There are two general mechanisms of competition. In 'exploitation competition', weeds (as with all plants) compete with one another by being better at exploiting the resources, i.e. obtaining more of them than others (Fig. 8.2). In 'interference competition', there can be a direct removal of a resource (taking it away from another who already captured it) or the occupation of a resource that may not be needed immediately, but denying it to others raises relative fitness (Murphy, 1999). In weeds, interference competition does not involve direct removal of a resource because this situation applies more to animals that physically confront one another over resources like nests or caches of food. Thus, interference competition in plants is generally the denial of resources to other plants.
While we will discuss general trends in exploitation competition for different types of resources, we caution that the relationship between the amount and type of resources available and competition is not simple because:
• weeds can compete for more than one resource at a time and may do so above and below ground;
• the outcome of competition is modified by abiotic factors (e.g. temperature, pH) and other biotic interactions (e.g. predation, parasitism) that influence the rate and efficiency at which the resources are consumed;
• weeds are genetically variable and thus all individuals of a species will not respond to competition in the same way.
Both light quality and quantity are important aspects of competition (e.g. Novoplansky, 1991). Weeds can compensate in a number of ways to avoid poor light conditions, all of these related to the basic processes of germination, emergence and adult phenology (Chapters 6 and 7). Many weeds germinate early and grow taller at a faster rate to acquire as much light as possible, e.g. self-heal (Prunella vulgaris) (Miller et al., 1994). Since the presence of dense leaf canopies reduces the quantity and quality of light available to weeds, competition for light is greatest when plant density is highest. The plasticity of weeds explains why:
• they tend to be taller when grown in high densities (Nagashima et al., 1995);
• the position and orientation of leaves changes to intercept more light (Alphalo et al., 1999);
• stems may elongate so that leaves are positioned above competing vegetation (Alphalo et al., 1999).
Plants compete mostly for nitrogen, phosphorus and potassium (but there are many others). Phosphorus is usually the most limited nutrient in aquatic ecosystems. Nitrogen is usually the most limited nutrient in terrestrial habitats. Potassium is often overlooked but some terrestrial weeds (e.g. dandelions) might be managed better if potassium-poor fertilizers were used because some weeds are limited by this nutrient (Tilman et al., 1999). In general, weeds compete directly for nutrients; however, weeds that germinate and emerge prior to other plants are able to deplete the available nutrient resource pool first. Because nitrogen is also one of the most mobile and absorbable nutrients, adding fertilizers may benefit weeds that are poor nitrogen com-
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