Fróðskaparrit - 01.01.2002, Side 124

122
THE MYCORRRHIZAL STATUS
IN MOUNTAINOUS VEGETATION IN THE FAROE ISLANDS
Mineralisation is slow in cold environ-
ments, and organic materials accumulate in
the soil, including organically bound phos-
phorus and nitrogen, which are unavailable
for the plants. Arbuscular mycorrhizal fun-
gi explore the soil better than the plant roots
do simply because the hyphae are crossing
the depletion zone and explore soils for nu-
trients, which physically are inaccessible to
the plant roots (Fitter and Hay, 2002). It is
increasingly evident that ericoid and some
ecto-mycorrhizal fungi produce extra-cel-
lular enzymes that break down organic
compounds, thereby exposing new sources
of organic nitrogen and phosphorus to at-
tack (Smith and Read, 1997). Until recent-
ly it was assumed that arbuscular mycor-
rhizal fungi do not utilise the organic
sources of nitrogen and phosphorus. How-
ever, Hodge et al. (2001) showed that the
arbuscular mycorrhizal symbiosis can both
enhance decomposition of and increase ni-
trogen capture from complex organic mate-
rial in soil. Further there is also evidence
that some of the carbon cost of mycorrhizal
infections may be met by heterotrophic car-
bon assimilation from organic sources
(Finlay et al., 1992).
The Faroes and the vegetation in general
The Faroese vegetation is mostly perennial
and is influenced by several factors. The
two most obvious are the oceanic climate
and the ubiquitous sheep. The landscape is
mostly dominated by steep mountains.
The temperature is relatively constant in
both winter and summer, with an average of
4°C in the coldest month (January) and
11°C in the warmest month (July). The cli-
mate is oceanic with much precipitation,
clouds and wind.
The sheep consist of 70,000 ewes, graz-
ing the mountains both summer and winter.
The number of animals is assumed to be
held just below the overgrazing limit.
In general the area is eroded, and accord-
ing to farmers the situation has become
worse during the last century. Probably
both weather conditions and overgrazing
are to blame for the erosion.
The vegetation consist mainly of herbs,
although some dwarf shrubs occur. Trees
are only present as single trees planted in
gardens or in plantations at some very shel-
tered locations.
Material and methods
In July-August 1999 and 2000 the vegeta-
tion on five mountains in the Faroe Islands
was investigated along five transects with
north and south aspects, from the top (be-
tween 750-850 m a.s.l.) down to 150 m
a.s.l. The dominant vegetation on these
mountains is open grassland vegetation,
Racomitrium vegetation and moist dwarf
shrub vegetation (Fosaa, 2002).
A total of 532 plots were sampled. With-
in 50 m altitudinal intervals along the tran-
sects, 10x10 m quadrats (macro-plots)
were placed. In each macro-plot, 8 smaller
(0.5x0.5 m) quadrates (sub-plots) were
placed. These subpiots were again divided
into 25 quadrate (micro-plots) and the fre-
quencies of the plant species were mea-
sured based on presence/absence in each
microplot, from 1 to 25. Soil samples were
taken at each macroplot (0-10 cm depth)
and the pH, humus (% loss of ignition),