How reproductive allocation and flowering probability of individuals in plant populations are affected by position in stand size hierarchy, plant size and CO2 regime

Aims We investigate the effect of position within a size-structured population on the reproductive allocation (RA) and flowering probability of individual plants of Sinapis arvensis. We also assess the effects of plant size and changing level of CO2 on both responses. Methods Sinapis arvensis L., (field mustard), an annual agricultural weed, was grown in monoculture at six densities under ambient and elevated CO2 in a study with 84 stands. Individual aboveground biomass and reproductive biomass were measured. Varying density produced a wide range of mean plant sizes across stands and size hierarchies within stands. Many (;40%) individuals had zero reproductive biomass. Employing a novel modelling approach, we analysed the joint effects of position in stand size hierarchy, plant size and CO2 on RA and flowering probability of individuals. Important Findings We found a strong effect of position within the size hierarchy of individuals in a population: for an individual of a given size, greater size relative to neighbours substantially increased RA and flowering probability at a single harvest time. There was no other effect of plant size on RA. We found a positive effect of elevated CO2 on RA regardless of position within the size hierarchy. These observed patterns could impact doubly on the reproductive biomass (R) of small individuals. First, because RA is not affected by size, smaller plants will have smaller R than larger plants; and second, for smaller plants lower down in a population size hierarchy, their RA and hence R will be further reduced. These results suggest that size relative to neighbours may be independent of and more important than direct abiotic effects in determining RA. Further studies are required to evaluate how these observed patterns generalize to other populations in non-experimental conditions.