• Below-ground interactions;
  • CO2 enrichment;
  • plant communities;
  • plant populations;
  • root allocation

1. Root competition can be an important determinant of the performance of neighbours within plant populations and communities. Because plants often maintain larger root systems and allocate more of their carbon to root systems under elevated atmospheric CO2 than they do at lower CO2 concentrations, root–root interactions could play an increasingly important role in determining competitive outcomes among individuals and plant species as global CO2 concentration continues to rise.

2. We established 12 pure stands of Linum usitatissimum (flax) and 12 mixed stands of Linum and its naturally co-occurring weed species Silene cretica in opaque plastic trays each filled with the same amount of nutrient-rich soil mix. In half of the trays from each of these stand types, vertical waterproof partitions separated the root systems of individual plants from each other to prevent root competition, while in the other half no partitions were present. Half of the trays from all treatments were allowed to grow under low atmospheric CO2 concentration (320μll–1) and the other half under elevated CO2 (600μll–1), in daylight growth chambers for 30 days from seedling emergence until harvest in mid-June. All trays received equal amounts of water so that soils in the low CO2 treatment were maintained at field capacity.

3. Our results indicate that under high soil fertilities: (1) intra-specific root–root interactions alone play a relatively insignificant role in determining plant biomass production within pure Linum populations and (2) the impact of an aggressive species (Silene) on co-occurring less aggressive species (Linum) becomes more severe under elevated CO2 as a result of amplified interspecific root competition.