb to a progressive increase in ecosystem productivity ( Hector et al., 1999; Tilman et al., 2001), while on the other hand, productivity has been shown to respond non-linearly to increasing plant species diversity, producing a 'hump-backed' response (Grime, 1973). However, in that location is a risk that these studies generate debates based on an entirely phytocentric view of 'ecosystem function'. It could exist argued, for example, that for many semi-natural grasslands, which are oft subjected to various degrees of grazing pressure, institute productivity is not a particularly useful or important measure of ecosystem function. Very few studies have attempted to take a more than holistic view past determining how found species diversity may affect wider and more than relevant indicators of ecosystem function.

In many semi-natural and natural ecosystems, mycorrhizal fungi are the most arable and functionally of import groups of soil micro-organisms (Smith and Read, 1997). Virtually all the plants in temperate grassland form mutualistic associations with arbuscular mycorrhizal (AM) fungi ( Read et al., 1976). AM fungi provide many benefits to plants, most notably the ability to access substantially greater amounts of nutrients. In limestone and calcareous grassland, the key food involved is phosphorus, because calcium phosphate is relatively unavailable to non-mycorrhizal plants. The energy required by AM fungi to exploit the heterogeneous soil environs so finer is supplied by the host found in the grade of carbohydrates. The carbon demand that this places on plants can be quite high; laboratory studies have shown that this can be as much as 20% of the total mountain of carbon stock-still by the plant (Pearson and Jakobsen, 1993), although information technology should be stressed that these values are derived from highly mycotrophic species such as cucumber, and the range almost usually seen is five–10% ( Johnson et al., 2005). Nonetheless, at that place tin be no doubt that AM fungi are important conduits of recent constitute photosynthate to soil. In addition, this pathway is a very rapid one. Field studies suggest that near carbon is allocated to AM fungi within 24 h of fixation ( Johnson et al., 2002a , b ) and the turnover of fine hyphae itself is probable to be in the gild of a few days ( Staddon et al., 2003).

In temperate and boreal woods communities, almost trees acquaintance with ectomycorrhizal (EcM) fungi. As with AM fungi, the significance of ectomycorrhizas for the major elemental cycles is enormous. In microcosms 20–30% of current digest from seedling hosts is allocated to EcM fungi (Söderström, 2002). Over fifty% of CO₂ released from boreal forest soils is accounted for past the respiration of tree roots and their associated EcM fungi ( Högberg et al., 2001) and at least xxx% of the microbial biomass in boreal forest soils is the extraradical hyphae of EcM fungi (Högberg and Högberg, 2002). EcM fungi are, therefore, significant sinks for establish photosynthate, the carbon beingness used to fuel the growth of extensive mycelial networks producing the so-chosen 'forest-wide web'. In return, EcM fungi have the ability to short-circuit conventional food-cycling pathways past taking up organic forms of nitrogen and phosphorus. Given the widely acknowledged fact that mycorrhizal fungi form ecologically important connections between plants and soils, it seems unusual therefore that they are rarely considered in studies investigating how plant communities can touch on ecosystems functioning.

While the response of ecosystems to changing establish diversity is clearly important, environmental perturbations or state use change may result in situations where replacement of a given plant species may occur merely establish diverseness remains similar. Because plants are the master pathway through which carbon enters the energy-impoverished soil environment, it seems reasonable to ask the question how the species composition of plant communities, not just their diversity or richness, may influence ecosystem and especially microbial functioning. Expanding this argument more widely, information technology may be desirable to make up one's mind how functional groups of plants or indeed the age of plants within a given community affect ecosystem processes. The latter point may be particularly applicable to establish communities created artificially and that accept existed for relatively short time scales, such as forestry plantations.

The advent of several new molecular and stable isotope techniques (such as stable isotope probing) in recent years and their application to the natural surround has significantly increased the understanding of microbial diverseness and functioning, especially in relation to carbon cycling. The aim of this paper is therefore to review the progress that has been made in understanding how the three chief factors of (i) diversity, (ii) composition, and (three) age of constitute communities in temperate and boreal biomes affect mycorrhizal community limerick and office, and how these new studies fit hypotheses derived from previous studies. These three factors are likely to have the greatest bear upon on mycorrhizal communities, but it is best-selling that there are undoubtedly other related processes not considered hither, such equally positive or negative institute interactions.

Species composition of plant communities

Of the iii factors discussed here, the importance of the composition of a given plant community in influencing mycorrhizal communities is probable to stand out as the most obvious. Indeed, the importance of plant customs composition of forests as determinants of EcM community composition is now well established largely because of the observations that many EcM fungal species show some level of host specificity or preference. Much of the pioneering piece of work demonstrating that sure EcM fungal species associate with specific plants, while others have broad host ranges, was undertaken in the coniferous forests of the Pacific Northwest (Trappe, 1962; Molina and Trappe, 1982). The observations past Trappe (1962) were based largely on distributions of sporocarps; the supposition being that the production of sporocarps was indicative of mycorrhizal association. This supposition has been challenged based on an assay of mycorrhizal morphotypes on root-tips (Taylor and Alexander, 1990) and, more recently, by molecular methods that enable their fungal endophytes to be identified more rigorously (Gardes and Bruns, 1996; Jonsson et al., 1999). Needless to say, the relationship between above- and below-ground species is, more often than not, poor. Molina and Trappe (1982) undertook a more rigorous approach to screening mycorrhizal specificity past testing the abilities of 27 EcM isolates to form mycorrhizas with vii conifer host plant species in pure culture microcosms. This piece of work demonstrated a wide range of 'EcM host potentials' ranging from, for case, the Alnus-specific symbiont Alpova diplophloeus, through the intermediate specificity of Cortinarius pistorius on Pseudotsuga menziesii, Larix occidentalis, Tsuga heterophylla, and Picea sitchensis, to the generalist symbiont Paxillus involutus. Molecular tools accept meant that host specificity and preference of EcM fungi can now be investigated in the field (Horton and Bruns, 1998; Horton et al., 1999; Cullings et al., 2000; Kennedy et al., 2003). By and large, these studies show that the nearly abundant fungi in the community are non host specific, but that a minority of species bear witness marked host specificity or preference (greater relative affluence on certain hosts).

Given the low number of AM fungal species thus far described (about 150), and the fact that they colonize effectually 200 000 establish species, conventional wisdom would suggest that species composition of broad types of plant communities, say temperate grassland, does not have much influence on the diversity or function of AM fungi. This view of low specificity has, not surprisingly, been assumed to agree truthful for most situations in which AM fungal associations occur, but there are exceptions to this. For example, Bidartondo et al. (2002) showed extreme host specificity by certain achlorophyllous mycoheterotrophic plants on AM fungi. Whether such extreme host specificity will ever exist encountered in the wider AM plant community seems unlikely. One must also consider that specificity tin can occur in either direction between plant and fungus. In the mycoheterotroph example in a higher place, for example, the fungus colonizes other species of neighbouring green plants from which the mycoheterotroph obtains its carbon. In his excellent review of the subject, Sanders (2002) was unable to depict any business firm conclusions about the being of specificity from either the constitute or the fungal perspective. He also questioned the relevance of identifying AM fungi at the species level, when in fact considerable variation, and possibly therefore specificity, could exist occurring at the genotype level. But does it really matter if tight host specificity is not seen universally, whether from the plant or fungal perspective? What is more of import, at least to address the question of the importance of constitute community composition in influencing AM fungal communities, is the beingness of a gradient from zero host specificity towards the extreme cases cited above. Perhaps more important still is the need to understand the many factors that may push detail fungal species/genotypes along this gradient and the management and magnitude of their movement.

The existence of host preference past AM fungi has been known for some time; Dhillion (1992) found that Glomus geosporum and M. fasciculatum differentially colonized 3 native prairie grasses. In addition, the observation that increasing diversity of AM fungi can help to maintain diverse assemblages of plants would suggest some caste of host preference ( van der Heijden et al., 1998a , b ). This blazon of study also provides back up for the argument that different AM fungi produce markedly different levels of root colonization, growth rates and nutritional responses in some plant species compared to others ( Streitwolf-Engel et al., 1997; van der Heijden et al., 1998a , b ). The ability to make up one's mind the identity of AM species within roots of naturally colonized plants has reinforced this view. Intensive analysis of neighbouring native woodland plants that grade AM symbioses revealed that the patterns of colonization of their roots were distributed in a not-random manner ( Helgason et al., 2002). For example, the combinations of Rubus fruticosus with Scutellospora and Acer pseudoplatanus with Glomus were significantly over-represented. Using molecular techniques, Vandenkoornhuyse et al. (2003) and Husband et al. (2002a) accept shown clear non-random associations of AM types with the unlike hosts in ecosystems as divergent as chalk grassland and tropical rain forest.

Johnson et al. (2004) studied AM fungal communities in grassland microcosms containing natural soil and artificial establish communities. The plants were naturally co-occurring and were obtained from the field and reconstituted to course four treatments comprising bare soil, Carex flacca just, Festuca ovina only, and a mixture 12 mainly mycorrhizal species. The diversity of AM fungi (based on T-RFLP patterns) colonizing the roots of Plantago lanceolata bioassay seedlings transplanted into the microcosms significantly afflicted the N and P concentration in the Plantago shoots. The diversity of AM fungi was itself only influenced past the limerick of the microcosms rather than other factors such every bit microcosm found biomass. Interestingly, the variety of AM fungi colonizing P. lanceolata seedlings in the bare soil treatment was greatest, while the multifariousness on P. lanceolata seedlings in the F. ovina monoculture was smallest. This provides further evidence of a degree of host specificity. This may have arisen through the authorisation of a restricted group of fungi that were able to form an extensive and vigorous mycelial network using carbon from the established community of mature F. ovina plants. In the bare soil, such a supply of carbon would not exist, instead, most colonization of the seedling roots would accept occurred from an assemblage of propagules. Because propagules in the blank soil treatment would non have been exposed to these selection pressures, the loss of multifariousness would be avoided. Kuszala et al. (2001) demonstrated that 20 isolates representing 16 genera were able to sporulate after storage in moist soil for up to 8 months. The power of mycorrhizal propagules to maintain their viability for relatively long-periods suggests a degree of resilience that can only be beneficial for restoring biodiversity and role. Despite the longevity of inocula, it is not yet known if mycorrhizal populations are able to reform on plant species re-introduced into a particular habitat after their loss.

One potential difficulty with many of the discussions made thus far on the topic of specificity is the assumption that only very few species of AM fungi colonize a single plant species (Fig. 1a, b). This is not surprising given that there is a very small pool of AM fungal species thus far described, although the consequence highlighted by Sanders (2002), and discussed higher up, regarding the usefulness of regarding AM communities only at the species level may complicate the style AM multifariousness on individual plants is characterized. While some plants are likely to be colonized by only a few species, there is at present a growing body of evidence suggesting extensive diversity in some grassland species. For instance, Vandenkoornhuyse et al. (2002) establish 24 distinct phylotypes (clades of closely related sequences) in two common co-occurring grassland species Trifolium repens and Agrostis capillaris. Merely three phylotypes were sectional to T. repens and six to A. capillaris. Thus, conceptual models of plant and AM fungal communities need to consider the possibility that a small number of host-specific species occur alongside many non-specific species (Fig. 1c). This has important consequences, because it implies some degree of functional redundancy or functional resilience in AM communities. On the other mitt, loss of a particular plant species may accept asymmetric impacts on the variety of the AM fungal community. A complicating cistron highlighted by Sanders (2002) is the need to chronicle presence with abundance, something that is currently impossible to do with whatsoever certainty for mycorrhizal fungi. For example, in the hypothetical situation in Fig. 1a, is it of import that AM fungal species A colonizes merely twenty% of the root length of plant X compared with ninety% in plant Y? In other words, the truthful diverseness, as opposed to species-richness, of AM fungi within plant roots needs to exist assessed. In reality, this argument needs to be taken one stride even further, so that some 'real' measure of role is quantified. It does non necessarily follow that percent root length colonized equates with greater functionality. Ane clear challenge that lies alee is the demand to relate the diverseness of AM fungi in the bulk soil to that in the plant roots. Later all, it is the external mycelium that exploits the patchy soil surroundings. In the case in Fig. 1c, loss of AM species F volition accept greater impacts on plant species Ten compared with Y. The fact that mycorrhizal fungi can form shared mycelial networks adds a farther complicating factor. If AM fungus species F is shared betwixt plant 10 and Y, then loss of plant X may accept knock-on furnishings for plant Y depending on the functions provided to the plant from AM fungus F (Fig. 1c). A striking contempo example of this blazon of effect for EcM fungi has been described by Haskins and Gehring (2004). They showed that trenching to forbid the intermingling of pinyon pine and juniper roots increased EcM abundance on the pinyon pine and altered the pinyon EcM community composition.

Fig. 1.

Models of host specificity in AM fungal symbioses. (a) No host specificity. AM fungal species A and B shared between each host plant. (b) Absolute host specificity. AM fungal species A and B colonize specific found species. (c) Express specificity as demonstrated in natural grassland species. Simply AM fungal species A–C and I–Yard are host-specific. In this case the length of external mycelium of fungus F differs betwixt constitute species, and may or may not form a mutual network.

The study by Vandenkoornhuyse et al. (2002) also showed that AM fungal customs composition differed according to the time of sampling, indicating that the community structure of AM fungi on the ii species investigated was very dynamic. This finding raises fascinating questions nearly the functional significance of these fungi. Are the different fungal communities at different time points reflecting different carbon or nutrient use strategies? The utilization of host-derived carbon is a fundamental component of all mycorrhizal symbioses that has the potential to exist investigated in great detail by sophisticated molecular-based techniques such as stable isotope probing (SIP; Radajewski et al., 2000). SIP utilizes conventional ^thirteenCO_two pulse-labelling in gild to enrich the DNA of micro-organisms that receive contempo photosynthate from plants. The labelled microbial DNA tin be separated from non-labelled DNA by ultra-centrifugation and the multifariousness and identity of the two fractions adamant using conventional PCR-based approaches, such as T-RFLP or DGGE. To date, in situ SIP has but been practical to bacterial communities in grassland systems ( Griffiths et al., 2004) and bioreactors ( Manefield et al., 2002). At that place are enormous technical hurdles to exist overcome for applying SIP to AM fungal communities, but the claiming is there and the showtime experiments demonstrating its application to mycorrhizas are eagerly awaited. The functional significance of AM community composition for phosphorus conquering could besides be tackled by isotope probing methods. For instance, given that a major component of Dna is phosphorus, the greater mass of the radionuclide ³³P in relation to the stable nuclide ³¹P could be exploited in similar means to ¹³C/¹²C separation using ultra-centrifugation. While this would not have the flexibility of ¹³C in terms of field applications, it would have the potential to provide important insights into the phosphorus use strategies of individual species/phylotypes in intact natural communities.

Diversity of plant communities

Grassland

The majority of studies that have attempted to understand the importance of institute species multifariousness to productivity take focused on grasslands of one sort or some other. Temperate grasslands can range from being floristically very rich (in temperate terms; i.eastward. >20 species m^−two) to floristically very poor. Differences in species richness can be seen in very close proximity in some situations, for example, the limestone grassland dales of the Tiptop National Park in Derbyshire, England. Despite the ubiquity and importance of AM fungi for elemental cycling, few studies have attempted to determine how found diversity can impact on AM fungi. This is partly because the community limerick, biomass, and abundance of AM fungi is hard to measure. The lack of a reliable index of AM fungal biomass contrasts with the array of methods bachelor to make up one's mind the biomass of soil microbial communities in general. In the last decade, conclusion of specific phospholipid fatty acids (typically 16:1ω5) has been seen as the near useful mensurate, because it has loftier specificity to AM fungi and is found in abundance in AM mycelium ( Olsson et al., 1995). Although criticisms can exist levelled at the general utility and specificity of the technique, specially in organic soils, it can nevertheless provide a useful comparative measure of AM fungal biomass. Hedlund et al. (2003) applied this technique to a pan-European experiment in which three levels of establish richness were imposed on grassland plots for three years. At one of their sites they constitute a potent positive linear relationship betwixt institute species richness and AM biomass (R ²=0.half-dozen; P<0.01) and a negative relationship between constitute biomass and AM biomass (R ²=0.viii; P <0.01). It seems remarkable, given the strength and clarity of the relationship, that a similar situation was not apparent at the other iv sites. Complementary measures of colonization of plant roots by AM fungi may accept indicated whether the observed differences in the amounts of the AM signature fatty acrid extracted from soil were reflected in planta. Nevertheless, this apparent positive relationship is supported by other work in which the product of AM fungal spores was correlated with plant species richness. For example, Chen et al. (2004) constructed a range of plots comprising 0, 1, two, 4, 8, and 12 species of weeds from a range of institute families including Leguminosae, Ranunculaceae, and Gramineae. The number of AM fungal spores extracted increased progressively with each increment of plant diversity. A similar situation was also observed at the famous Cedar Creek biodiversity experiment utilizing a similar plant diversity gradient (1, ii, 8, and 16 species; Burrows and Pfleger, 2002). Here, AM fungi in the 16-species plots produced from xxx–150% more spores and from 40–seventy% greater spore volumes than AM fungi in 1-species plots, although it should exist noted that soil nitrate concentration was too a significant predictor of AM sporulation in parallel with plant species richness.

Testify from signature fatty acids and spore abundance certainly suggests that establish species richness does impact on AM fungi. To exist really useful, however, the identity of the AM fungi concerned is essential. Practise some species of AM fungi respond more readily to plant richness than others? Does constitute richness really stimulate AM fungal species richness (or vice versa), or is the relationship with AM fungal sporulation? This latter question is of considerable importance given the wealth of show that has accumulated showing how diverseness of AM fungi can feedback positively and stimulate host plant richness, productivity, and nutrient concentrations ( van der Heijden et al., 1998a , b ). The ability to identify individual species of AM fungi accurately is, therefore, of utmost importance.

Traditionally, this has been achieved by the painstaking analysis of AM fungal spore communities. Recently, Landis et al. (2004) applied this approach to try and unravel the linkages between plant diversity and AM fungal diversity. The authors fabricated use of a natural gradient in institute species richness through an oak woodland and found strong positive correlations between AM fungal, and plant, species richness. Notwithstanding, equally with all correlative studies, the authors correctly point out that it is impossible to decide cause and outcome when using such natural gradients. Of grade, the reliability of using the morphological features of spores for the determination of AM species identity is highly contentious (Sanders, 2004). The very fact that merely about 150 AM fungal species accept thus far been identified and yet they colonize tens of thousands of plant species and show host specificity, suggests that spore morphology is a poor discriminator. Revisiting the experimental gradient used by Landis et al. (2004) with the advisable molecular tools would be an obvious next step. Nevertheless, despite its limitations, the study by Landis et al. (2004) appears to contradict the views of Allen et al. (1995) who state that 'the diversity of mycorrhizal fungi does non follow patterns of plant diversity'.

Of import insights into AM species identification have recently been attained using molecular methods targeting the pocket-size sub-unit (SSU) of rRNA. The development of primers to identify AM fungi colonizing establish roots ( Helgason et al., 1998; Vandenkoornhuyse et al., 2002) has made it possible to report the diversity of AM fungi with a greater caste of certainty and precision than was hitherto possible. These studies ( Helgason et al., 1998, 1999; Daniell et al., 2001; Hubby et al., 2002a ) are revealing a greater diversity of AM fungi in roots than could be recognized by morphometric studies. They have also resulted in the discovery of previously unrecorded species ( Helgason et al., 1998, 1999; Daniell et al., 2001; Husband et al., 2002b ; Vandenkoornhuyse et al., 2002, 2003) and show of hitherto hidden host specificity ( Bidartondo et al., 2002; Vandenkoornhuyse et al., 2002, 2003). Furthermore, such studies take revealed that both the limerick of AM fungal communities in roots ( Helgason et al., 2002; Vandenkoornhuyse et al., 2002, 2003; Johnson et al., 2004) and the product of their spores (Bever, 2002) tin be influenced past host plants. One important factor is that, as with whatever PCR-based analysis, the results are prone to a degree of bias. For example, it is virtually impossible at nowadays to quantify the abundance of an AM fungal genotype based on T-RFLP, and and so information technology is non possible to determine fifty-fifty the extent of root colonization. On its own, variety has niggling value and then complementary data on abundance, biomass, and function should exist obtained where possible.

Boreal forests

Approximately 6000 species of EcM fungi have been described, considerably more than AM fungi. This has led to the assumption that EcM fungi accept greater host specificity than AM fungi ( Allen et al., 1995). Most EcM fungi are basidiomycetes and many produce distinctive sporocarps although peradventure 5% are ascomycetes, the identification of which is more challenging. The influence of plant species richness on ectomycorrhizal fungi is rather less clear than for AM fungi, primarily because the required experiments have however to be done (which is not surprising given the obvious difficulties involved), but also because the multifariousness of EcM fungi contrasts markedly with their host constitute communities. It seems unusual, therefore, that a full general consensus seems to have appeared that suggests EcM richness is non in any fashion related to found richness ( Allen et al., 1995). These views may have arisen past taking a viewpoint at the calibration of the biome, at which it is difficult to see detailed patterns emerging confronting other variables that undoubtedly affect mycorrhizal customs structure. Certainly, in that location is information detailing how both edaphic and climatic variables affect EcM diversity. Despite this consensus, there is, withal, some bear witness to suggest that establish species richness may be important. For example, just manipulating a establish community to contain either a monoculture or a two-species mixture can accept consequences for EcM customs construction. Jones et al. (1997) found that EcM communities had greater species evenness (relative abundance) when paper birch and Douglas fir were grown in combination compared to when they were grown in monoculture, although this finding did not concur for EcM richness or diversity.

Contempo work at a larger scale (upwards to 100 m⁻²) in boreal forests suggests that positive relationships between plant and EcM diversity can be ( Kernaghan et al., 2003; Ferris et al., 2000). Kernaghan et al. (2003) morphotyped fine root tips and demonstrated that around 38% of the variation in EcM diversity was attributable to overstorey institute diversity. The fact that only overstorey plant diverseness was a meaning factor from the range of abiotic and biotic factors tested is not only interesting but also very surprising for two reasons. Firstly, the influence of the species richness and biomass of understorey vegetation seems not to have been meaning for the richness of the community as a whole. I might expect competition for resources from fungi associated with non-ectomycorrhizal plant species to exist an important commuter of ectomycorrhizal diversity. Secondly, no relationship was seen between the diverseness of plant species determined from root samples. The implication of this is that the diversity of ectomycorrhizal communities are being driven, at least to some extent, by indirect process such as litter chemistry, rather than directly processes such as carbon allotment. Indeed, it must be seen every bit a priority to understand the mechanisms through which plant diversity regulates ectomycorrhizal fungal diversity (or vice versa). The hypothesis postulated higher up can readily be tested through the application of methods that link the office and diversity of micro-organisms, notably SIP. The application to forest systems where the soil microbial biomass is dominated by EcM fungi would seem an obvious and relatively straightforward extension of the engineering, peculiarly given the likelihood that more carbon is allocated to EcM fungi than AM fungi. Given that the molecular tools are now condign increasingly bachelor for soil fungi (Anderson and Cairney, 2004), the plea made past Allen et al. (1995), to explore the interactions betwixt plant and mycorrhizal diverseness through rigorous experimental design, tin can only exist reiterated.

Influence of host found age

The structure and function of mycorrhizal communities in response to the historic period of host plants is of principal concern in forestry, peculiarly where it is intended for timber production. At that place are undoubtedly many other instances where stand age is influenced naturally, primarily by fire, and an understanding of the factors that drive changes in EcM community limerick and function is of great ecological interest. Notwithstanding, some of the all-time testify of the importance of host plant age on mycorrhizal communities has arisen from studies of AM fungi in tropical forests, and then this give-and-take will include some of these findings.

The notion that the historic period of a stand up of trees could influence the customs structure of EcM was first postulated in the early 1980s ( Stonemason et al., 1982, 1983). The concept generated considerable involvement because Mason and colleagues suggested that some species of EcM fungi were found but when trees were in their pioneer phase, and so-called early-stage fungi and others were specific to climax vegetation, and so-chosen late-stage fungi. This was after refined by Danielson (1984) to include a third category known as multi-stage fungi. Numerous studies have utilized forests with a slope of stand up ages to examination Bricklayer's hypothesis with varying degrees of agreement. As a full general betoken, it should exist noted that several of these studies are compromised by lack of true replication. This essential requirement for meaningful statistical analysis is not always easy to achieve in chronosequences. Many other factors are oftentimes correlated with stand age and conscientious experimental design, field observation, and statistical analyses are required to try and dissever the various factors tested.

Critics of the EcM succession hypothesis have argued that the hypothesis is only likely to concur true for pioneer plant species because the original study utilized a stand of birch (Betula pendula) that had recently colonized agricultural soil. Molina et al. (1992) go on to suggest such a simple model is wholly inadequate to stand for the situation in natural woods systems because of the vast array of abiotic and biotic factors that interact to influence EcM succession. This is borne-out by some studies that accept shown Douglas fir to be routinely colonized past the genus-specific fungi Rhizopogon whether they are seedlings or mature trees (Borchers and Perry, 1990). Using molecular techniques, Jonsson et al. (1999) provided evidence that the EcM fungal communities on Scots pine root tips were like to neighbouring trees, regardless of their age, suggesting that the external fungal mycelium may be important as a determinant of EcM community structure on seedlings. This mechanism, which has been suggested for AM fungi in grassland, is not surprising, given the extent of hyphal networks in forests. The ability of a seedling to tap into an established mycelial network supported past carbon from mature overstorey plants would seem to be of considerable reward, particularly in the comparative gloom of the forest floor. This mechanism supports the view that early-stage fungi are only likely to dominate when spores are the principal source of inoculum.

Visser (1995) studied a chronosequence of Jack pino (Pinus banksiana) that had regenerated naturally after wildfire disturbance. Her data showed that the number of EcM morphotypes increased progressively in the starting time 65 years before increasing at a much-reduced charge per unit until 122 years. The communities included early-stage species such every bit Coltricia perennis, multi-stage species such every bit Suillus brevipes, and late-stage species such as Cortinarius spp. The view of the EcM customs did non show the predicted pass up in species richness post-obit canopy closure ( Final et al., 1987). A similar trend was seen in stands of Pinus kesiya during the initial (2–17 year) growth phase ( Rao et al., 1997). Here, species richness of EcM fungi was direct proportional to the age of the stand. All of the studies cited above, withal, are discipline either to the vagaries of the relationship between sporocarp presence and mycorrhiza presence or from the uncertainties of EcM morphotype identification. Past dissimilarity, Husband et al. (2002b) used molecular methods to study AM fungal communities in the roots of seedlings of 2 co-occurring species from tropical rain woods on Barro Colorado Island. AM types that were dominant in newly germinated seedlings were replaced by other types in those seedlings which survived to the following twelvemonth, and seedlings of different ages sampled at the aforementioned time signal supported significantly different AM communities. Lee and Alexander (1996) obtained similar data for the EcM fungi in tropical pelting forest to demonstrate that EcM community on the roots of dipterocarp seedlings changed in the 7 months following germination. Equally well equally fungi inbound the mycorrhizal community, as fourth dimension progressed some fungi were lost or declined in relative abundance, clear evidence of succession. As far as is known, there accept yet to exist any studies using Dna-based methods to investigate the successional processes in EcM communities, despite the recognition 10 years agone that this would exist a useful line of inquiry (Egger, 1995). Some preliminary data are presented hither on species richness (represented by the number of dominant bands on DGGE gels) of fungi colonizing EcM roots of Scots pine in five stands ranging from 13 to 116 years one-time (Fig. 2). Species richness in the surface organic (Fifty, F, and H) horizons was to the lowest degree in the youngest stand (13 years) and was greatest in the 59 and 116 yr-quondam-stands. By contrast, species richness in the mineral horizon (in this case a compatible sand horizon several metres deep) did non differ betwixt stand ages. These data advise a rather idiosyncratic response of root-associated basidiomycete fungal communities to host constitute age.

Fig. 2.

Upshot of stand age on the species richness (dominant bands on DGGE gels) of basidiomycetes colonizing Scots pine (Pinus sylvestris) roots extracted from organic (articulate bars) and mineral soil (shaded bars) horizons at Culbin forest National Nature Reserve, Morayshire, Scotland. For each age class, roots were extracted from four cores removed from each of three spatially separated stands. Nucleic acids were extracted according to Griffiths et al. (2000) and bulked to give three replicate pools of Deoxyribonucleic acid per stand age. The internal transacribed spacer (ITS) regions of soil basidiomycetes were amplified past nested PCR using the primer pairs ITS1f/ITS4b and ITS1f-GC/iTS2 and bands were subsequently separated by DGGE ( Anderson et al., 2003). Dominant band richness was analysed using Gelcompar saftware with 20% profiling.

It is clear that tree historic period can accept impacts on EcM fungal communities, merely that these may be more or less apparent in particular woods types, notably immature plantations versus erstwhile growth. However at present that so much painstaking observational work has been achieved, perhaps a far more important question to ask is what is actually causing the EcM communities to change in plants of different age and what are the consequences of these changes for diverse aspects of functioning? There seem to be ii processes occurring; changes in mycorrhizal communities on individuals with time every bit that private samples the inoculum available, and also changes at the stand level associated with a range of edaphic factors. Several authors have alluded to the latter signal. Visser (1995) highlighted that differences in host carbon supply could have driven the changes seen in the EcM fungal communities. This hypothesis arises from the notion that carbohydrate supply can impact EcM colonization (Björkman, 1949). The isotope tracer techniques required to determine if EcM community limerick is related to host carbon supply are readily available and have been highlighted already.

Conclusions

The emerging view seems to be that host plant diversity, species composition, and age do take a office to play in regulating mycorrhizal communities. This view contrasts with previous models and assumptions and has, in the chief, been possible only past the rapid development of molecular biology techniques and their application to mycorrhizal fungi. However, there is still some manner to go in providing a business firm experimental basis, rather than relying on correlative approaches, in order to address some of these relationships, particularly with respect to plant species diversity. The actually fascinating questions about what these differences mean for various aspects of ecosystem functioning, nigh notably transfers of carbon, have the potential to be addressed in forthcoming months and years as a result of the integration of isotope tracers and DNA-based methods of assessing species and genotypic identity. Another futurity key line of enquiry, peculiarly for EcM fungi, relates to the importance of host establish genotypic variation, the general molecular typing methods discussed for fungi being applicable to plant communities. Variations in the formation of EcM fungi on host found progenies have been known for some time (Marx and Bryan, 1971). Some studies besides suggest that plant secondary metabolites tin can bear upon EcM colonization, and that production of these compounds is at least in part under plant genetic command.

Nosotros give thanks Pamela Parkin for skilled technical assistance and the Forestry Committee for access onto their land. DJ is currently supported by an NERC Advanced Fellowship and ICA receives funding from the Scottish Executive, Environment and Rural Diplomacy Department (SEERAD).

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In Addition to Abiotic Factors, Community Composition of Plants Can Be Severely Compromised by

How exercise plants regulate the function, community structure, and diversity of mycorrhizal fungi?

Abstract

Introduction

Species composition of plant communities

Diversity of plant communities

Grassland

Boreal forests

Influence of host found age

Conclusions

References

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