SUCCESSION FROM HEATHER MOORLAND TO BIRCH WOODLAND .1. EXPERIMENTAL ALTERATION OF SPECIFIC ENVIRONMENTAL-CONDITIONS IN THE FIELD

(1) The effects of experimental alteration of light intensity, nutrient availability and simulated grazing on plant communities beneath different ages of birch are examined. Plant responses are discussed in relation to their role in the succession from heather moorland to mature birch woodland. (2) A generalized sequence of changes in species dominance as the birch (Betula pendula and B. pubescens) ages is identified. Calluna vulgaris gradually declines and is replaced by Vaccinium myrtillus as the birch canopy closes. This is subsequently replaced by Deschampsia flexuosa and then Agrostis capillaris as the woodland matures and the canopy opens out. (3) The growth of Calluna was greatly reduced in shaded plots and nutrient addition had no significant compensatory effect. Empetrum nigrum showed similar growth responses. Vaccinium myrtillus and Deschampsia flexuosa also grew less well in reduced light but D. flexuosa responded positively to nutrient addition. (4) Clipping reduced the growth of Calluna, Empetrum nigrum, Vaccinium myrtillus and Deschampsia flexuosa in most plots. Only Galium saxatile showed increased growth after clipping at one site. (5) Competitive interactions between the main species were apparent and are further examined by the second paper in this series.