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The Ecological Significance of Mycorrhizae in Plant Communities
Many fungi form underground arbuscular mycorrhizal (AM) relationships with larger
plants. These fungi facilitate nutrient absorption in the plant root, and the plant serves as
a carbohydrate source for the fungi in return (Merryweather & Fitter 1995; Douds et al.
1988). Perhaps two-thirds of all plants form relationships with AM fungi in their root
systems (Fitter & Moyersoen 1996). Some AM fungal relationships are obligatory, with
plants relying on fungi to provide sufficient levels of phosphorus (Merryweather & Fitter
1995), whereas other relationships are merely facilitory. Phosphorus absorption is not the
only benefit derived by the plant from the mycorrhizal relationship: certain AM fungi
protect plant roots from pathogens. One study showed that fecundity reduction from such
pathogens may be reduced up to 50% when mycorrhizae are present (Newsham et al.
1995).
Mycorrhizal relationships are complex, subtle, and affect aboveground
community structure in a variety of ways. Much research has been performed in the field
of AM fungal diversity and its effect on aboveground plant diversity, as well as
mycorrhizal roles in community successional responses to disturbance. This aboveground
influence will be the primary focus of the current discussion, but one hotly debated
subject regarding AM fungi must be explored to fully understand the ecological effects of
mycorrhizae, and that is host specificity.
Several researchers have found that AM fungi have a low specificity, that is,
fungi arbitrarily form mycorrhizal relationships with a wide variety of plants; plants
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likewise form arbitrary relationships with a variety of fungi (Smith & Read 1997, as cited
in Helgason et al. 2002). However, there is also evidence that although some AM fungi
lack specificity, other fungi form mycorrhizal relationships with only a few certain
species of plants (Helgason et al. 2002). The presence of mycorrhizae can be difficult to
observe (Bever et al. 2001), and so the specificity of a certain fungi may be troublesome
to pinpoint. More research into host specificity is warranted before generalizations can be
made. Regardless of how narrow a range of hosts fungi may colonize, it is clear that AM
fungal growth rates are linked to their host species. AM fungi may colonize many
different types of plants, but fungi produce more spores more rapidly when associated
with certain species of hosts (Bever et al. 2001; Castelli & Casper 2003). Evidence for or
against if this increased fungal growth is beneficial to the host plant is unclear. More
compelling is evidence that increased fungi diversity increases plant success and
community diversity.
van der Heijden et al. (1998a) hypothesized that if AM fungi facilitated superior
plant growth when paired with certain species of plants, then fungal diversity would
affect plant community diversity. The researcher’s logic behind this statement was that if
different AM fungi facilitate growth at different rates in a species of plant, the cumulative
benefit to the plant would be higher with more types of fungi present. Specifically, plants
would absorb more of the available phosphorus in the soil. More efficient use of
resources in the environment would lead to less competition between plants, leading to
higher diversity. This general trend of AM fungi diversity influencing community
diversity was shown to hold true in several different studies. The abundance of two coexistent dominant grassland plant species was influenced by the level of diversity in
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mycorrhizal root relationships (van der Heijden et al. 1998b). Additional study has also
shown mycorrhizal relationships to affect competition among pairs of plants (Kytöviita et
al. 2003). Further studies revealed that greater numbers (eight or more) of AM fungi
species led to higher plant biodiversity within controlled plots (van der Heijden et al.
1998a), and that a lack of AM fungi decreased species diversity (Gange et al. 1990).
Hartnett and Wilson (1999) observed aboveground species composition in a tallgrass
prairie to change after artificially suppressing AM fungal growth over several growing
seasons. This research indicates that although out of sight, AM fungi can highly influence
diversity in more obvious plant communities.
Mycorrhizal relationships are also important in maintaining endangered species
populations. Several endangered species from Florida form symbiotic mycorrhizal fungal
relationships. When fungi were removed from the environment, the endangered species
lacked the means to assimilate adequate amounts of phosphorus (Fisher & Jayachandran
2002). When the level of AM fungi associated with plant roots was experimentally
increased, the growth of the two endangered species also increased (Fisher &
Jayachandran 2002). Research on endangered Hawaiian species indicates that species
have greater biomass and leaf tissue when paired with AM fungi (Gemma et al. 2002).
Additionally, mycorrhizal dependence was found to range between 44% and 88% when
species were grown in phosphorus-sparse soil (Gemma et al. 2002). This suggests that
protecting endangered plant species may involve protecting AM fungi or even artificially
increasing fungi abundance.
Mycorrhizae may play an important role in the process of community succession.
Effects of this relationship, however, are unclear. Titus and del Moral (1998) found that
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AM fungi did not have a significant effect on plant biomass in primary succession. They
hypothesized that this was due to the notable lack of nutrients in the successional site.
Titus and del Moral (1998) also hypothesized that AM fungi are not of great importance
in primary succession. Other research (Gemma & Koske 1990) suggests that in primary
succession sites, species of plants that are obligately symbiotic may be slow to arrive due
to a lack of AM fungi. In one Hawaiian study (Gemma & Koske 1990), 57% of the
species sampled in an eight-year-old site were mycorrhizal, whereas 100% of the species
sampled in a 137-year-old site were mycorrhizal. In secondary succession, however,
mycorrhizae may be more influential. In one area in Western Pennsylvania, most
dominant plant species occurring shortly after a secondary disturbance are associated
with AM fungi (Medve 1984). Gange et al. (1993) experimentally demonstrated that AM
fungi presence affects seed germination in secondary succession sites. Gange et al. (1993)
also points towards phosphorus facilitation as means for increased plant growth (in this
case seedlings) due to mycorrhizae. Annual forbs with mycorrhizal relationships are
quick to dominate the successional site, the researchers state, because of the plants’
increased ability to absorb phosphorus from the soil (Gange et al. 1993).
In summary, it is apparent that subtle underground mycorrhizal relationships can
strongly impact aboveground plant life. Community diversity, plant fitness, preservation
of endangered species, and secondary succession are all influenced by the presence or
absence of AM fungi and mycorrhizal relationships. Mycorrhizae illustrate a common
theme in ecology: small, easily ignored organisms are critical for community success.
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