Studies in the grazing of heather moorland in north-east Scotland. V. Trends in Nardus stricta and other unpalatable graminoids.
Abstract
Botanical composition, grazing treatment and trends in Calluna vulgaris were recorded for periods of 4-15 years at moorland sites ranging in alt. from 70 to 700 m. Tussocks of N. stricta in permanent 1 m2 areas were monitored yearly at 3 sites. Changes in the cover of Juncus squarrosus, Molinia caerulea and Nardus were inversely related to cover trends in Calluna, but only weakly so. J. squarrosus was unresponsive, with only one significant change at its twenty sites; Molinia increased significantly at four sites where Calluna declined. Nardus showed many small increases in cover which were counterbalanced by large decreases at 2 of 23 sites, giving a negligible overall trend. Increases were largest at sheep-grazed sites, but no general relationship could be shown with grazing type. Nardus changes were more consistently related to the state of Calluna as expressed by percentage cover × height than to the trends of Calluna cover. Nardus declined once Calluna mean height was >15 cm. At one site heavy cattle grazing reduced Nardus cover. Many seedlings of Nardus became established, mostly in positions close to mature tussocks. Increment in Nardus tussock diam. was little influenced by grazing, even though tussocks grazed in one summer were more likely to be grazed in the next. It was concluded that Nardus will spread in grazed moorland unless soils are very dry or Calluna is present and allowed to grow tall for periods of several years.