Influence of arbuscular mycorrhizal infection on competition between Holcus lanatus and Dactylis glomerata

1 Holcus lanatus and Dactylis glomerata were grown in monocultures and mixtures to determine the influence of mycorrhizal fungi on intra- and interspecific competition. 2 In monoculture, shoot biomass per plant of both species was increased over all densities by mycorrhizal infection. Increased tiller and leaf numbers per plant for H. lanatus, but not for D. glomerata, suggested that enhanced biomass was a result of greater tiller and leaf production in Holcus, but to increased tiller weight in Dactylis. Increased competition reduced shoot biomass, tiller number and leaf number in both species. 3 Although the response was variable, mycorrhizal infection generally enhanced shoot biomass of both species in mixed cultures. Tiller production in D. glomerata was generally unaffected by infection but reductions in leaf number were observed in mycorrhizal stands. In contrast to its effects on monocultures, infection had no effect on tiller production when H. lanatus was in mixture. 4 Relative yield totals (RYT) calculated for grasses grown in mixture suggest that, overall, mycorrhizal infection resulted in a reduction of resource complementarity, i.e. it increased the level of competition for the same resources. This was reflected in the relative yields which showed that the shoot biomass for each species in mixture was lower than that expected from growth in monoculture. 5 Aggressivity indices suggested that H. lanatus was more aggressive than D. glomerata when present in equal or greater numbers. Mycorrhizal infection altered the degree of aggressivity in favour of the already more aggressive plant within the combination, although, at very low densities, D. glomerata was more aggressive than H. lanatus when mycorrhizal.