Fróðskaparrit - 01.01.2007, Síða 181

HVUSSU SEYÐABIT ÁVIRKAR FJALLAVØKSTUR OG ARBUSKUL SOPPRÓT
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tion on AM fungi have shown both posi-
tive, negative, and neutral responses. Daft
and Al-Giahmi (1978), Bethlenfalvay and
Dakessian (1984), Bethlenfalvay et al.
(1985), Gehring and Whitham (1994) all
found a decrease in the colonisation due to
herbivory. However, more recent studies
have suggested an increase in the colonisa-
tion due to herbivory (Eom et al., 2001,
Gehring et al., 2002, Kula et al., 2005).
Bentivenga and Hetrick (1992) and Lugo
et al. (2003) found no change in the coloni-
sation. Hokka et a/. (2004) found that the
effects of grazing depended on the plant
species, as they found a decrease in some
grasses, and increase in others. Hokka et
al. (2004) concluded that there is an inter-
action between mycorrhizal infection and
defoliation, which is likely to be extremely
complex.
In the present study, we investigated
one grassland site in the lowland and one
alpine vegetation type, where sheep has
been excluded from both sides. The aim
was to follow the changes in the vegetation
composition and arbuscular mycorrhiza
symbiont and to see if there was an interac-
tion between these changes. No previous
experimental study of the grazing impact
on the vegetation in the Faroe Islands has
been carried out before. Neither has the in-
teraction between the vegetation and the
mycorrhizal symbiont been studied. Thus,
there was limited knowledge, which plant
species would be the best indicators for
grazing impact. We chose the most com-
mon grass in the Faroes, Agrostis capil-
laris, as a representative for palatable
grasses. With this material we have tried to
answer the following questions: How does
sheep grazing change the vegetation com-
position? Does grazing by sheep change
the arbuscular mycorrhizal colonisation
pattern? And, lastly: Does grazing by
sheep change the interactions in arbuscular
mycorrhizal symbiosis in terms of com-
bined plant and fungal responses?
Description of sites
The study sites used in this experiment are
two enclosures, established in 2000. One
of the sites is located at 600 m a.s.l., the
other enclosure at 70 m a.s.l. The study
sites are on the mountain Somfelli
(62°04'10"N, 6°57'25"W) on Streymoy
in the central part of the Faroe Islands.
The vegetation at 600 m a.s.l. reaches
from sparsely vegetated open grassland to
richly vegetated Racomitrium heath. The
plant communities found on the site are the
species-poor Koenigia islandica-commu-
nity, and the species-rich communities
Festuca vivipara-Agrostis capillaris-com-
munity, Racomitrium-community and
Racomitrium-Salix herbacea-community
(Fosaa 2004).
The vegetation at 70 m a.s.l. is moist
grassland vegetation. The most common
plant community at this site is Nardus
stricta-Potentilla erecta community but
Festuca vivipara-Agrostis capillaris-com-
munity, Racomitrium-community are also
present (Fosaa 2004).
The whole area is grazed by sheep,
about 45 ewes per km2 (Thorsteinsson,
2001).