Competitive interactions between Nardus stricta L. and Calluna vulgaris (L.) Hull: the effect of fertilizer and defoliation on above- and below-ground performance

1) The role of nutrient supply and defoliation on the competitive interactions between pot-grown Calluna vulgaris and Nardus stricta plants was investigated. 2) Young plants were grown alone and together in pots under a combination of fertilizer and defoliation treatments. After 18 months, parameters reflecting both above- and below-ground performance were measured, namely: total above-ground biomass, shoot nitrogen and phosphorus content, root length and the extent of mycorrhizal infection of the roots. 3) In the pots that received fertilizer, the shoot nutrient content and above-ground biomass of Nardus plants increased to a greater extent than those of Calluna plants; this effect was more marked for Nardus plants growing with Calluna plants than for those growing with other Nardus plants. In contrast, Calluna plants growing in competition with Nardus failed to respond to the addition of nutrients. However, in unfertilized pots, Calluna gained more above-ground biomass during the experimental period than Nardus. 4) Calluna had greater root length than Nardus, but Nardus had a higher proportion of its root length infected by mycorrhizal fungi. In both plants, the addition of fertilizer reduced the mycorrhizal infection and increased the root length. Nardus root length was decreased when grown in competition with Calluna only in pots where no nutrients were added. Defoliation decreased the extent of mycorrhizal infection in Calluna roots but not in those of Nardus; defoliation decreased the shoot nutrient content in Calluna plants, but not in Nardus plants. 5) These results suggest that the competitive balance between Nardus and Calluna may be altered by the addition of nutrients, and by defoliation, which may have serious implications for the future dominance of Calluna in heathland ecosystems, particularly those where nutrient inputs are increasing significantly or where grazing pressures are high.