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improve soil quality or attract organisms that attack weeds (Jordan et al., 2000).

Many weeds are not mycorrhizal (Goodwin, 1992). This is why they are found in highly disturbed habitats, i.e. where the mycorrhizal 'system' has been disrupted by human activities. When mycorrhizal fungi are absent from soil, non-mycorrhizal species are favoured, e.g. most mustard weeds (Brassicaceae) and lambsquarters (Chenopodium album). However, non-mycorrhizal weeds may use an existing mycor-rhizal network in a relatively undisturbed habitat to its advantage. For example in North American prairie, weedy spotted knapweed (Centaurea maculosa) is able to outcompete the native Idaho fescue (Festuca idahoensis), but only if mycorrhizal fungi are present (Marler et al., 1999) (Fig. 9.3). Normally, neither species benefits individually from mycorrhizae as both are infected with the fungi but do not provide carbohydrates to them (hence the fungi constitute merely a passive infection). If, however, an extensive mycorrhizal network is formed among prairie natives, spotted knapweed will 'activate' its mycorrhiza and 'steal' nutrients from the normally dominant Idaho fescue via their common fungal 'pipeline'.

Plants often form associations with mutualistic bacteria that use ('fix') atmospheric nitrogen. Nitrogen in this form is normally unavailable to plants because they lack the enzymes needed to capture it. The plant gets another source of nitrogen while the bacteria receive carbohydrates and protection. These nitrogen-fixing bacteria are generally ubiquitous (Richardson et al.,

2000) and weedy nitrogen-fixing plants usually have no difficulty finding the required bacteria. e.g. gorse (Ulex europeus) and scotch broom (Cytisus scoparius) (Peterson and Prasad, 1998; Clements et al.,

2001). Nitrogen-fixing weeds may have a strong influence on their new habitat because they may change the nutrient dynamics. This is especially true in places like Hawaii where nitrogen-fixers are not a part of the native flora. For example, the fire tree (Myrica faya) enriches soil nitrogen and provides shade that improves germination and seedling growth of the native "ohi'a lehua (Metrosideros polymorpha). However, the increased amount of nitrogen in the soil also permitted the invasion of weeds formerly constrained by limited nitrogen, e.g. strawberry guava (Psidium cattleianum) (Wall and Moore, 1999). The same phenomenon can occur when nitrogen-fixing crops like clover (Trifolium) or lucerne (Medicago) are used; the increased soil nitrogen is of benefit to non-nitrogen-fixing crops (like maize) that will be planted the next year in rotation, but it also benefits weeds. Further,

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