Mycorrhizae are mutualistic symbioses between plants and fungi. As such, mycorrhizal fungi play an important role in structuring plant communities by improving the performance of individual plants. Improved growth and survival of host plants is mostly attributed to the ability of fungal partners to improve plant nutrition and rooting structure (Allen and Allen 1986; Allen et al. 1989; Lewis 1973) and enhance resistance to environmental stresses and soil pathogens (Sylvia and Williams 1992). However, a variety of mycorrhizal types differentially affects their host plants. If the appropriate group of fungi is unavailable to form the type of mycorrhizal association to which a plant belongs, that group of plants may not remain or recover from a disturbance (Allen et al. 1993). Disturbance alters fungal communities in different ways and fungal recovery typically occurs after initial plant establishment. As a consequence, the formation of functional mycorrhizae may require decades or may never occur.
The importance of mycorrhizae to the restoration of arid plant communities has received considerable attention, most notably because successful establishment of mycorrhizae has implications to rates and patterns of plant succession (Allen and Allen 1990). It is critical, therefore, to understand the key factors that regulate the natural reestablishment of mycor-rhizae (Allen et al. 1992), and determine whether mycorrhizal inoculation is a viable alternative to managing existing fungal populations or manipulating their natural reinvasion. Although the specific factors that regulate natural reestablishment of mycorrhizae are not entirely known, they involve survival of residual fungal propagules, dispersal of propagules from adjacent undisturbed areas or from local sites of residual survival, and available microsites where plants and fungi can survive and contact each other (Allen et al. 1992). Alternatively, inoculations of a disturbed site with native or exotic fungi facilitate formation of mycorrhizae in a shorter period, given certain circumstances. The simple application of inoculum, however, does not guarantee the formation of a functional mycorrhiza. The potential benefits of inoculation could be outweighed by cost and efficiency limitations, if the probability of plants benefiting from mycorrhizae is low (Findlay and Kendle 2001). Understanding mycorrhizal function and limits of mycorrhizal function are critical in studying ecosystem response to disturbance and recovery from disturbance (Allen et al. 1999).
In addition to examining the integral role of mycorrhizal fungi in ecosystem restoration, there has been considerable interest in determining the influence of mycorrhizal fungi on revegetation of severely polluted soils. The use of mycorrhizal fungi in bioremediation of metal-polluted soils has received increasing attention. Recent studies suggest that mycorrhizal fungi may exhibit some degree of heavy metal tolerance and as a result, confer heavy-metal tolerance in host plants. However, under natural conditions, the extent to which mycorrhizal associations benefit plants, in terms of alleviating metal toxicity, remains largely uncertain (Leyval et al. 1997). In this chapter, we will address in more detail the response of mycorrhizal fungi to disturbance and the factors that inhibit or facilitate their natural recovery. We will also discuss management options for enhancing native fungal reestablishment and the potential benefits and drawbacks of using mycorrhizal inoculum. Lastly, we will briefly discuss the potential role of mycorrhizal fungi as bioindicators and their application to bioremediation.
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