CONTROLLED GRAZING STUDIES ON NARDUS GRASSLAND - EFFECTS OF BETWEEN-TUSSOCK SWARD HEIGHT AND SPECIES OF GRAZER ON NARDUS UTILIZATION AND FLORISTIC COMPOSITION IN 2 FIELDS IN SCOTLAND

1. The utilization of the tussock-forming grass Nardus stricta was inv estigated in controlled grazing experiments in which various sward hei ghts of between-tussock grasses were maintained thoughout the growing season using either sheep, cattle or goats. 2. The utilization of Nard us increased as the height maintained for the between-tussock grasses was reduced. The relationship between sward height and level of utiliz ation of Nardus, however, differed greatly among herbivore species. 3. Under similar sward conditions, cattle and goats utilized more Nardus than did sheep. Utilization of Nardus declined over successive season s under grazing by sheep but was sustained under grazing by cattle or goats. 4. The severity of defoliation of Nardus achieved by cattle or goats when between-tussock grasses were maintained at 4.5 cm (mean swa rd surface height above ground) resulted in disappearance of tussocks from the sward and in reduced size, leaf extension growth and nutrient reserves of Nardus tillers. Under cattle grazing, the cover achieved by Nardus decreased from 55.4% to 30.0% over 5 years. 5. Under sheep g razing, both the biomass of individual Nardus tussocks and the cover o f Nardus increased. Cover increased from 58% to over 86% at a between- tussock height of 4.5 cm, while at a height of 3.5 cm, after initially declining, it increased to reach 72%. 6. The broad-leaved grasses sho wed a trend towards increase in cover when between-tussock sward heigh t was maintained at 4.5 cm whether grazing was by cattle or by sheep; the trend was not evident at 3.5 cm (sheep grazing only). 7. Changes i n the floristic composition of the sward are discussed in relation to the roles of the inherent growth characteristics of the species presen t, to selective defoliation and uprooting of the shoots, to altered nu trient cycling pathways under grazing and to the nutrient status of si tes.