Can Mycorrhizal Fungi Influence Plant Diversity and Production in an Ecosystem?

Abstract

The functioning and stability of terrestrial ecosystem are determined by plant biodiversity and species composition. So far, little attention has been paid to the effects of microbe-plant interactions, particularly the mycorrhizal symbiosis, on ecosystem variability, productivity and plant biodiversity. The most common type of mycorrhizal association is the arbuscular mycorrhizal (AM) type. The role of AM fungi in the uptake of diffusion limited nutrients like P and in conferring resistance to soil-borne-diseases is well documented.
One of the major components determining the success of early colonizing plants during plant seral succession is the availability of nutrients. In this context the ability of plants to associate with mycorrhizal fungi and enhance their ability to sequester nutrients from a limited resource is of benefit to the success of the plant species in the community. Tree species of the matured forest canopy and sub-canopy tend to be obligately mycotrophic or non-mycorrhizal. Mineral nutrient availability, especially of P, and the type of mycorrhizal fungi, in different habitats, are probably the primary selective factors which produced different degree of dependence on mycorrhizae.
There are two hypotheses to explain the examples of successional changes in AM fungal species in an ecosystem. One of the hypothesis suggests that the mycorrhizal fungi are the “drivers” to determine the plant species; the second suggests that the changes in mycorrhizal species which are “passengers” are dependent on the plant and environmental conditions. The role of AM fungal diversity in contributing to the maintenance of plant bio-diversity and to the ecosystem function has been studied recently. The results showed that both plant bio-diversity and ecosystem productivity increases with the increasing number of AM fungi. Thus, it appears that the diversity of AM fungi in soil is a major factor contributing to the maintenance of plant bio-diversity and the ecosystem functioning.

Full text

1
Can Mycorrhizal Fungi Influence Plant
Diversity and Production in an
Ecosystem?
Bagyaraj, D.J. and Ashwin, R.
Abstract
The functioning and stability of terrestrial ecosystem are determined by
plant biodiversity and species composition. So far, little attention has been
paid to the effects of microbe-plant interactions, particularly the
mycorrhizal symbiosis, on ecosystem variability, productivity and plant
biodiversity. The most common type of mycorrhizal association is the
arbuscular mycorrhizal (AM) type. The role of AM fungi in the uptake of
diffusion limited nutrients like P and in conferring resistance to soil-borne-
diseases is well documented.
One of the major components determining the success of early colonizing
plants during plant seral succession is the availability of nutrients. In this
context the ability of plants to associate with mycorrhizal fungi and
enhance their ability to sequester nutrients from a limited resource is of
benefit to the success of the plant species in the community. Tree species
of the matured forest canopy and sub-canopy tend to be obligately
mycotrophic or non-mycorrhizal. Mineral nutrient availability, especially
of P, and the type of mycorrhizal fungi, in different habitats, are probably
the primary selective factors which produced different degree of
dependence on mycorrhizae.
There are two hypotheses to explain the examples of successional changes
in AM fungal species in an ecosystem. One of the hypothesis suggests
that the mycorrhizal fungi are the “drivers” to determine the plant species;
the second suggests that the changes in mycorrhizal species which are
“passengers” are dependent on the plant and environmental conditions.
The role of AM fungal diversity in contributing to the maintenance of
plant bio-diversity and to the ecosystem function has been studied recently.
Microbes for Restoration of Degraded Ecosystems, pp 1-17
© 2017, New India Publishing Agency, New Delhi, India
Editors: D.J. Bagyaraj and Jamaluddin

2 Microbes for Restoration of Degraded Ecosystems
The results showed that both plant bio-diversity and ecosystem productivity
increases with the increasing number of AM fungi. Thus, it appears that
the diversity of AM fungi in soil is a major factor contributing to the
maintenance of plant bio-diversity and the ecosystem functioning.
Keywords: Ecosystem, Mycorrhizal Fungi, Plant Community Composition, Plant
Diversity, Plant succession
1. Introduction
The functioning and stability of terrestrial ecosystem are determined by plant
biodiversity and species composition (Schulze and Mooney 1993, Huang 2004).
The mechanisms that control plant biodiversity are still being debated. The
ability of many plant species to co-exist, and thus to determine plant biodiversity,
can be explained by competitive interactions (Grace and Tilman 1990, van der
Heijden et al. 1998b), by spatial or temporal resource partitioning (Tilman 1982,
Kneitel and Chase 2004), by disturbance creating new patches for colonization
(Huston 1979, Kneitel and Chase 2004)), and by interactions among different
functional groups of organisms that constitute ecosystems (Brown and Gange
1989; van der Heijden et al. 1998b). So far, little attention has been paid to the
effects of microbe-plant interactions, particularly the mycorrhizal symbiosis, on
ecosystem variability, productivity and plant bio-diversity. The word ‘Mycorrhiza”
literally mean ‘fungus root’. It is a symbiotic association between certain fungi
and plant roots (Desai et al. 2016). Approximately 95% of all vascular plants
have a mycorrhizal association (Brundrett 1991).
Traditionally, mycorrhizal associations have been divided into a range of
categories, based on the taxonomy of the fungal associate and the physical
forms of interactions between the root and the fungus in the mycorrhizal
structures that are produced in the symbiosis. A list of mycorrhizal forms, their
plant associates, and the key features of the mycorrhizae is given in Table. 1.
Among the most common types of mycorrhizal association are the AM types,
which are formed mainly by the fungi belonging to the phylum Glomeromycota
(Sturmer 2012). These fungi are mainly associated with herbaceous vegetation,
grasses, and tropical trees although a limited number of temperate woody plants
may also associate with arbuscular mycorrhizae. The association is characterized
by fungal penetration within the host cortical cells and the development of a
variously developed, tree like branching of the hyphae between the host cell
wall and plasmolemma called an arbuscule. It is here that the surface area of
the interface between plant host and fungus is optimized for nutrient and
carbohydrate exchange. In most of the arbuscular mycorrhizal fungi AMF,
vesicles are formed in cortical cells. These consist of a swollen hyphum

Can Mycorrhizal Fungi Influence Plant Diversity and Production 3
Table 1: Outline of some of the features of different types of mycorrhizal associations
Mycorrhizal type Host plant group Characteristics Fungal associate
Arbuscular mycorrhizae Herbaceous plants, grasses; Formation of arbuscules within cortical cells of Glomeromycetes
some trees host root
Ectomycorrhizae Coniferous and deciduous trees Formation of a sheath or mantle of fungal tissue Basidiomycetes
around the root surface and a Hartig net of fungal Ascomycetes
penetration between the cortical cells to the Deuteromycetes
endodermis.
Ericoid mycorrhizae Ericaceae Hyphal coils within the host root cortical cells Ascomycetes
Arbutoid mycorrhizae Arbutus Hyphal coils within the host root cortical cells Basidiomycetes
Ascomycetes
Orchidaceous mycorrhizae Orchids Fungal propagule carried in the seed of the plant. Deuteromycetes
Basidiomycetes

4 Microbes for Restoration of Degraded Ecosystems
occupying a large volume of the cell. This structure contains storage material.
The AM association is formed with a large number of plant species and a
relative small diversity of fungal species. Though there are 26 genera of
Glomeromycetes fungi, the predominant genus forming arbuscular AM all over
the world is Glomus.
The ectomycorrhizal habit consists of an association between, mainly, tree
species and a range of fungal taxa consisting of basidiomycetes, ascomycetes
and some zygomycetes. In this type, the fungus does not penetrate into the host
cortical cells, but only between them, forming a Hartig net. The Hartig net
exists outside the endodermis of the root. On the surface of the root, a sheath
or covering of fungal material develops. This surface structures may be of
varying degrees of complexity from a loose weft of hyphae to highly organized
pseudoparenchymatous structures. It is the structure of the sheath, degree of
branching, (induced by change in cytokinins), and nature of emanating hyphae
or hyphal strands that allow morphological identification of these mycorrhizae
(Agerer 1987 – 1999; Ingleby et al. 1990; Goodman et al. 1996-2000, Dighton
2016). Ectomycorrhizal associations are formed between a limited number of
plant species and a huge number of fungal species.
Ericoid mycorrhizae are similar in structure to arbuscular mycorrhizae, but are
associated solely with members of the Ericales (Ericaceae, Empetraceae,
Epicaridaceae, Diapensiaceae and Prionotocaceae). All of these groups are
sclerophyllous evergreen and reside in habitats where both nitrogen and
phosphorus are sparsely available. The root system of these plants consist of
very fine roots containing a single layer of cortical cells, which the mycorrhizal
fungi penetrate to form hyphal coils, rather than arbuscules (Read 1996, Dighton
2016). The fungi associated with this type of symbiosis are Hymenoscyphus
and Oidiodendron. Closely associated with these mycorrhizae are the arbutoid
mycorrhizae.
Orchidaceous mycorrhizae are unique in terms of the obligate nature of the
association. The importance of the mycorrhizal association for seed germination
and the initial establishment of the plant has been reviewed by Zettler and Mclnnes
(1992) and Rasmussen and Wigham (1994). The fungal partner is usually ascribed
to the genus Rhizoctonia, and there has been such evolution of the obligateness
of the association that the fungus is transported in the seed of the plant.
2. The Basic Functions of Mycorrhizae
Mycorrhizal association can alter morphology, wherein root hair development is
suppressed and the function of the root hair is replaced by fungal hyphae.
These hyphae have 2 major benefits for sequestring nutrients. They are of

Can Mycorrhizal Fungi Influence Plant Diversity and Production 5
smaller diameter than root hairs and can penetrate more easily and to a greater
distance from the root into the soil, thus exploring a greater volume of soil and
presenting a greater surface area for nutrient absorption than could the root-
root hair system alone (Marschner and Dell 1994, Dighton 2016). The second
benefit is that it is energetically more efficient to produce a long, thin hyphum
than a root hair. Further in AM type number of lateral roots produced is greater
than non-mycorrhizal plant, suggesting dual benefits of mycorrhizal habit, one
of immense root branching and the other of the fungal exploitation of soil for
nutrients. Using radiotracer phosphate, it was shown that the phosphate depletion
zone was greater (up to 110 mm) in mycorrihzal plant compared to non-
mycorrhizal plant (10-20 mm), indicating that AM hyphe exploit a larger volume
of soil than root hairs (Fig. 1), (Jacobson 1992).
Thus the role of AM fungi in the uptake of diffusion limited nutrients like P, Zn,
Cu etc. is well documented (Bagyaraj et al. 2015). In addition to their role in
conferring resistance to soil-borne diseases, alleviating the effects of salt and
water stress, synergistic interaction with beneficial soil organisms, enhancing
the abundance of soil aggregation and in improving plant growth, leading to the
“big plant – little plant” syndrome is well documented (Jumpponen et al, 1998).
Further, the derived benefit of a mycorrhizal association is predominantly nutrient
acquisition if the host plant root system is poorly branched (Fig. 2) (Newsham
1995). In contrast, the benefit shifts toward pathogen prevention where the
host root system is highly branched.
The ecology of mycorrhizal fungi has been reviewed by several workers
(Bagyaraj 1990; 2014; Allen et al. 2003). The information on the role of
mycorrhizal fungi influencing plant diversity and production in natural ecosystem
remains scanty and disjointed.
Fig. 1: Increasing P depletion zone from a root surface created by the addition of root hairs
and AM hyphae as protrusions from the root surface in to the soil.
P depletion zone P depletion zone P depletion zone
Root
Hair Mycorhizal
Hyphae

6 Microbes for Restoration of Degraded Ecosystems
3. Dependence on Mycorrhiza
Plants can be grouped into three categories, based on their dependence on
mycorrhizae: (1) non-mycorrhizal; (2) facultative mycotrophs; and (3) obligate
mycotrophs (Bagyaraj 1989).
Non-mycorrhizal plants are those which grow normally without forming
mycorrhizae, even under infertile conditions. The growth of these plants is not
affected by mycorrhizal inoculation (Janos 1975). Species of the Aizoceae,
Amaranthaceae, Brassicaeae, Caryophyllaceae, Chenopodiaceae,
Commelinaceae, Cyperaceae, Fumariaceae, Juncaceae, Nyctaginaceae,
Phytolaccaceae, Polygonaceae, Portulaceae, and Urticaceae do not usually
form mycorrhizae (Gerdemann 1968, Bagyaraj 1989, Sharma 2014). Non-
mycorrhizal plants usually have well-developed root systems with highly branched
fine roots, as well as numerous root hairs. Some of them, like Banksia, Hakea,
Persoonia and Viminaria, produce proteoid roots (Lamont 1982; Janos 1983).
Non-mycorrhizal species may secrete organic acids, thereby utilizing the bound
forms of phosphorus (Graustein et al. 1977), or have tissues with low mineral
requirements, or have slow growth rates or durable tissues which compensate
for the limited nutrient supply. Non-mycorrhizal species reject mycorrhizal
association either because it is incompatible with their physiology, or in order to
favor their persistence in competition with mycotrophic species (Janos 1980b).
They are the only species which are the effective competitors on infertile soils
in the absence of mycorrhizal fungi.
Facultative mycotrophs are those plant species which are able to grow without
mycorrhizae, but in which mycorrhizal colonization improves growth. In fertile
soil, facultatively mycotrophic species reject mycorrhizae which are of no benefit
for mineral nutrient uptake (Bowen 1980). They also produce fine roots as well
Fig. 2: Nature and magnitude of the benefit (P nutrition) derived from plant associations with
arbuscular mycorrhizae, depending on the branching pattern of the host plant root system

Can Mycorrhizal Fungi Influence Plant Diversity and Production 7
as profuse root hairs (St. John 1980c). They may root more deeply, have higher
root / shoot ratios or lower total mineral requirements than obligate mycotrophs.
Some facultatively mycotrophic species are able to take up minerals from
relatively low substrate concentrations when uninfected, and may fail to support
much infection.
Obligate mycotrophs are those which can neither grow nor survive without
mycorrhizae. Janos (1980a) defined them as species which would not survive
to reproductive maturity at the fertility levels encountered in the natural habitats,
if not colonized by mycorrhizal fungi. Obligate mycotrophs do not respond to
fertilizer applications (Vozza and Hacskaylo 1971) suggesting therein obligate
dependence on mycorrhizae for nutrition. Obligately mycotrophic tree species
tend to have large seeds (Janos 1980b, Muthukumar and Udaiyan 2007) which
favour the persistence of uninfected seedlings and the formation of large pre-
infection root systems, maximizing the probability of infection. Obligate
mycotrophs have coarse hairless roots. They are often light-demanding, partly
because of the energy requirements of mycorrhizal association. Thus, under
shade mycorrhizae become a liability; but they are still retained because of
their importance to seedling growth when canopy gaps open. Tree species of
the mature forest canopy and sub canopy tend to be obligately mycotrophic,
while many pioneer and early-successional species are facultatively mycotrophic
or non-mycorrhizal (Janos 1980b). Mineral nutrient availability, especially of
phosphorus, and the type of mycorrhizal fungi in different habitats, are probably
the primary selective factors which produced different degrees of dependence
on mycorrhizae.
4. The Impact of Plant Community Composition on Mycorrhizal Fungi
In tropical rainforests AMF produce fewer spores (saving energy for sporulation)
and exist as hyphae spreading through hyphae connecting roots. The distribution
of mycorrhizal types is dependent upon the geographic distribution of the plant
species and the nature of the soil. Read (1991) showed the geographical distribution
of the main mycorrhizal types in the world, demonstrating that the AM habit was
dominant in the temperate and tropical grasslands, tropical forests, and desert
communities. Ectomycorrhizae were dominant in temperate and arctic forested
ecosystems, and ericoid mycorrhizae were most common in the boreal heathland
ecosystems. Mycorrhizal fungi are likely to be affected by plant-community
composition (Janos 1980a, Johnson et al. 2003).
Frequent or extensive disturbance can favour non-mycorrhizal species or
ectomycorrhizal species, which are effective colonizers. Non-mycorrhizal
species reduce mycorrhizal fungus populations indirectly by not sustaining
mycorrhizal infection. In addition, some non-mycorrhizal species can have a

8 Microbes for Restoration of Degraded Ecosystems
direct antagonistic chemical effect
on the fungi (Hayman et al. 1975,
Finlay 2008). The amounts of
mycorrhizal fungus in the soil will
thus be low where non-
mycorrhizal plants dominate the
communities, and in fertile soils
where facultative mycotrophs are
dominant. High populations of
mycorrhizal fungi are expected in
stands of facultative mycotrophs
on infertile soils or where obligate
mycotrophic plants are dominant.
In field crops there are reports to
show that in addition to the host,
soil also plays an important role in determining the quality and quantity of AM
population (Sreeramulu and Bagyaraj 1986). This analogy can be extended to
natural ecosystem, a fungal species forming AM mycorrhiza probably dominates
in a certain soil because of its ability to outgrow other species in that environment
(Dayala Doss and Bagyaraj 2001). The three way interaction between
mycorrhizal fungi; host plants and environment / soil conditions are illustrated
by three overlapping regions in Fig. 3.
The distribution of fungal species in a mixed community of AM plant species is
not homogenous. Eom et al. (2000) showed that the different species of plants
in a tall grass prairie ecosystem have differing AMF associates (Fig. 4). Johnson
et al. (1992) showed that the AM community differed among five plant species
in garden plots in a native grassland. This information lends credence to the
idea that there are feedbacks between the mycorrhizal fungal associate and the
Fig. 3: Mycorrhizal associations result from three
way interactions between mycorrhizal fungi, host
plants and environment / soil conditions.
Fig. 4: Cluster Analysis of the similarity of AMF species associated with 5 host plants in
a mixed species tall grass prairie ecosystem.
Environment /
soil conditions Mycorrhizal
fungus
Host plant
Mycorrhizal
association

Can Mycorrhizal Fungi Influence Plant Diversity and Production 9
plant that enable the plant species to dictate the fungal species assemblage and
vice versa.
5. Impact of Mycorrhizal Fungi on Plant Community Composition
The varied dependence of different plant species on different types of
mycorrhizae can influence the composition of both seral and mature plant
communities. Although there is a lack of host specificity among arbuscular
mycorrhizal fungi, host preferences have been recorded (Mosse 1977; Bagyaraj
2007). Thus, the species of mycorrhizal fungus present can influence the
competitive abilities of plant species. In the absence of mycorrhizal fungi, only
the species which do not need mycorrhizae will be able to grow. Therefore,
non-mycorrhizal species are most likely to dominate plant communities on poor
soils, with no or few mycorrhizal fungi. Even if mycorrhizae form slowly, the
growth lag of mycotrophic plants before infection can confer a competitive
advantage on non-mycorrhizal plants. On fertile soils, the facultative mycotrophs
can also occur in the absence of mycorrhizal fungi. Regardless of the availability
of mycorrhizal fungi, facultatively mycotrophic species are those most likely to
dominate plant communities on fertile soils.
Obligately mycotrophic species are most likely to dominate plant communities
on poor soils with abundant mycorrhizal propagules. They are probably better
competitors than facultative mycotrophs on infertile soils, because the
adaptations of facultative mycotrophs for mineral uptake, without mycorrhiza,
are redundant. For example, the hyphae can reach much farther beyond the
zone of phosphorus depletion than root hairs (Janos 1980a, Smith and Read
2008). Most of the plants dependent on mycorrhizae, in tropical rain forests,
do not have root hairs. Species obligately dependent on arbuscular mycorrhizae
probably have similar mineral-uptake characteristics within a habitat because
there are few fungal species available for this type of association compared
to the large number of host species (Janos 1975). Consequently, the great
species richness of tropical forests in which AM associations predominate is
unlikely to reflect niche differentiation with respect to mineral uptake. In
mixed plots of nine tropical tree species, including three which were non-
mycorrhizal, three facultative mycotrophs, and three which were strongly
mycorrhiza-dependent, the seedling survival of the last group was increased
by arbuscular mycorrhizae (Janos 1983). This experiment suggested that
arbuscular mycorrhizae reduce differences in competitive ability among the
species, and thereby contribute to the high within-habitat species diversity
characteristic of most tropical forests.

10 Microbes for Restoration of Degraded Ecosystems
Non-mycorrhizal species and some ectomycorrhizal species may have the
demographic characteristics of pioneer or fugitive species because non-
mycorrhizal plants can quickly establish without awaiting infection, and
ectomycorrhizal inoculum can build up very rapidly. Obligate mycotrophs
probably are the best competitors under mature forest conditions.
6. Mycorrhizae and Plant Fitness
The effects of mycorrhizae on the increase in reproductive potential of plants
has been noted by Koide et al. (1988). Stanley et al. (1993) and Barea et al
(2002) reported increased reproductive potential leading to improvement in
offspring vigor by increased seedling germination, leaf area, root: shoot ratio,
and root enzyme production. Heppel et al. (1998) showed that offspring of AM
infected Abutiolon theophrasti were significantly larger than offspring of non-
mycorrhizal parents (Table 2). This can be a significant factor in determining
plant species composition in the tropics.
Table 2: Plant fitness parameters of Abutiolon theophrasti offspring of mycorrhizal or non-
mycorrhizal parents.
Offspring age (days) Fitness parameters Mycorrhizal Non-mycorrhizal
parent parent
20 Shoot height (cm) 12.5 9.4
Shoot dry mass (g) 61.2 30.9
Leaf number 3.6 3.0
47 Shoot height (cm) 30.6 19.8
Shoot dry mass (g) 521 154
Leaf number 4.4 3.4
94 Survivors per box 59.1 26.6
Seeds per survivor 17.9 10.6
Source: Data from Heppel et al. (1998)
7. Mycorrhiza and Plant Succession
Pederson and Sylvia (1996) suggest that one of the major components
determining the success of early colonizing plants during plant seral succession
is the availability of nutrients. In this context the ability of plants to associate
with mycorrhizal fungi and enhance their ability to sequester nutrients from a
limited resource is of benefit to the success of the plant species in the community.
Indeed, it has been shown that the dispersal of spores of hypogeous fungi by
rodents and other animals like squirrels, mountain goat, mule deer is an important
determinant of mycorrhizal inoculum for plants in the early stages of succession
on bare ground. Small mammals defecate and are likely to deposit more spores
in areas of active feeding sites than in other localities. The patchy distribution

Can Mycorrhizal Fungi Influence Plant Diversity and Production 11
of mycorrhizal spores, and hence inoculum potential, would allow the
establishment of both mycorrhizal and non-mycorrhizal plant species in the
community.
Working with ectomycorrhiza Jumpponen et al. (2002) found that
ectomycorrhizal species diversely increased to a maximum where tree canopies
started to overlap. This observation at canopy closure may be related to both
the relative paucity of available nutrients (P) and an increasing proportion of
nutrients locked up in organic forms. Such studies with AM fungi are scanty.
Indeed, Hart et al. (2001) propose two hypothesis to explain the examples of
successional changes in AMFspecies. One of these hypotheses suggests that
the mycorrhizal fungi are the driving force (drivers); the second suggests that
the changes in mycorrhizal species are dependent on the plant and environmental
conditions and the mycorrhizae are considered “passengers” (Fig. 5).
Fig. 5: A model proposing two alternate mechanisms for changes in community structure of AMF
communities through time. The “passenger hypotheses” proposes that mycorrhizal communities
are determined by the plant community, whereas in the “driver hypotheses” the mycorrhizae
determine the plant species by interspecific differences in colonization and persistence potential
of the fungi. Source: Hart et al. (2001)
8. Can Mycorrhizal Fungi Determine Plant Biodiversity in an Ecosystem?
The role of AMF diversity in contributing to the maintenance of plant biodiversity
and to the ecosystem function has been studied by a few workers (van der
Heijden et al. 1998a). They did two experiments: in the first experiment, 4
different native AMF which were all isolated from soil of a calcarious grassland,
and of a combination of these 4 AMF on the species composition structure of
48 microcosms simulating European calcareous grasslands was compared. It
was found that 8 viz. Brachypodium pinnatum, Centaurium erythrea,
Passenger hypotheses Driver Hypotheses
DISTURBANCE
Some plants become established AMF with superior coloniz-
ing ability dominate
Only certain compatible AMF colonize plants Compatible plants establish
Plants modify environment AMF with superior persistence
replace AMF with superior
colonization ability
Other plants establish and persist Other plants become
established and persist

12 Microbes for Restoration of Degraded Ecosystems
Hieracium pilosella, Lotus corniculatus, Prunella grandiflora, Prunella
vulgaris, Sanguisorba officinalis and Trifolium pratense of the 11 plant
species were almost completely dependent on the presence of AMF to be
successful. Different plant species benefited to different extent from different
AMF suggesting host preferences by different AMF. Carex flacca, the only
plant species that does not have a symbiotic relationship with AMF, had the
highest biomass in non-mycorrhizal control treatment. Altering the AMF taxa in
the soil had no significant effect on the biomass of Bromus erectus, the dominant
plant species, in the ecosystem. From the results it was concluded that a
reduction in AM biodiversity from 4 to 1 leads to a decreasing biomass of
several plant species and hence it was proposed that both plant biodiversity and
ecosystem productivity will increase with increasing numbers of AMF, because
of added beneficial effect of each single AMF species.
In another field experiment, simulating North American old-field ecosystem in
70 macrocosms, 23 AMF species isolated from the site were inoculated
containing 1,2,4, 8 or 14 AMF species (van der Heijden et al. 1998a). Each
macrocosm [(1 x 0.75 x 0.25 m3) containing 90 kg of β-ray irradiated sand: soil
(1: 1 v/v)] was showered with a seed rain consisting of 100 seeds from each of
the 15 most abundant plant species of the research site. After one growing
season, plants were harvested and root and shoot biomass were determined.
Plant biodiversity was assessed using a Simpson’s diversity index on individual
species shoot – biomass data. Total shoot and root biomass, plant P, soil P and
hyphal length/ g soil were also measured (Fig. 6). The results brought out lowest
plant biodiversity and productivity occurred in plants without AMF or with only
a few AMF species. In contrast, plant biodiversity and productivity were highest
when 8 of 14 AMF species were present. Increasing AMF biodiversity lead to
increased hyphal foraging capacity and more efficient exploitation of soil P,
improved resource use and increased productivity and plant diversity. Increasing
plant biodiversity has been shown to lead to greater ecosystem productivity
(Stuwe et al, 1994).

Can Mycorrhizal Fungi Influence Plant Diversity and Production 13
Conclusion
In conclusion it can be said that the functioning and stability of terrestrial
ecosystems are determined by plant biodiversity and species composition
(Schulze and Mooney 1993; Hooper and Vitousek 1997, Isbell et al. 2011).
However, the ecological mechanisms by which plant biodiversity and species
composition are regulated and maintained are not well understood. These
mechanisms need to be identified to ensure successful management for
conservation and restoration of diverse natural ecosystems. It appears that the
below ground diversity of AMF is a major factor contributing to the maintenance
of plant biodiversity and to ecosystem functioning. The loss of AMF biodiversity,
which occurs in agricultural systems (Helgason et al. 1998; Johnson 1993,
Jeffries and Barea 2012) could, therefore, decrease both plant biodiversity and
ecosystem productivity while increasing ecosystem instability. The loss of
biodiversity in soils represents an under studied field of research which requires
more attention. The present reduction in biodiversity on Earth and its potential
threat to ecosystem stability and sustainability (Schulze and Mooney 1993; Tilman
Fig. 6: The effect of AM fungal (AMF) species richness on different parameters in macrocosms
simulating North American old field ecosystems. a = Simpson’s diversity index, b= Shoot biomass,
c= Total Plant P content, d = Length of external mycorrhizal hyphae in soil.

14 Microbes for Restoration of Degraded Ecosystems
et al. 1996, Berbara et al. 2013) can only be reversed or stopped if whole
ecosystems, including ecosystems components other than plants, are protected
and conserved.
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