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Hidden Partners: Mycorrhizal Fungi and Plants

Mycorrhizal Fungi and the Future

For life as we know it to continue, it is essential that mycorrhizae adapt to the changing conditions presented to them as a result of human and global activities. Conditions and events to consider are global warming, deforestation and development, loss of host species, introduced and invasive species, and chemical pollution.

Since the beginning of the industrial revolution, the quality of Earth's air has steadily decreased. One of the major components of that decrease in quality is nitrogen, a contributing factor of smog that results from the burning of fossil fuels (for more information please go to When excess nitrogen enters the environment, it is stored in the ground, acidifying the soil where it damages plant and fungal tissues, and eventually leaches nutrients out of the soil, particularly potassium and calcium, thus degrading its fertility. Nitrogen deposition, as the process is known, may have a profound impact on mycorrhizal (particularly ectomycorrhizal) and plant ecology. Early research has shown that an increase in nitrogen deposition decreases the colonization rate of plant roots by ectomycorrhizae, and decreases the biodiversity of ectomycorrhizal species. With rates of colonization and species diversity decreasing, the plants which rely on ectomycorrhizae lose their evolutionary advantage over their neighbors, becoming less well nourished and more prone to drought stress. This could result in the degradation or loss of certain environments, such as oak forests, and the advancement of others, such as grasslands (which do not rely on ectomycorrhizae). However, as this research is still ongoing, much is still uncertain, such as whether the ability of mycorrhizal fungi to function is directly affected.

The general public does not often consider the possibility of fungi acting as invasive species and/or allowing invasive plant species to spread in a non-native area. From European colonization in the late 1500's and continuing today with massive ocean freighters and airplanes, humans have been the major vector for plant and fungal movements on a intercontinental scale. Ballast, agricultural and horticultural introductions, accidental releases, and contaminated shipping material have all contributed to the introduction of invasive species. Although these introductions can often have drastic and disastrous consequences, invasive mychorrhizal fungi often spread unnoticed, beneath the ground and out of sight.

Many cases of invasive introduced fungi involve ectomycorrhizae and forest plantations used for lumber and fuel. Introductions of ectomycorrhizae have been both deliberate and accidental. When soil is moved, fungi can be easily and unknowingly transported. In other cases, ectomycorrhizae have been introduced precisely because they allow timber trees to perform better (and thus produce more timber in a shorter amount of time, cheaply). In well documented cases involving pine and eucalyptus in California, Spain and the Southern Hemisphere, this has had profound consequences.

Eucalyptus trees naturally occur only in Australia, Papua, New Guinea and Timor and are highly host specific with particular fungi. When seeds and seedling trees were transported to Spain from Australia in the early 1900's, it is believed that ectomycorrhizal fungi endemic to Australia (found as spores on seeds and in potting soils) had already colonized the eucalyptus. This allowed for a quick establishment time in their new surrounding. By 2005, Spain had planted approximately 1.5 million hectares (2.5 million acres) with various species of eucalyptus used for paper products, lumber, windbreaks, shelter trees, and intercropping with arable crops. As a result of both massive plantings and colonization by their Australian native ectomycorrhizal fungi, eucalyptus was easily able to escape cultivation and invade many local Spanish ecosystems which happen to closely match that of Australia.

To date, invasion of Spanish ecosystems by Eucalyptus has been mostly contained to locations close to the original plantations despite easy pollination and seed dispersal, and high seed production rates. This moderate spread can be directly tied to the simultaneous invasion of the Australian ectomycorrhizal fungi. Underground fruit bodies which do not allow for much wind dispersal of spores, and the need for a spore bank to build up to sufficient levels are believed to be some limiting factors in the Eucalypts spread. Yet, it is only a matter of time before the fungi have had enough time to spread, after which the Eucalyptus can run free across the Spanish countryside, its mycorrhizal fungi ensuring its survival.

"So what," you say, "if the Eucalyptus colonizes the ecosystems of Spain, they're pretty trees." Although they may be pretty and smell nice, Eucalyptus are heavy water consumers and can easily drain small streams thereby denying the native flora of water, and once colonized by their ectomycorrhizal fungi, Eucalyptus can easily out-compete native trees such as Spanish fir (Abies pinsapo), and Holm and Cork Oak (Quercus ilex and Q. suber respectively).

Most of the ecosystems of Spain are already extremely fragile due to over development and harvesting for wood and crops. Many species of native Spanish plants are endangered and found only in fragment relic forests such as Sierra de Grazalema (declared a Reserve of the Biosphere by UNESCO) and los Alcoronocales. The economic impacts could be enormous on Spanish tourism in Huelva and Cádiz provinces as the world renown flora and fauna of their national parks begins to degrade.

A case involving the mycorrhizal facilitated invasion of Eucalyptus in the United States can be found in California. For many decades Eucalyptus was a favorite street and lawn tree for the cities and towns of California. However, as in Spain, the trees escaped with their mycorrhizal fungi into the surrounding hills and countryside and thrived. Their full environmental and economic impact is now coming to light as small, naturally occurring fires are turning into massive fire storms by the exploding oils in the leaves of the Eucalyptus. As the leaves of the Eucalyptus burst into flames, their crowns allow for easy access into the forest canopy or onto the roof of a home or business. Though not all destructive and large fires can be linked to Eucalyptus, it is known that Eucalyptus helps in the spread and intensity of wild fires.
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