Summary
Competition is generally regarded as an important force in organizing the structure of vegetation, and evidence from several experimental studies of species mixtures suggests that larger mature plant size elicits a competitive advantage. However, these findings are at odds with the fact that large and small plant species generally coexist, and relatively smaller species are more common in virtually all plant communities. Here, we use replicates of ten relatively large old-field plant species to explore the competitive impact of target individual size on their surrounding neighbourhoods compared to nearby neighbourhoods of the same size that are not centred by a large target individual. While target individuals of the largest of our test species, Centaurea jacea L., had a strong impact on neighbouring species, in general, target species size was a weak predictor of the number of other resident species growing within its immediate neighbourhood, as well as the number of resident species that were reproductive. Thus, the presence of a large competitor did not restrict the ability of neighbouring species to reproduce. Lastly, target species size did not have any impact on the species size structure of neighbouring species; i.e. they did not restrict smaller, supposedly poorer competitors, from growing and reproducing close by. Taken together, these results provide no support for a size-advantage in competition restricting local species richness or the ability of small species to coexist and successfully reproduce in the immediate neighbourhood of a large species.
Methodology
Sites and Selection of ‘Target’ Species
Samples of relatively ‘big’ herbaceous plant (target) species were collected from old-field vegetation at Queen’s University Biological Station (QUBS) (44°33′N, 76°21′W) located north of Kingston, Ontario, Canada during the spring and summer of 2010. A candidate ‘big’ species was regarded as one that has a typical adult body size that is within about the top 20% of the body size of the resident species within a community, based on visual estimation. Ten different ‘big’ species were selected based on local availability and abundance (Table 1). For convenience, these will be simply referred to as ‘big’ species. Historically, the fields chosen have been used periodically for haying and some cattle grazing but have otherwise been left relatively undisturbed for at least ten years prior to the start of this study. Neighbour competition within this vegetation type is intense; a previous neighbour-removal study at QUBS [38] showed that target individuals (selected randomly) that had near-neighbours left in place were 75–85% smaller in size (dry mass) at the end of the growing season compared with target individuals that had near neighbours removed.
Selection of Large ‘Target’ Individuals
Sampling was restricted to relatively uniform parts of the site in terms of topography to maximize the chance that the Target neighbourhood and Randomly-chosen neighbourhood (Fig. 1) did not differ noticeably in this respect. Target size was assessed based on visual estimation of above-ground biomass, taking account of both height and lateral extent; i.e. the largest individuals were not necessarily the tallest. The objective was to assess the effect that a relatively large individual (not a clump of individuals) of a particular ‘big’ species has on the composition of its immediate neighbourhood. Accordingly, dense clumps of large plants belonging to the target species were avoided and target neighbourhoods generally contained few or no other members of the target species. The three largest target individuals of each of the chosen ten species within the population were selected, but with the proviso that the target individual could not have any other big individuals of any species as near neighbours. In other words, within both the inner (TA) and outer (TB) target individual neighbourhood (see Fig. 1), there were no other resident plants belonging to any other species that were any larger than half of the size of the target individual–based on visual estimation, and taking both height and lateral extent into account. This was done in order to ensure that potential effects on the composition of the resident species within the target neighbourhoods could not be attributed to any other relatively large plants nearby, i.e. other than the target individual. If this condition was not satisfied, then the potential target individual was rejected for sampling, and the next largest potential target individual available within the local population was chosen. The target eventually chosen, however, was no smaller than ¾ of the size of the largest individual plant in the local population (based on visual estimation).