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Plants, Soil, and Fungi

 

One of the most active parts in a forest takes place in the soil where insects and other small invertebrate animals start to decompose fallen leaves, branches and animal remains. Fungi and bacteria complete the decay processes that return nitrogen, phosphorus and trace minerals to the soil to be taken up by roots and once again incorporated into living plants. These nutrient cycles support all the plants and animals of the forests and other natural areas.

 

Fungi play another vital role in the forest as symbiotic (sym -together, bios – living) partners of roots. Certain types of fungi are incorporated into the structure of roots and help them take up nutrients such as nitrogen and phosphate. In return, the host plant supplies the fungus with carbohydrates (sugars and starches).

 

When speaking of fungi, there are a couple of critical things to note. First, plants and fungi form two different kingdoms of living things, i.e. fungi are NOT plants. Fungi have very different life cycles, means of reproduction, and do not contain chlorophyll, the green chemical that plants use to convert carbon dioxide and water into sugars and starches using the energy from sunlight.

 

Second, although they are not at all closely related, plants commonly are intimately associated with fungi both in constructive and detrimental ways. The roots of most plants form associations with fungi called mycorrhizas (my-cor-RY-zas. See: http://mycorrhizas.info).  Many plants also contain other symbiotic fungi within their systems called endophytic (literally “in plant”) fungi. Some of these are fungi are helpful and some are disease-causing. Some actually create toxins that protect the plant against insects and other predators.

 

“Mycorrhizas, not roots are the chief organs of nutrient uptake by land plants and recent work has amply confirmed…that the earliest land plants, that had no true roots, were colonized by hyphal fungi that formed vesicles and arbuscules strikingly similar to modern arbuscular Mycorrhizas.”  (S. E. Smith and D. J. Read. 2008. Mycorrhizal Symbiosis, 3rd ed. Academic Press, New York, NY). Arbuscular mycorrhizas, the most common type, live inside roots, while other, less common, ectomycorrhizas, found in many trees and shrubs, live wrapped around the outer layers of root tissue. In both cases the fungus grows out into the soil in the form of thin, thread-like structures called hyphae. These can reach tiny spaces in the soil, not available to roots, in search of water and nutrients. In exchange, the fungus derives carbon-based nutrients (sugars, starches) from the plant. Mycorrhizal fungi can often protect against root parasites or predators  such as nematodes.

 

The best known, and possibly most common, genus of arbuscular mycorrhizas (MA) is Glomus, although there are other genera. Glomus belongs to the Phylum Glomeromycota; class Glomeromycetes, all of which form mycorrhizas. In pine and oak forests, the bright red or yellow “mushrooms” are often in the genus “Russula,” and are the fruiting bodies of the ectomycorrhizal fungi associated with the forest trees.

 

When large areas of forest are clear-cut, the most serious loss may be the soil fungi that support the trees and other plants. Without the shelter of the forest the fungi die, and without the fungi the forest cannot reestablish itself even when the land is no longer cultivated or grazed. Erosion of soil, no longer held in place by roots, further degrades land and makes re-establishment of a forest more difficult.

 

Plants and Bacteria

While most plants must get their nitrogen from the soil in the form of nitrates, some plants can use nitrogen gas from the air to sustain themselves. These plants are legumes, (families Fabaceae/Caesalpinioideae, Fabaceae/ Faboideae, Fabaceae/Mimosoideae) recognizable by their fruit pods containing bean-like seeds. Legumes, and a few other plant families, have symbiotic bacteria that grow in small nodules in their roots. These bacteria take nitrogen gas from the air and chemically modify it into a form usable by the plant. With these bacterial partners, legumes can grow in low nutrient, degraded or eroded soils that can’t support most other plants. The most common nitrogen fixing soil bacteria are in the genus Rhizobium.

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