Competitive size hierarchy, reproductive allometry, and adaptive strategies of annual plants

Background/Question/Methods

Plant size and reproductive allocation (RA) determine reproductive output, an important plant fitness component.  Assuming a tradeoff between plant size and RA, many studies have emphasized RA in plant reproductive strategy.  However, if RA changes with plant size, reproductive allometry is more appropriate as a plant reproductive strategy.  Several studies have shown differing patterns of size dependency of RA of annual and perennial plant species: positive size dependency for annuals and negative size dependency for perennials.  This paper describes how competitive interaction influences plant reproductive strategies by fostering a reproductive hierarchy in a plant population.

Results/Conclusions

Reproductive outputs of eight families of Abutilon theophrasti, raised under different nutrient treatments in the absence and presence of competition, were compared.  The eight families’ outputs showed a highly significant difference without competition, but the significant difference did not pertain under competition because competition amplified stochastic variation included in individual size variation at an early developmental stage.  A Monte Carlo simulation was conducted to elucidate long-term influences of competition and stochastic variation on the population structure.  The following results were obtained. (1) Small stochastic variation relative to the difference between the competitors in asymmetric competition intensifies their mutual reproductive difference, causing competitive exclusion of inferior competitors and dominance of specialists. (2) Given large stochastic variation, competition mitigates their mutual difference and engenders long-term coexistence.  However, that coexistence was attained differently for small and large magnitudes of competitive asymmetry: through coexistence of competitors within a patch under low asymmetry, but through dominance of different competitors between patches under high asymmetry.  The results suggest that large stochastic size variation and a large magnitude of competitive asymmetry, which are typical habitat conditions for annual plant species, favor generalists with high colonizing ability and broad physiological tolerance.  Positive size dependency of RA in annual plants apparently reflects an adaptive strategy to cope with stochastic environmental variation and asymmetric competition.