Fróðskaparrit - 01.01.2007, Síða 180

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THE IMPACT OF GRAZING ON MOUNTAIN VEGETATION
AND THE ARBUSCULAR MYCORRHIZAL SYMBIONT
to changes in habitat conditions (Adler et
al., 2001).
Herbivores do not only have direct ef-
fect on plants by removing biomass but
they may also result in changed competi-
tion for light when biomass is increased as
a result of excluding herbivores. Moderate
grazing in productive areas such as tall
herb and grass meadows normally increas-
es local plant diversity (Austrheim and
Eriksson, 2001; Dupre and Dickmann,
2001) compared to areas where grazing is
excluded. Huisman et al. (1999) consid-
ered the interaction between herbivores
and competition for light. He predicted that
competition for light is more evident in
communities with high productivity. Here
small palatable herbs are out-shaded by
taller less palatable species.
In less productive habitats, grazing will,
however, reduce species richness (Austr-
heim and Eriksson, 2001) although the im-
pact of grazing is seen to be strongest in
areas with low biomass where competition
for light is less (van der Wal et al., 2000).
There is an increasing recognition that
many of the herbivore driven ecosystem
changes are predictable stepwise transi-
tions from one vegetation state to another.
In northem and arctic areas, herbivory
changes lichen-dominated vegetation to
moss-dominated vegetation (Thing, 1984;
van der Wal et al., 2001) and from moss-
dwarf shrub heath to grass-dominated tun-
dra (Olofsson et al., 2001; 2004; Olofsson,
2006). These transitions are also seen in
alpine Skandinavian areas (Wielgolaski,
1975; Austrheim and Erikson, 2001) as
well as Scottish uplands (Alonso et al,
2001, van der Wal et al, 2003). These tran-
sitions toward a grass dominated vegeta-
tion increase the capacity of the area to
sustain a greater population of herbivores
(van der Wal, 2005).
Similar results are seen from Scottish
Calluna vulgaris heath, where the mosses
replace lichens in the understorey vegeta-
tion under moderate grazing pressure
(Welch and Scott, 1995), and when the
heather is replaced by grass dominated
communities after increased grazing pres-
sure.
Relatively few studies have attempted
to quantify the interacting effects of her-
bivory and resource availability of the
competitive balance between plant species
in terms of below-ground growth parame-
ters as well as above-ground parameters,
but knowledge of the differential effects of
herbivory and nutrient availability on plant
performance is important in understanding
the mechanisms by which one species re-
places another (Hartley and Amos, 1999).
One such mechanism is mycorrhizas,
which are benefícial symbioses between
fungi and plants. The arbuscular mycor-
rhizal (AM) symbiosis is the most fre-
quent. In the Faroes, where the current
study is conducted, around 70% of the na-
tive plant species are AM mycorrhizal,
while other forms of mycorrhiza account
for 15% (Olsen and Fosaa, 2002). It >s
known that the symbiosis can provide a
range of benefits to plants, including pro-
tection from pathogens, uptake of micro-
nutrients, and possibly improvcd drought
resistance (Newsham et al., 1995).
Studies examining the effect of defolia-