5.4.1 Fungal friends and foes: saprotrophic, parasitic, mycorrhizal, edible and poisonous species
Fungi resemble animals in having heterotrophic nutrition. Unlike the autotrophs, which fix their own energy by photosynthesis (green plants) or chemosynthesis (certain colourless bacteria), fungi are dependent on living or dead organic matter for their sustenance. Saprotrophs obtain their energy from dead organic matter, which they digest. Many are completely harmless and some are edible, like the ubiquitous field mushroom Agaricus campestris, but others cause a great deal of damage such as the dry rot fungus Serpula lacrymans which establishes in damp timbers and then uses its mycelial strands, which can transmit water and nutrients, to invade drier parts of a building. Parasites, on the other hand, attack living organisms. Fungal parasites such as those causing Dutch elm disease (DED, see Section 5.4.5) and chestnut blight (see Section 5.4.6) have caused very serious damage to tree populations and have become increasingly difficult to control with the development of global trade. As mentioned in Section 5.1, many fungi can continue to live saprotrophically on the dead tissues of plants they have parasitized and killed.
The white-spotted red fruiting body of the highly poisonous fly agaric Amanita muscaria var. muscaria can often be seen growing on sandy soils. This fungus receives photosynthate from the birches with which it forms a mycorrhizal association. A mycorrhiza is an association between a root and a fungus (or fungi) to the benefit of both (symbiosis), found in over 80% of the world's vascular plants. Mycorrhizal fungi, such as the highly poisonous fly agaric, live in association with higher plants from which they receive photosynthate, while supplying in return water and important nutrients (particularly phosphorus but also nitrogen and any others for which competition is intense), derived from the soil. The main role of the fungus appears to be in increasing the effective absorbing surface area of the roots; Rousseau et al. (1994) found that fungus contributed 80% of the absorbing area of pine seedlings. Several forms of mycorrhizas exist (see Box 5.1). Of these, ectomy-corrhizas (ECM), in which fungal sheaths envelop the root, are of particular interest to forest ecologists (judged by the number of published studies) since they are confined almost entirely to woody plants. Ectomycorrhizal fungal species frequently live on more than one host plant and this makes possible the formation of common mycorrhizal networks between understorey and canopy trees, as Kennedy et al. (2003) have demonstrated in mixed evergreen forest stands in northern coastal California. Douglas fir Pseudotsuga menziesii dominated the canopy and tanoak Lithocarpus densiflora the understorey trees. Of the 56 ECM fungi found in this forest, 17 were found on both hosts, 27 solely on Douglas fir and 12 on tanoak. Although the number offungal species found on both hosts was fairly low, their abundance was sufficiently high that 13 of the 17 multi-host fungal species were present on both hosts within at least one of the 18 soil samples taken. This suggests that it is very likely that tanoak is connected to the Douglas fir trees above by the mycorrhizal net. Do such common mycorrhizal networks result in a transfer of photosynthate from the roots of the canopy trees to those of the understorey? This problem was not investigated in the study discussed above, but Simard et al. (1997) found that significant amounts of carbon were transferred from paper birch Betula papyr-ifera seedlings growing in full sun to the roots of experimentally shaded Douglas fir seedlings. This suggests that carbon may move preferentially to plants in shaded environments; in any event individual higher plants tapping into ECM networks will have access to larger nutrient pools. Evidence of such facilitation was provided by Horton et al. (1999), who showed that Douglas fir seedlings shared many ECM fungi with mature Arctostaphylos spp., and that in the area concerned Douglas fir seedlings would establish successfully only when near Arctostaphylos individuals. Similar studies in a volcanic desert in Japan have shown that ECM fungi associated with pioneer willow Salix reinii were essential in facilitating the establishment of subsequent Erman birch Betula ermanii and Japanese larch Larix kaempferi (Nara and Hogetsu, 2004).
The importance of ectomycorrhizas to seedling success was demonstrated by Dickie and Reich (2005). They looked at how the ECM community changed with distance from the forest edge into two abandoned agricultural fields in
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