Competition in Heathland along an Experimental Gradient of Nutrient Availability

In a three year field experiment competition between Erica tetralix and Molinia caerulea and between Calluna vulgaris and Molinia was studied at four levels of nutrient (NPK) availability using replacement series. In the monocultures of the unfertilized control productivity of both Erica and Calluna exceeded that of Molinia by a factor two. At the highest fertilisation level the rank order of productivity was Erica 1) and Molinia was outcompeted. Only in the highest nutrient treatment was RYem smaller than 1 and Erica was outcompeted by Molinia. In all nutrient treatments Calluna was the superior competitor (RYcm > 1) and Molinia was outcompeted by Calluna, despite its higher potential growth rate and its greater maximum foliage height. The high competitive ability for light interception of Erica and Calluna with respect to Molinia can be attributed to their evergreen habit, which permits canopy closure early in the growing season. The results emphasize the importance of vertical canopy structure and timing of canopy development in competition for light. However, the abilities to compete for light and mineral nutrients are probably closely inter-dependent.