MYCORRHIZAE AND REVEGETATION OF DISTURBED LANDS
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Introduction
Many plant ecologists and restoration practitioners have realized that soil
microorganisms can have a significant impact on how vegetation communities establish
and develop. Much of this attention has been focused on mycorrhizae, an important
symbiotic relationship between plant roots and specialized fungi. This document is
intended to provide basic biological information about mycorrhizae, explain ways that
mycorrhizae may enhance revegetation projects, and provided some advice and resources
to guide decisions about incorporating mycorrhizae into your revegetation strategies.
What are mycorrhizae?
Mycorrhizae are fungal-plant root symbioses that occur in members of more than
90% of vascular plant families. In general, the fungus benefits from photosynthate (fixed
carbon) supplied by the plant, while the plant benefits from improved nutrient uptake and
other mechanisms described below. There are seven types of mycorrhizae, which can
loosely be classified as endomycorrhizae (which include arbuscular mycorrhizae, and
endosymbionts of the Orchidaceae, Epacidaceae and Ericaceae), and ectomycorrhizae.
The most common type of
endomycorrhizae are the arbuscular
mycorrhizae (AM, also known as VAM),
which involve the fungus penetrating the plant
cells and forming structures (vesicles and
arbuscules) within plant cortical cells (Figure
1). AM are generally associated with
herbaceous plants and a few shrubs and trees.
Ectomycorrhizae have fungal hyphae
intertwined with plant cells (Hartig net) which
extend outside the root, forming a sheath
around the root and extending outward into the
soil matrix (Figure 2). Ectomycorrhizae are
generally associated with trees.
This paper focuses on AM and
ectomycorrhizae, but it is important to note that
there are other specific types, such as
specialists that associate with ericacious plants,
and those that associate with only orchids. An
important component of all mycorrhizal
associations is a dense network of fungal
hyphae extending outside of the plant, which
makes it possible for the mycorrhizae to
harvest resources from a greater soil area.
Figure 1. Arbuscular mycorrhizae
structures inside a plant root.
Figure 2. Ectomycorrhizal sheath
How do mycorrhizae form?
Mycorrhizal formation occurs through a series of biochemical, morphogenetic,
and electrophysiological changes in both the plant root and the fungal partners. Infective
fungal propagules can be in the form of spores, hyphae or hyphal fragments, or colonized
root fragments. If living fungal propagules are not present at a disturbed site, they must
be brought in by animals, natural elements, or restoration efforts.
How are mycorrhizae important to plants in ecological systems?
Mycorrhizae can provide a
crucial link between plants and the
surrounding soil environment, which
Potential Plant Benefits from Mycorrhizae
leads to many direct and indirect
benefits to plant communities. Plant
 Improved P uptake
ecology literature describes many
 Improved N uptake
important benefits of mycorrhizae,
 Improved water status
which are highlighted in the inset
 Improved toxic metal resistance
textbox. These benefits, the best
 Root pathogen protection
recognized of which is enhance
 Production of plant growth regulators
phosphorus uptake, is primarily due to
 Improved soil structure
the ability of fungi to penetrate and
harvest resources from smaller pore
spaces and greater soil volume.
In addition to individual plant effects, recent plant ecology literature shows that
mycorrhizae can affect plant communities by increasing diversity and altering plant
community structure. This may have important implications for revegetation because it
suggests that how the seeded vegetation develops may depend on the mycorrhizal fungi
that are present in the soil.
Can the effects of mycorrhizae vary in different situations?
The benefits from mycorrhizae can vary greatly and will depend on several
factors. Mycorrhizal effects can depend on both the plant and fungal species. Some
plant species require mycorrhizae to survive (obligate hosts), while others can benefit
from mycorrhizae but do not require them (facultative hosts), and still others do not form
mycorrhizae at all (non-host plants).
Environmental conditions can greatly affect the benefits of mycorrhizae. For
example, when phosphorus is abundant in the soil, the fungus is less-beneficial to the
plant and the mycorrhizae may be inhibited or may be a drain to the plant. Mycorrhizae
can also be inhibited or less-beneficial in low light conditions, extreme soil
environmental conditions, and in the presence of toxins, such as heavy metals. When
evaluating the potential benefits of mycorrhizal inoculation, it is important to remember
that mycorrhizae will function according to their environmental context.
Why are mycorrhizae important in revegetation?
Mycorrhizal propagules and infectivity can be substantially reduced in metalcontaminated/disturbed systems, sometimes as much as 100%. AM abundance and
diversity can also be reduced in metal-contaminated systems. Mycorrhizal recovery in
disturbed systems can be enhanced with soil amendments, irrigation, inoculation, preinoculated plants, and with varying lengths of recovery time.
There are two primary reasons that you should consider including mycorrhizae in
your revegetation strategies. First, mycorrhizae are an important component of natural
soil ecosystems that interact with soil, microbes, soil fauna, and plants, and should be a
part of complete reconstructed soil environments. Second, because of numerous potential
benefits to the plant, mycorrhizae could substantially improve revegetation success.
Should I inoculate my revegetation project?
Whether inoculation is appropriate as part of restoration should be evaluated for
each new case. Planners should examine whether revegetation species are mycorrhizal
hosts, and which type of mycorrhizal fungi they associate with (ex. ectomycorrhizae or
endomycorrhizae). The soil should be examined to be certain it is not toxic and will be
conducive to plant and mycorrhizal establishment.
It is important to determine whether mycorrhizae are already present at the site.
This can be achieved by sending soil to a lab for a mycorrhizae infectivity potential
assay, which uses plants growing in soil samples to determine whether fungal propagules
are present and infective.
What are my options for inoculation sources?
The mycorrhizal inoculum source that will be most appropriate for revegetation
will depend on the goals of the restoration project. In some cases, land managers are
charged with strict genetic guidelines, so a native inoculum will be most appropriate. In
other cases restorationists may want to buy pure cultures that tend to exhibit specific
characteristics, such as those that produce large plant growth effects. The most common
sources for mycorrhizal inoculum are listed beow.
Native Topsoil
Topsoil is commonly used as a very good source of native mycorrhizal inoculum.
Advantages of using topsoil are that the fungal species are likely to be adapted to
environmental conditions of the area. Inoculum costs may be lower because the
inoculum is collected rather than purchased. Disadvantages include problems associated
with disturbing the topsoil collection site and logistics of transporting soil. It is important
to realize that other soil biota are added with topsoil, which can be beneficial as well.
Custom-produced native inoculum
Site-specific inoculum can be cultivated by mass-producing mycorrhizal plant
species in the greenhouse with a small amount of native soil inoculum. This method is
more labor-intensive and expensive, but can be done in a way to protect against
pathogens and causes less disturbance to the inoculum collection site. There are several
nurseries specializing in mycorrhizal plants that can provide this service.
Colonized Transplants
Some nurseries can produce transplants that are inoculated with mycorrhizae in
the greenhouse. When these plants are planted, they introduce mycorrhizae to the site.
This is a way to use a small amount of native inoculum to recolonize the disturbed site.
It is very important to note that many normal greenhouse practices are not
conducive to mycorrhizal growth. Nursery high fertilization practices, watering regimes,
low light levels, and soil sterilization can inhibit mycorrhizae. There is a great deal of
literature on this subject and it is important to contract plant production work with
knowledgeable and experienced nurseries.
Commercial Inoculum
Commercial inoculum has become increasingly available for restoration
applications. Some of these are likely to be very valuable to soil restoration, while others
may not even contain the organisms they claim. Commercial inocula come in many
varieties, and it is important to gather as much information as possible before selecting a
product.
How can I evaluate and compare commercial inoculum products?
It can be difficult to sort through marketing materials promoting commercial
inoculum products. The questions below may provide some guidance for evaluating and
comparing mycorrhizal inoculum products.
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What types of fungal propagules are included in the product?
In general, fungal and root fragments may be more infective, but spores may be
more resistant to harsh environmental conditions. Some products contain a
combination of propagule types, which may provide both benefits.
Is the propagule count and infectivity of the product independently tested?
Are those qualities garanteed?
Because commercial inoculum products are not regulated for quality assurance in
the United States, it is important to verify your product quality with your own
independent analysis if possible.
What is the cost per acre? ...per propagule?
Products may vary in recommended application rates and propagule density,
which should be considered when comparing products for cost and effectiveness.
Does the product contain other organisms or active abiotic ingredients?
Be sure you know what the product contains, so you can evaluate whether effects
are due to the mycorrhizae or other ingredients, such as fertilizers or nitrogenfixing bacteria.
How should it be applied?
Application methods may determine feasibility and/or additional costs for your
project.
Has it been tried in sites/projects similar to yours?
Advice from experienced restoration practitioners may provide valuable
information and guidance for your project.
Can they provide convincing data with proper design and controls???
Review experimental data using the product carefully. Be sure that you are able
to determine whether plant growth effects are due to mycorrhizae or other factors
that may not be controlled in the experiment.
Final Thoughts:
Mycorrhizae are not a magic bullet for restoration, but rather they are common
associations that most plants employ to support their growth in natural systems.
Mycorrhizal survival and function will depend on plant and fungal species in the context
of their environment. Including mycorrhizal associations as part of careful revegetation
efforts can lead to more complete and more stable restoration projects.
Valuable Web Sites for more information:
 http://mycorrhiza.ag.utk.edu – great general mycorrhizae information exchange
 http://invam.caf.wvu.edu/invam.htm – especially good for technical and
taxonomy information, as well as information about commercial inocula
 http://www.ifas.ufl.edu/~dmsa/refman.htm – searchable database of
mycorrhiza literature and information about commercial inocula
 http://www.mycorrhiza.org – a good place to start with simple descriptions,
good links, and excellent documents to download
 http://www.ffp.csiro.au/research/mycorrhiza/ – a great resource for
information, methods, photos and definitions