Fróðskaparrit - 01.01.2007, Blaðsíða 186

184
THE IMPACT OF GRAZING ON MOUNTAIN VEGETATION
AND THE ARBUSCULAR MYCORRHIZAL SYMBIONT
Discussion
Several short term studies of exclusions of
herbivores have shown that the vegetation
biomass increases (e.g. Austrheim and
Eriksson, 2001). We found some changes
in the vegetation, some of them significant,
as a result of sheep grazing. During the
study period, the biomass increased con-
siderably in the enclosures, which can be
seen by the increase in length of grass-
leaves and vegetation cover.
Our short term results gave a significant
increase in both cover and vegetation
height in the lowland site as well as the
alpine site for thc ungrazed plots. As the
vegetation cover and the vegetation height
is much higher in lowland plots than in the
alpine plots it would be expected that the
gain in biomass was higher in the lowland
site compared to the alpine site. This was
also the case, but the difference was much
smaller than would be expected. We must
also take into consideration here that the
alpine site was sampled after five years of
enclosure and the lowland site after six
years of enclosure. These fmdings indicate
that the alpine site also is a productive site
(Korner, 1999) and that the herbivores
have a profound effect on the alpine eco-
system.
We have seen that five plant species
changed their abundance significantly.
Only small changes in loss and gain of new
species had occurred at this stage, and
maybe the time to detect such changes
must be longer. The abundances of
graminoids increased in grazed plots in the
lowland while the opposite was the case in
the alpine area. In the lowland plots, this
increase was due to increased abundance
of the non-palatable Nardus stricta while
other grass species were decreasing. This
fínding could be explained by competition
between species instead of the grazing ef-
fect. This is also seen by others (e.g. Welch
and Scott, 1995). At the alpine site, Nardus
stricta was not found, but here all grass
species increased their abundance in the
enclosures. This result can be explained by
the stronger effect of grazing in areas of
low biomass and lesser effect due to com-
petition (van der Wal et. al., 2000).
In the Scottish heathland, a 20-years
study showed that Calluna declined at four
sites where there were increases in herbi-
vores. At these sites, graminoids and herbs
showed a general rise in cover, and eri-
coids and lichens decreased. Species in-
creasing significantly included Agrostis
capillaris, Anthoxantum odoratum, Fes-
tuca ovina, Galium saxatilis, Luzula multi-
flora, Nardus stricta and the moss Rhytidi-
adelphus squarrosus. In contrast, De-
schampsia flexuosa was reduced in cover
at the same sites (Welch and Scott, 1995).
The response of Agrostis capillaris to
grazing is variable. Some studies have
found the abundance of Agrostis capillaris
to decrease when grazed (Magnússon and
Magnússon, 1990; Crawley, 1990). Ac-
cording to Magnússon and Magnússon
(1990), the abundance and cover of
Agrostis capillaris decreases with increas-
ing grazing intensity. This study from Ice-
land might be the best predictor for what to
expect in the Faroe Islands, as the climatic
conditions are quite similar.
Other studies have found the abundance