Although shifts in the ranking of species were apparent between high and low nutrient conditions, there were predictable patterns at the multispecies scale, and statistical comparison between treatments indicated that there was a significant positive relationship between species’ competitive performance at high and low fertility. In general, irrespective of nutrient level, large leafy species typical of fertile habitats (e.g. Lythrum salicaria, Sparganium eurycarpum, Typha xglauca) emerged near the top of the hierarchy, as would be predicted from other studies (Day et al. 1988; Weiher & Keddy 1995; Keddy et al. 1997). Small rosette species (Lobelia dortmanna, Viola lanceolata) and partially evergreen species typical of infertile habitats (Juncus pelocarpus, Eleocharis acicularis, Ranunculus reptans) were near the bottom of the hierarchy, which is consistent with other studies (Goldsmith 1978; Wilson & Keddy 1986b; Givnish 1988; Grime et al. 1997). Therefore, the main axis appears to represent growth form, which in turn varies with habitat.
While our study was not set up to explore mechanisms underlying competitive performance, these size-related patterns do suggest such an interpretation. Large plants have larger surface areas for extracting resources from the environment, and therefore also have a greater ability to create nutrient-depletion zones irrespective of resource levels (Campbell et al. 1991). In the case of light, large plants, being generally taller, can intercept light, thereby denying it to smaller neighbours, leading to asymmetric interactions (Weiner 1985, 1986; Keddy & Shipley 1989; Keddy 1989). Superimposed differences in nutrient uptake rates per unit surface area, or differences in efficiency of use or conservation, may contribute to the residual variation.
It is noteworthy that the most obvious shifts in competitive performance occurred under low nutrient conditions between years 1 and 2. For example, Eleocharis acicularis, a small lanceolate-leaved plant, induced a 48% reduction in the phytometer in the first year at low fertility, but promoted growth (competitive performance =−18%) in the second year. On the other hand, the relative performance in the presence of Euthamia galetorum, a large leafy plant that can reach a height of up to 1 m, was −5% in the first year, compared with 69% in year 2. One possible interpretation is that position in the competitive hierarchy is more variable under low fertility conditions. This might be expected if competitive hierarchies under fertile conditions result from strong asymmetric competition for light (Weiner 1986; Keddy 1989), leading to a clear winner and loser. In contrast, under infertile conditions, root competition may predominate, which may be more symmetric (Weiner 1986) with no pronounced dominance/suppression. Shifts in competitive rankings may result when there is no asymmetric advantage that is maintained and reinforced over time.
At low productivity, competitive hierarchies may also be influenced by mycorrhizae. When mineral nutrient concentration is high, the rate of mycorrhizal infection is relatively low (Smith & Read 1997), whereas at low productivity mycorrhizae play a much more important role in the acquisition of mineral nutrients. Grime et al. (1987) grew seedlings of 20 grassland plants in a mixture with larger Festuca ovina plants, both with and without mycorrhizae. Although the mycorrhizae had little impact on the performance of Festuca ovina seedlings that were competing with larger plants of the same species, the balance between Festuca ovina and seedlings of other species was affected. Thus, mycorrhizae may play a role in interspecific competition (Fitter 1977; Allen & Allen 1990; Watkinson & Freckleton 1997).
The reduced intensity of competition under low nutrient conditions may also be important in accounting for shifts in ranking. As competition intensity decreases, other factors may become increasingly important as determinants of plant biomass. However, an alternative explanation is that under conditions of low fertility, plants may simply take longer to establish an hierarchy, and this is supported by the increasing correlation between ranking at high and low fertility over the course of the experiment.
Wilson (1988) suggested that root and shoot competition vary independently, but out results do not support this view for shoreline plant species. However, root and shoot competition were not studied independently, and it may be that if species had been prevented from competing above-ground, below-ground competition alone would have resulted in a different hierarchy. The results, however, do support the work by Twolan-Strutt & Keddy (1996), which showed that below-ground competition intensity did not vary between different shoreline habitats.
Contingency and scale
The contrasting views about competitive hierarchies and contingency appear to represent results from two different scales rather than opposing inconsistent views, and it is therefore important to distinguish between coarse- and fine-scale relationships. The current study was designed to explore a general coarse-scale pattern across a large number of species in two very different habitats. In contrast, fine-scale work on small numbers of species, particularly ones with similar ecological requirements, may emphasize contingency and possibly miss general predictive relationships. In fact, one could argue that the past emphasis upon contingency has arisen because the majority of experiments used a few pairs of species in varied environments and were by their very design capable of detecting only contingency. However, although larger scale comparative work may provide general predictive relationships, it might overlook the details of species’ interactions that contribute to the understanding of fine-scale pattern and mechanism in the field. The fine-scale pattern seen in small plots may depend on competitive interactions between pairs of species that are similar in competitive performance, as it has been suggested that such equivalence is likely to foster coexistence of species (Aarssen 1983, 1985). It is at this scale, where competitive interactions may be increasingly symmetrical (sensuWilson 1988; Keddy 1989), that contingency may be most pronounced and minor fluctuations in the environment may have a major influence on dominance. For example, Grace & Wetzel (1981) found that water depth played a critical role in determining competitive dominance between two closely associated species of Typha.
Our results suggest that, at the broad community scale, the outcome of competitive interactions may be relatively predictable and independent of the environment. Such studies therefore provide a useful tool for exploring and understanding community pattern, but cannot address questions related to the outcome of competitive interactions between similar species or within fine-scale pattern. Further empirical work is needed to sort out the factors that may be operating at different scales of organization.
Intensity of competition
Our results indicate that, while competitive hierarchies do exist at low nutrient levels, the intensity of competition is reduced by approximately 25% relative to fertile conditions. Similar results have been obtained for shoreline plant communities by Wilson & Keddy (1986a) in a field study of diffuse competition, as well as for grassland species (Campbell & Grime 1992; Peltzer et al. 1998) and moorland species (Hartley & Amos 1999). The hypothesis (Grime 1979; Huston 1979) that factors other than competition may be increasingly important in structuring natural communities under infertile conditions therefore receives general support. Although this experiment was not designed to test directly for the importance or intensity of competition (sensuWelden & Slauson 1986), its results indicate that competition intensity, measured as the relative decrease in biomass of the phytometer across all species, was lower under stressful infertile conditions, as suggested by Grime (1979, 1988). This may in part explain why species of high relative competitive performance are not equally dominant throughout the gradient. However, the fact that competitive performance, although reduced, is still significant under low nutrient conditions means that declining soil fertility altering the nature of competition from above-ground to below-ground resources (Newman 1973; Tilman 1988, 1990; Wilson 1988) is an alternative explanation. However, the supposition that competition will become more intense or important as resources are increasingly limited (Fowler 1986) is not supported, at least with respect to nutrient levels.
The results were based on a comparison of only two nutrient levels, and it is not known how results would have varied with other nutrient regimes. Nor do we know how the results would have changed over a longer period of time [Tilman (1988), and Berendse & Elberse (1990) show that time span may be an important consideration when evaluating competitive performance].
More studies at the community scale are needed to assess how other environmental variables or different nutrient levels will influence plant hierarchies. There are few studies using large numbers of species in contrasting environments, so generalization about the consistency of competitive hierarchies is not yet possible. However, the phytometer approach makes such studies more feasible than past pair-wise designs, which increased the size of experiments by the square of the number of species examined. It may be that different patterns will emerge, particularly in traits associated with competitive performance, if different floras are examined. If wetlands are generally light limited, it may be that hierarchies are produced by different competitive abilities for light based on above-ground traits, whereas in other habitats, such as prairies and deserts, hierarchies could be associated with below-ground traits.
The results of this experiment have two important implications for community ecology. First, they illustrate that scale must be considered. General questions about the community level of organization are probably best answered by experiments examining simultaneously large numbers of species of contrasting ecology (Keddy 1992). Secondly, our results show that position in competitive hierarchies is relatively consistent. This has two further implications for the study of plant competition: first, that it is reasonable to test for specific traits that can predict competitive performance, and secondly, that competitive hierarchies can provide a general framework for exploring the structure of communities.