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 site has been analysed using a range of techniques. Vegetation change after spraying appears to depend on three things - the species present at the time of control, the initial effectiveness of treatment and the amount of subsequent disturbance (mainly by sheep). Where grazing pressure was low, there was little invasion or spread of plants, except on sites with an initial moderate cover of Vaccinium myrtillus. Where high sheep activity disturbed the litter layer, invasion by the moss Campylopus introflexus characteristically occurred, followed by an increase in dominance by grasses, especially Deschampsia flexuosa. Where grazing pressure was high enough, it appeared that bracken regeneration was slowed, otherwise the bracken canopy and the stand recovered. However, in the timescale of the study, sufficient variety and cover of other species remained after the canopy regenerated to prevent the complete progression to vegetation characteristic of untreated areas.