Effect of NH4-fertilization on the maintenance of a Calluna vulgaris vegetation

NH4-fertilization in a natural, undamaged Calluna vegetation did not result in an immediate replacement of Calluna vulgaris by grasses. However, obvious changes were found in plant and soil composition that may lead to a greater stress sensitivity, but also to a lower regeneration capacity and a lower growth of C. vulgaris. Vegetative expansion of Molinia caerulea and Deschampsia flexuosa was higher in a Calluna vegetation damaged by a severe frost in the winter of 1986/1987 compared to growth in undamaged plots. The higher stress sensitivity of C. vulgaris due to nitrogen deposition may accelerate opening of the vegetation canopy. Once the vegetation has been opened, the negative effects of the high nitrogen input by the lower regeneration capacity and lower plant growth may speed up even more the transition of heathland into grassland.