Fjölrit RALA - 20.11.1992, Blaðsíða 82

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The ordination and classification revealed that the vegetation in the study area was
homogeneous and that vegetational differences between plots and sections, which could
be related to the grazing history were not substantial. The vegetational differences found
between plots were more closely related to variation in soil parameters than to grazing
intensity. The plots were divided into two main groups acconding to their vegetation
composition. In the first group were plots with relatively deep soil, rich in organic matter
and with limited extent of bare ground. In this group species, e.g. Carex bigelowii and
Poa pratensis, which make some demand for soil nutrient status were more common. In
the second group were plots with shallower soil, poorer in organic matter content and
greater extent of bare ground. In the group species, e.g. Carex rupestris, Dryas
octopetala and Tofieldia pusilla, which are characteristic of open ground or nutrient
poorer soils, were more prominent.
The sheep were strongly selective in their choice of diet and about half of the
vascular plant species showed signs of having been grazed. Grasses and sedges were
most frequently grazed but shrubs the least. The most preferred species at light grazing
were Carex vaginata, Bartsia alpina, Salix callicarpaea, Poa pratensis and Agrostis
vinealis, while the least preferred species were Thalictrum alpinum, Empetrum nigrum,
Equisetum variegatum, Silene acaulis and Cerastium alpinum. With increasing grazing
intensity the selectivity was reduced and the sheep started to graze species which were
littie or not grazed at light intensity. In the intensely grazed section the sheep grazed
Betula nana considerably leading to the reduction of its cover. The abundance of
individual species in the plant community and the study of diet selection indicates that
Salix callicarpaea, Carex bigelowii, Bistorta vivipara and Armeria maritima are the most
important grazing plants of the range, making up a considerable proportion of the food of
the sheep.
The average carcass weight of twin lambs was 13.8, 12.7 and 11.7 kg in the lightly,
moderately and intensely grazed sections respectively, over the experimental period
1975-1987. In the intensely grazed section where each ewe had on the average 1.6-2.6
ha to graze, the carcass weight of twin lambs dropped to 10.0 kg when the grazing
intensity was at its peak in 1981. The present study showed that the vegetation in the
section had been over-grazed. In the moderately grazed section where the stocking rate
was 2.8-4.0 ha/ewe, the vegetation did not show signs of over-grazing and did not differ
markedly from the vegetation in the lightly grazed section where the stocking rate was
6-13.5 ha/ewe.
The vegetation in the study area has been influenced by sheep grazing for centuries
and it is characterized by species which are tolerant of defoliation or not preferred by the
sheep. The plant communities studied appear to be relatively stable and changes occur
slowly. The vegetation in the sections which had been protected from all grazing for 8