According to traditional theory, success in competition between plant species generally involves a ‘size-advantage’. We predicted therefore that plants with larger body size should impose greater limits on the number of species — especially relatively small ones — that can reside within their immediate neighbourhoods. Species composition was compared within local neighbourhoods surrounding target plants of different sizes belonging to one of the largest herbaceous species found within old-field vegetation in eastern Ontario Canada — Centaurea jacea. Resident species density was generally greater within immediate ‘inner’ target neighbourhoods than within adjacent circular ‘outer’ neighbourhoods, and mean body size of resident neighbour species was unrelated to increases in target plant size. As target plant size increased, the proportion of resident neighbour species that were reproductive increased. Relatively big plants of C. jacea do not limit the number or the proportion of reproductive species that can coexist within their immediate neighbourhoods, nor do they cause local exclusion of relatively small species from these neighbourhoods. These results fail to support the ‘size-advantage’ hypothesis and are more consistent with the ‘reproductive economy advantage’ hypothesis: success under intense competition is promoted by capacity to recruit offspring that — despite severe suppression — are able to reach their minimum body size needed for reproduction, and hence produce grand-offspring for the next generation. The latter is facilitated by a relatively small minimum reproductive threshold size, which is generally negatively correlated with a relatively large maximum potential body size.
The three largest plants of C. jacea within the population were selected, and 16 additional C. jacea plants were selected to obtain a large range of body size among ‘target’ plants — based on visual estimation, taking account of both height and lateral extent. The three largest plants were easily more than twice as large as the largest plants of any other resident species within the community. For the 16 target plants with a broad range of sizes, individuals were chosen without bias, with the exception that the target plant could not have any other relatively large near-neighbouring plants that belonged to any other species. In other words, within both the inner (TA) and outer (TB) target plant 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 plant-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. If this condition was not satisfied, then the potential target plant was rejected for sampling. In addition, the objective was to assess the effect that a relatively large individual (not a large clump of individuals) of the study species has on the composition of its immediate neighbourhood; accordingly, dense clumps were avoided by selecting only target plants that had no conspecific neighbours that were larger than half of the target plant size residing within its inner or outer target neighbourhood (see Fig. 1). We used dry biomass as our measure of target plant size; however, biomass was strongly positively correlated with height (r = 0.931, P < 0.0001) and lateral extent (r = 0.933, P < 0.0001) which determined the radius of the inner target neighbourhood (all variables log transformed).
For each of target plant, data collection involved 4 stages:
- (1) When the study species (C. jacea) was in the flowering stage (with visible open flowers, but before any flowers/dry mass was lost), a suitable target plant (see criteria above and Fig. 1) was located and its lateral extent (distance from the rooted location to the point of the furthest reaching outer shoots) was recorded. Flag A (see Fig. 1) was inserted at this point and then the target plant was cut at ground level and placed in a paper bag for later dry-weight measurement in the lab. A flag (C, Fig. 1) was placed where the target plant was rooted; then flag B (Fig. 1) was inserted and the radius rA and radius rB were recorded (see above and Fig. 1).
- (2) The perimeter for the two scales of the target neighbourhood was delineated, TA and TB, as described above (Fig. 1): The radius measurements rA and rB were used to calculate the circumferences (2πr) for TA and TB and these circular perimeters were marked with large adjustable metal hose clamps.
- (3) A list of all species residing within TA (the ‘inner’ target neighbourhood), was recorded, with notes indicating, for each species, whether or not at least one of the individuals were reproductive (showing flowers or fruits or evidence of their recent attachment to the plant, e.g. a peduncle).
- (4) A list of all species residing within the donut-shaped ‘outer’ target neighbourhood was recorded, with notes indicating, for each species, whether or not at least one of the individuals was reproductive.