Fig. 8.3. (a) Effect of nitrogen addition and two weeds (barnyardgrass and the annual broadleaf monchoris) on rice yield (redrawn from data from Moody, 1981 in Zimdahl, 1999) and (b) the effect of nitrogen and increasing wild oats density on wheat yield (redrawn from data in Carlson and Hill, 1986).
petitors at a cost to crops (Fig. 8.3). However, crops such as maize have been shown to outcompete weeds for the extra nutrients and as a result suffer less from weed competition (Tollenaar et al., 1994). Agricultural weeds may be poor competitors at low levels of nitrogen, although there are exceptions (Iqbal and Wright, 1997; Hashem et al., 2000).
Competition for water occurs below ground between roots. The ability to absorb water is related to rooting volume. However, not only are the dimensions (breadth and depth) of rooting zones important: so is the degree of water extraction. The ability of a plant to use water depends on its water use efficiency (WUE). This is the ability to minimize water use for a given amount of carbon assimilation. When water is abundant, plants with low WUE (and flood tolerance) are more competitive, whereas in arid conditions, plants with high WUE (and drought tolerance) have the advantage (Di Tomaso, 1998; Gealy, 1998; Hunt and Beadle, 1998; Iqbal and Wright, 1998; Walch et al., 1999; Schillinger and Young, 2000). The examples of weedy saltcedars (Tamarix spp.) and their impact on the southwestern USA are particularly illustrative as these weeds have invaded 600,000 ha, in part, because of their superior WUE (Di Tomaso, 1998). Similarly, trees in forest plantations of Tasmania, Australia, suffer because some weeds have a high WUE and can outcompete the tree for water (Hunt and Beadle, 1998). Plants in more temperate climates are less likely to compete for water because rain and snowmelt are usually abundant. However, we should not ignore the importance of WUE, because this may be an important adaptation needed to survive climate changes and a tendency for temperate areas to receive less precipitation (Norris, 1996).
It is easier to understand how weeds compete for a resource such as nutrients, water or light because we can imagine this being consumed by the plant. It is harder to imagine space as something that is competed for, and yet the lack of physical space creates consequences for individuals. Root restriction experiments are one way to test the effect of limiting space. Although not all species react equally, restricting a plant's rooting space generally decreases shoot bio-
mass, height and/or growth rate, even when ample water and nutrients are supplied (Richards and Rowe, 1977; Gurevitch et al., 1990; McConnaughay and Bazzaz, 1991; Matthes-Sears and Larson, 1999). Schenk et al. (1999) argue that plants are 'territorial' because they defend space for their exclusive use. A territorial individual may not be better at exploiting limited space; it may be better at preventing others from using it. This might apply to forestry weeds competing with willows (Salix) (Sage, 1999) and pasture weeds like Paterson's curse (Echium plantagineum) (Grigulis et al., 2001).
Exploiting different resources: competition above and below ground
Above- and below-ground competition may be considered separately because plants use different structures (e.g. roots vs. leaves) to compete for different resources (e.g. nutrients vs. light) above and below ground. Below-ground root competition is more complex because individuals compete for space, water and many nutrients, and these all differ in distribution, mobility, molecular size and other aspects. In above-ground competition, individuals normally compete only for light or space (Casper and Jackson, 1997). Below-ground competition usually reduces plant performance more than above-ground competition, except in weed-crop competition where shoot competition tends to be more intense (Wilson, 1988). Root competition is more prevalent in arid and nutrient-poor systems because plants tend to be spaced further away from neighbours and therefore do not shade each other; however, root zones may overlap.
From a physiological perspective, roots and shoots are so integrally related that it is practically impossible to separate them. In weeds, this creates problems because they must trade-off allocating resources between tissues involved in above- and below-ground competition. For example, if the weedy rice cockspur (Echinochloa oryzoides) increases leaf area to compete for light, it reduces allocation of resources to roots and is vulnerable to competition for nutrients (Gibson and Fischer, 2001). Many weed species experi ence such trade-offs (e.g. McLachlan et al., 1995).
The relationship between above- and below-ground competition is not usually additive: that is, the total competitive effect is not simply the above-ground effect plus the below-ground effect, although this is often presumed (Wilson, 1988). Root and shoot competition may have opposing effects, or be subject to complex interactions, and this may not be evident when measured together. One species may benefit from below-ground interaction and this may counter any negative effect of above-ground competition (Wilson and Tilman, 1995).
While the term 'interference competition' is still used, all plant competition might be considered exploitation if denial of resources is caused by overexploiting a common resource pool (Schoener, 1983). For example, Rebele (2000) grew weedy feather-top (Calamagrostis epigeios) and Canada goldenrod (Solidago canadensis) in mixed stands for several years. For the first 3 years, the much taller and large leaved goldenrod dominated feathertop. Subsequently, Canada goldenrod seedlings or regenerating shoots began to be buried by the accumulation of litter from feathertop. Eventually feathertop dominated because it interfered with the space that Canada goldenrod needed. However, the outcome could be interpreted as exploitation competition if the key process was that the litter prevented Canada goldenrod from competing for light.
'Allelopathy' is an interaction that might be considered to be interference competition (Aarssen, 1989, 1992; Murphy, 1999). Allelopathy is the direct effect of one individual on others through the release of chemical compounds from roots, shoots, leaves or flowers (Rice, 1995). Allelopathy can require resources in the form of the chemicals used or can create autotoxic effects that harm the allelopathic plant; since neither the allelopathic plant nor its targets gain absolute benefits, allelopathy can fit this part of the definition of competition. However, because no resource is being exploited and because the release of beneficial compounds can occur, allelopathy is sometimes described as a unique interaction that is not related to competition (Rose et al., 1984; Aarssen, 1989; Inderjit and del Moral, 1997; Murphy, 1999; Olofsdotter et al., 1999). It is even unclear how often allelopathy occurs since many chemicals released by plants only become toxic after being transformed by other species in soil or water. Such indirect effects may be considered to be allelopathy but whether the plant that exuded the original chemical benefits at all is unclear (Connell, 1990; Williamson, 1990).
While claims that allelopathy exists in many weeds (like velvetleaf, quackgrass, redroot pigweed) should be treated sceptically, it does occur (Rice, 1984, 1995). Generally, allelopathy seems to be easier to demonstrate and more ecologically important in species-poor habitats where one species may 'dominate the biochemistry of the soil' (Wardle et al., 1998). Additionally, if some species adapt and resist allelopathic chemicals, then these 'resistant' species may dominate, e.g. pink flower (Lantana camara) (Gentle and Duggin, 1997; Mallik and Pellissier, 2000). When nodding thistle (Carduus nutans) dies, its decomposing leaves release allelopathic chemicals that suppress or kill only white clover (Trifolium repens). However, white clover increases soil nitrogen, and loss of white clover reduces nitrogen available to all other species. Nodding thistle may be less affected by loss of nitrogen and benefits from the reduced competition by all other species, hence nodding thistle populations increase greatly (Wardle et al., 1998). In pollen allelopathy, the chemicals are released from pollen that lands on the stigma of the 'wrong' species (see Chapter 4). This reduces the success of sexual reproduction in recipient species, and can allow the allelopathic species to dominate. Parthenium weed (Parthenium hysterophorus), hawkweeds (Hieracium species) and timothy (Phleum pratense) are the only weedy species demonstrated to have allelopathic pollen (Murphy and Aarssen, 1995a,b; Murphy, 1999).
Even with well-designed experiments, an observer may conclude that competition is occurring between individuals, even though some other reason may explain the outcome. For example, common sage (Salvia offici-nalis) attracts small herbivores because it provides shelter for them. The herbivores, in turn, consume other vegetation around the sage plant. The gross result is that other species do poorly while sage is healthy; if you did not look for the herbivores, you might erroneously conclude that sage was outcompeting other species. This is called 'apparent competition' (Holt, 1977; Connell, 1990) (Fig. 8.2). In another example, the survival, growth and reproduction of the parsley fern (Botrychium australe), native to New Zealand decreased after the introduction of the non-native colonial bentgrass (Agrostis capillaris). This occurred because the colonial bentgrass provided habitat for non-native slugs (Deroceras reticulatum), which then predated on the parsley fern (Sessions and Kelly, 2002)
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