Fróðskaparrit - 01.01.2007, Side 187

HVUSSU SEYÐABIT ÁVIRKAR FJALLAVØKSTUR OG ARBUSKUL SOPPRÓT
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of Agrostis capillaris to decrease when the
grazers were removed (Hartley and
Mitchell, 2005; Huline et al., 1999). This
decrease has been explained by a change in
the plant coinposition to more grazing-in-
tolerant plant species such as Deschampsia
flexuosa and Molinia caerulea (Hulme et
al., 1999).
The palatable grass Agrostis capillaris
did not have a signifícant response to graz-
ing, but the response was very similar to
the response of Agrostis canina, which had
signifícant increase in abundance inside
the enclosure, indicating that these palat-
able grasses were affected by grazing. The
response of grazing was more root growth
and greater %RLC. Both these factors are
known to increase nutrient uptake by the
plant. While keeping sheep out of the plots,
the enclosure also had one more unin-
tended impact: it also eliminated one nutri-
ent resource otherwise available for the
plants, namely sheep urine and sheep
dung. This could make species like
Agrostis capillaris more vulnerable than
other species, as A. capillaris has the high-
est nutrient requirements in Table 1. A
study using Agrostis capillaris to examine
the impact of defoliation on the distribu-
tion of nitrogen between shoot, roots, and
soil nitrogen pool found that root biomass
was decreased by regular defoliation and
this coincided with greater amounts of ni-
trogen stemming from sheep urine (in this
case l5N-labelled synthetic sheep urine)
(Williams et al., 2003).
The good correlation between %FR and
%RLC might suggest that greater colonisa-
tion would be expected if increased fíne
root production occuned. The observed
differences in mycorrhizal colonisation in
response to grazing may have been due to
changes in the fraction of fine roots in re-
sponse to grazing, rather than a direct re-
sponse of the mycorrhizal fungi them-
selves.
Conclusions and future perspectives
Three questions were posed. Firstly, how
sheep grazing changes the vegetation com-
position. In this study we have found that
sheep grazing decreased the biomass of the
vegetation in both lowland and alpine
areas in tenns of lower vegetation cover
and lower vegetation height. We also
found that the abundance of palatable
graminoids decreased in grazed plots.
Secondly, whether grazing by sheep
changes the arbuscular colonisation pat-
tem. This can also be answered positively
as we found a higher fraction of fme roots
as well as greater colonisation in grazed
plots, in particular at the alpine site.
This study did not give a clear answer
to the third question, whether grazing by
sheep changes the interactions in arbuscu-
lar mycorrhizal symbiosis in terms of com-
bined plant and fungal responses. The
grass species used may not be appropriate,
as A. capillaris might be more dependent
on nutrient availability than other grasses.
Future studies should include nutrient
availability as a factor, as well as include
several plant species, which should be
studied both in terms of specific biomass,
grazing preferences as well as mycotrhizal
colonisation.