The relationship between individual seed quality and maternal plant body size in crowded herbaceous vegetation

In most natural plant populations, there is a strong right-skewed distribution of body sizes for reproductive plants—i.e. the vast majority are relatively small, suppressed weaklings that manage not just to survive effects of crowding/competition and other hazards but also to produce offspring. Recent research has shown that because of their relatively large numbers, these relatively small resident plants collectively contribute most of the seed offspring production available for the population in the next generation. However, the success of these offspring will depend in part on their quality, e.g. reflected by seed size and resource content. Accordingly, in the present study, we used material from natural populations of herbaceous species to test the null hypothesis that there is no significant relationship between body size variation in resident plants—resulting from between-site variation in the intensity of crowding/competition—and variation in the mass or N content of their individual seeds. Using populations of 56 herbaceous species common in eastern Ontario, total above-ground dry plant mass, mean mass per seed and mean nitrogen (N) content per seed were recorded for a sample of the largest resident plants and also for the smallest reproductive plants growing in local neighbourhoods with the most severe crowding/competition from near neighbours. Mass per seed was numerically smaller from the smallest resident plants for most study species, but with few exceptions, this was not significantly different (P > 0.05) from mass per seed from the largest resident plants. The results therefore showed no general effect of maternal plant body size on individual seed mass, or N content. This suggests that the reproductive output of the smaller half of the resident plant size distribution within these populations is likely to contribute not just most of the seed production available for the next generation but also seed offspring that are just as likely—on a per individual basis—to achieve seedling/juvenile recruitment success as the seed offspring produced by the largest resident plants. This conflicts with the traditional ‘size-advantage’ hypothesis for predicting plant fitness under severe competition, and instead supports the recent ‘reproductive-economy-advantage’ hypothesis, where competitive fitness is promoted by capacity to produce offspring that—despite severe body size suppression imposed by neighbour effects—in turn have capacity to produce grand-offspring.

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