MOORLAND VEGETATION SUCCESSION AFTER THE CONTROL OF BRACKEN WITH ASULAM

A range of moorland sites in the North York Moors, North Yorkshire, UK , where bracken control with the herbicide asulam has been carried out , has been re-surveyed. The trajectory of vegetation change at each si te has been analysed using a range of techniques. Vegetation change af ter spraying appears to depend on three things - the species present a t the time of control, the initial effectiveness of treatment and the amount of subsequent disturbance (mainly by sheep). Where grazing pres sure was low, there was little invasion or spread of plants, except on sites with an initial moderate cover of Vaccinium myrtillus. Where hi gh sheep activity disturbed the litter layer, invasion by the moss Cam pylopus introflexus characteristically occurred, followed by an increa se in dominance by grasses, especially Deschampsia flexuosa. Where gra zing pressure was high enough, it appeared that bracken regeneration w as slowed, otherwise the bracken canopy and the stand recovered. Howev er, in the timescale of the study, sufficient variety and cover of oth er species remained after the canopy regenerated to prevent the comple te progression to vegetation characteristic of untreated areas.