Studies in the grazing of heather moorland in north-east Scotland. VI. 20-year trends in botanical composition.
Botanical composition and herbivore (red deer, sheep, hares, rabbits and red grouse) usage were monitored over a 20-year period at 15 moorland sites; point quadrats were recorded in fixed positions. Although composition reflected soil type and altitude, Calluna vulgaris was initially the main species at all sites, with cover averaging 61%. Grazing pressures varied from light to heavy, causing wide variation in the utilization of Calluna shoots. Hence, Calluna declined at 4 sites, stayed in balance or showed negligible trend at 4 sites, and increased at 7 sites. At sites with Calluna decline, graminoids and forbs showed a general rise in cover, and ericoids and lichens decreased. Species increasing significantly included Agrostis capillaris, Anthoxanthum odoratum, Festuca ovina, Galium saxatile, Luzula multiflora, Nardus stricta and Rhytidiadelphus squarrosus; Deschampsia flexuosa was reduced in cover. At one site with agricultural reseeding nearby, Cynosurus cristatus, Dactylis glomerata and Lolium perenne became established. At sites with Calluna steady, changes in the main plant groups were small. Bryophytes increased modestly, the chief contributor being Pleurozium schreberi which replaced Hypnum cupressiforme. At sites with Calluna increase, changes were greater when the Calluna sward was continuous rather than patchy. At the former sites graminoids and forbs declined sharply, and bryophytes increased, particularly the pleurocarpous mosses Hylocomium splendens, Hypnum cupressiforme and P. schreberi. Species richness, as measured by the number of contacts with vascular plant species per point-quadrat pin, was much more affected by soil type than by Calluna trend. Species number declined somewhat at sites with Calluna static and increasing; at sites with Calluna decline, an increase in the number of forbs was offset by reduced numbers of bryophytes and lichens.