Within crowded natural plant populations, the traditional prediction is that most of the offspring from which future generations are drawn will be contributed by the relatively few individuals belonging to the larger size classes. Yet, the extent to which this is true should depend on the extent to which the inevitably more numerous, but relatively small suppressed plants within the same population manage not only to survive suppression, but also to reproduce before death. We recorded the above-ground dry mass for mature reproductive plants from natural populations of 21 species of herbaceous angiosperms. The size distributions of these reproductive plants were all strongly right-skewed, and in every case, the vast majority of the estimated offspring production within the population was contributed by the three, four, or five smallest deciles of the plant size distribution. Our data suggest, in contrast with traditional theory, that most of the coexisting species within crowded vegetation are successful residents not because they are relatively large, but because they produce numerous descendants from numerous offspring that have ‘reproductive economy’—i.e. offspring with the ability, despite suppression to a very small size, to also produce offspring of their own for the next generation.
Natural populations of 21 herbaceous plant species were sampled from the Kingston, Ontario region during the summer growing season of 2006. Species were chosen based on local availability, habitat accessibility, population size, and absence of major habitat disturbance for the duration of the growing season. Collections sites included grasslands, old fields, and woodland edges where the study species were coexisting with other species. Species were also chosen to obtain representation of all three major plant life histories: 10 annual species, 5 biennial species, and 6 perennial species were sampled (Table 1).
Each population was monitored for reproductive maturity, and harvesting was carried out when the plants had finished flower production for the season and seed dispersal had begun. Within each population, a 1 × 1 m plot was placed within that portion of the population that had the highest density based on visual estimation, assuming that this was where the effects of crowding would be most intense. Within the plot, reproductive plants (with dispersing seeds) were harvested individually above-ground and placed into separate paper bags. Harvesting involved the collection of all reproductive plants occurring within one quadrant of the plot at a time, until a minimum sample size of 200 reproductive individuals was obtained. In some cases, additional plots were required to reach this minimum sample size. In the case of Pilea pumila, an entire 1 m2 plot was harvested, yielding 788 reproductive plants. In two cases, fewer than 200 plants were available (Erechtites hieracifolia, N = 121; and Ambrosia artemisiifolia, N = 189). The number of suppressed plants that had died (e.g. due to self-thinning) during the growing season could not be assessed. However, when non-reproductive plants were present within plots at final harvest, they were virtually always smaller than reproductive plants, and presumably therefore below the size-threshold for reproduction.