Note: NS = step variable did not meet the 0.15 significance level for incorporation into the model.

Source: Data from Baxter and Dighton (2001).

Note: NS = step variable did not meet the 0.15 significance level for incorporation into the model.

Source: Data from Baxter and Dighton (2001).

ectomycorrhizae to exploit resources and enzyme expression, nutrient uptake, and translocation within the mycorrhizal system (Cairney, 1992).

Changes in the availability of photosynthate can also influence the mycorrhizal community and its function on the roots of host plants. Cullings et al. (2001) showed that the effect of defoliation of a mixed Pinus contorta (lodgepole pine)/Picea engelmannii (Engelmann spruce) forest in Yellowstone National Park was to alter the ectomycorrhizal species composition on the tree roots (Fig. 3.24). Lodgepole pine trees were defoliated to 50%, while Engelmann spruce were left untreated. There were no significant effects of defoliation on either ectomycorrhizal colonization (142.0 mycorrhizal tips/core in both defoliated and control plots) or species richness (5.0 species/core in controls and 4.5 in treatments). The ecosystem-dominant ectomycorrhizal species, Inocybe sp., however, was rare in defoliation plots, whereas both Agaricoid and Suilloid species were dominant in both the defoliated plots and the control plots. Ectomycorrhizal fungal species associating with both lodgepole pine and

FIGURE 3.24 The effect of faunal defoliation (right column of each pair of bars) on the ectomycorrhizal community structure of lodgepole pine (Pinus controrta) [left] and Englemann spruce (Picea engelmannii) [right]. Data from Cullings et al. (2001).

Engelmann spruce were affected by defoliation, which suggests that changing the photosynthetic capacity of one species can affect the mycorrhizal associations of neighboring trees of a different species. A study of the effects of winter browsing of willow by elk (Peinetti et al., 2001) showed that browsing induces higher shoot biomass production but similar leaf biomass and leaf area per plant, a lower number but larger shoots, a lower number and bigger leaves, and flower inhibition. In addition to the changes in aboveground plant parts, they inferred that browsing induces lower allocation of resources belowground, resulting in higher soil N uptake. Although they did not discuss the effects of grazing on the mycorrhizal condition of the trees, it is apparent that the changes in resource allocation within the plant would place different demands on a mycorrhizal community colonizing the roots. Reduced carbohydrate allocation belowground would reduce the ability of the trees to support mycorrhizae, but the increase demand for nitrogen would require the presence of an active mycorrhizal flora. We know little about the influence of aboveground herbivory on the mycorrhizal status and activity of plants.

As we stated earlier, fungi do not exist alone in the environment. Especially in soil, mycorrhizal fungi are in close juxtaposition with a range of other fungi, bacteria, and fauna. The effect of fungi may thus be considerably affected by their interactions with these other organisms. The interaction between mycorrhizae and saprotrophs in litter decomposition is one example of an interaction that may alter the rates of decomposition, nutrient mineralization, and the ultimate fate of the nutrients released during the decomposition process. We have seen that many ectomycorrhizae have the capacity to act as decomposers. Gadgil and Gadgil (1971; 1975) first suggested that there could be strong interaction between mycorrhizal tree roots and the saprotrophic community in soil, in which

the presence of roots can suppress the rate of decomposition of leaf litter (Table 3.18). Berg and Lindberg (1980) repeated Gadgil and Gadgil's experiment in a northern coniferous forest and found the opposite effect—that the presence of tree roots did not influence the rate of leaf litter decomposition. In a laboratory study of controlled mycorrhizal inoculation of trees in the presence and absence of saprotrophic fungi, Dighton et al. (1987) showed that the presence of the saprotrophic fungus Mycena galopus reduced the decomposition potential of the ectomycorrhizal fungi Suillus luteus and Hebeloma crustuliniforme, which are associated with seedling pine roots. More recently, Zhu and Ehrenfeld (1996) revisited this argument in an oligotrophic forest system to show that the presence of roots increased the activities of saprotrophic fungi and soil fauna to increase the rate of leaf litter decomposition and nutrient mineralization. The nematode faunal population of rooted chambers was significantly enhanced. The difference in results among studies is probably related to the overall fertility of the soil and the relative dependence of the system on readily available or unavailable sources of nutrients. The balance between the relative abilities of the saprotrophic fungal community and the ectomycorrhizal community to effect leaf litter decomposition is also dependent upon the species composition of the two groups of fungi. Colpaert and van Tichelen (1996) showed that the decomposition of beech leaf litter is much less in the presence of Scots pine tree seedlings colonized by the ectomycorrhizal species Thelephora terrestris, Suillus bovinus, or Paxillus involutus than in the presence of the saprotroph Lepista nuda (Table 3.19). Indeed, nitrogen mineralization only occurred in the presence of Lepista. The authors suggest that ectomycorrhizal fungi are capable of effecting leaf litter decomposition (Durall et al., 1994) in the absence of a competing saprotroph, but that saprotrophic fungi are superior competitors for the organic resources and suppress the decomposing abilities of the ectomycorrhizal fungi. Lindahl et al. (1999) showed that the interaction between the ectomycorrhizal fungi Suillius luteus and Paxillus involutus and the wood decomposing saprotroph Hypholoma

Table 3.18 Effect of Combinations of Ectomycorrhizal Tree Seedlings and Saprotrophic Fungi on the Decomposition of Leaf Litter


Dry weight of litter after 6 months (g)

Mycorrhizal plant + litter + saprotrophs Nonmycorrhizal plant + litter + saprotrophs No plant: + litter + saprotrophs Mycorrhizal plant + litter Nonmycorrhizal plant + litter

Source: Data from Gadgil and Gadgil (1975).

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