Fróðskaparrit - 01.01.2002, Side 127

SOPPARÓT í FØROYSKUM FJALLAVØKSTRI
125
low, decreasing from the mountain-tops
downward. Average pH is 5.5.
The methods used to measure soil phos-
phorus concentrations are based on the
work of Olsen et al. (1954) and Banderis et
al. (1976). It is a spectrophotometric esti-
mate of extractable phosphorus in soils.
The phosphorus levels are low, approxi-
mately 10 mg kg'1 at high altitude, but are
increasing downslope (Fig. 1).
The level of soil total nitrogen is on av-
erage 0.6% and is a measure of both inor-
ganic and organic nitrogen (Fig. 1).
The humus content is relative high. The
loss of ignition was on average 25% (Fig.
1).
Thus the Faroese soils are phosphorus-
limited, while it is not known whether the
nitrogen sources are directly available for
the plants.
Arbuscular mycorrhiza
Arbuscular mycorrhiza is considered as the
most ancient form of mycorrhiza, and is
more or less invisible, except microscopi-
cally. This symbiosis occurs mainly with
herbs. The fungal partner in this mycor-
rhiza belongs to the zygomycetes, and pro-
vides the plant with mainly phosphorus
(Fitter, 1985), by exploring the soil better
than the plant roots are able to do.
One striking thing with this symbiosis is,
that while perhaps 200,000 plant species
worldwide are involved, less than 200 fun-
gal species have been described.
Sixty-one plant species were found in
this vegetation survey which are known
able to form arbuscular mycorrhiza. In ad-
dition to these there are 10 plant species,
% of total AM plant species % of total AM plant tYequency
Rarely AM 21.1 11.2
Occasionally AM 19.7 9.3
Normally AM 45.1 74.3
Probably AM 14.1 5.2
Table 6. The distribution ofarbuscular mycorrhizal
(AM) plant species. First column shows how tlie
AM categories are distributed based on the number
of AM plant species involved. The second column
shows how the different groups of AM plants are
distributed based upon tlie sum offrequencies.
which probably form arbuscular mycor-
rhiza, estimated from their relatives (Table
1).
Attempts have been made to clarify how
likely a species is to form mycorrhiza
(Harley and Harley, 1987; Fitter and Peat,
1994). In the Ecological Flora database
from the University of York, Fitter and Peat
(1994) used the coding:
never mycorrhizal
rarely mycorrhizal
occasionally mycorrhizal
normally mycorrhizal
Using this coding, the group »Normally ar-
buscular mycorrhizal« is the most dominat-
ing (Table 6). 45% of the arbuscular myc-
orrhizal species (31% of the total plant
species in this survey) are normally arbus-
cular mycorrhizal. In addition, they ac-
count for 74.3% of the arbuscular mycor-
rhizal frequency (54% of the total sum of
frequencies). From this follows, that it
seems to be beneficial for the plants to be
normally arbuscular mycorrhizal.