Many essential elements such as calcium and potassium are easily leached out by the heavy tropical rainfall, further reducing soil nutrient levels. There are few nutrients more than 5cm (2 inches) below the surface of the soil in tropical rainforests. This poverty of soils (which is common but not universal in rainforests) has the consequence that the forest is dependent on the recycling of nutrients, most of which are contained within the vegetation and not in the soil, unlike temperate forests. Because many rainforest trees are evergreen and drop their leaves infrequently, there is relatively little “litterfall” in comparison with temperate forests. Leaves and dead plants and animals which fall on the forest floor are rapidly decomposed by fungi and bacteria, and the resulting chemical compounds are quickly reabsorbed by the living plants. Plants on tropical soils typically recycle 60% to 80% of nutrients, and in the case of calcium and phosphorus, more than 99% of these minerals appear to be recaptured from the soil by the roots of forest trees. The remainder of necessary nutrients must come from soil or from rainfall. [See also Section G5.]
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Rainforests are very fragile habitats. In many places they are “wet deserts,” which grow on soils poor in nutrients. In many tropical regions, the bedrock is very old and weathered, and, consequently, depleted in minerals and nutrients. Mineral release is also inhibited by the acidic nature of many tropical soils. The soil types derived from the bedrock underlying tropical forests are mainly soils called oxisols and ultisols. (There are many kinds of tropical soils, each with its own characteristic array of minerals; see Richter and Babbar, 1991, for a detailed discussion). Oxisols have a high aluminum and iron oxide content and a low silica content. Ultisols are highly-weathered, acidic soils and are less frequently found than oxisols. These two types of soils, generally of low fertility, comprise about 43% of the soils under tropical rainforests (Hoffman and Carroll, 1995). Another 40% consists of variably fertile soils, some of which are suitable for agriculture, but many of which have low pH, poor physical structure, low phosphorus and other nutrient deficiencies, or high salt or aluminum levels. Interestingly, tropical soils can vary a great deal within a relatively small area, which leads to a variety of vegetation types because of differences in nutrient concentrations and availability, variations in the ability of the soil types to retain water, and the like. (Many other factors are also involved in determining the vegetation which grows in any particular area.)
Effect of arbuscular mycorrhizal (AM) fungi on heavy …
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Oxygen is used in respiration in most organisms. The fungi include species that are obligately aerobic or obligately anaerobic (eg rumen fungi). However many fungi are in between, with the capacity to function facultatively in aerobic and anaerobic conditions. Oxygen is used for oxidative metabolism, to generate energy. However, it is also essential for biosynthesis of sterols, unsaturated fatty acids and some vitamins. Thus, while many fungi can exist in anaerobic conditions and respire fermentatively, they also have the capacity to transport oxygen or the products of respiration through their cytoplasm, or air in air spaces of the rhizomorph.
Orchids and fungi: An unexpected case of ..
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with other photosynthetic plants or with saprotrophic fungi, ..
Fungi follow a pattern of growth and development which is much less predictable than for plants and animals. If you plate out a hypha from within a compound structure, it is likely that hyphae will emerge and grow on the agar. In other words, fungi do not become fixed into a developmental pathway. All compartments (cells) of a viable hypha may initiate a colony (totipotent). It also means that the lifecycle of a fungus is unpredictable and flexible.