Fróðskaparrit - 01.01.2004, Blaðsíða 234

232 ALTITUDINAL DISTRIBUTION OF PLANT COMMUNITIES IN THE FAROE ISLANDS
by structural features such as vegetation
height and density.
The uniformity of Faroese vegetation
makes it difficult to classify it into plant
communities and also makes the differenc-
es between communities small. The main
reason for this is the extreme oceanic envi-
ronment (0 on the Conrad scale: Crawford,
2000). This results in a uniformity of the
composition of life-forms (Fosaa, 2003)
due to the absence of trees and to the ubiq-
uitous presence of species like Racomitrium
lanuginosum, Festuca vivpara and Agrostis
capillaris. The extreme oceanic environ-
ment and centuries of heavy grazing have
modified the vegetation, giving it the tun-
dra like appearance that it has today.
Despite a lack of trees, the islands are
situated south of the limit of tree growth,
since the mean temperature for the warm-
est month in the Faroe Islands is over 11 °C,
one degree higher than the mean July iso-
therm (10°C), which is usually used to de-
fine the limit for tree growth as well as the
lower limit of the alpine zone in temperate
areas (Billings and Mooney, 1968; Troll,
1973; Korner, 1998). Therefore, one would
expect to find vegetation types in the Faroe
Islands comparable to oceanic areas in the
temperate zone (Tuhkanen, 1987), rather
than the arctic zone.
The altitudinal distribution of vegetation
into the three main vegetation zones found
in this study (Fig. 3), largely corresponds
with the climate zones defíned by Humlum
and Christiansen (1998) and Christiansen
and Mortensen (2002) based on temperature
and periglacial activity. They propose a low
arctic zone frorn 200 m a.s.l. and an arctic
zone from around 400 m a.s.l. This altitude
level for the low arctic zone corresponds
well with the upper limit of the moist dwarf
shrub vegetation and the lower limit of the
moist grassland vegetation. A 400 m a.s.l.
boundary for the arctic zone corresponds
with the upper limit of the moist grassland,
and the lower limit of the Racomitrium veg-
etation and open grassland vegetation. With
a lapse rate of 0.8°C (Koppen, 1920), the
10°C isotherm is found in the upper part of
the temperate zone in the Faroe Islands.
Alpine vegetation zone
Characteristic species for the alpine zone
are species with their main distribution in
the arctic area and in the mountain areas
of the temperate zone. Examples are Bis-
torta vivipara, Carex bigelowii, Salix her-
bacea, Sibbaldia procumbens, and Thalic-
trum alpinum (Hulten and Fries, 1986). In
this zone, two main vegetation types were
found - open grassland vegetation and
Racomitrium vegetation.
In the open grassland vegetation, there
are four plant communities. The Koenigia
islandica community is characteristic for
two types of habitats, fellfields and screes.
This reflects the fact that the annual life-
form of Koenigia islandica favours areas
with no competition from other species and
unstable soil. Similar communities have
been described for the Faroe Islands from
high altitudes (Koenigia islandica-Ranun-
culus glacialis community: Bocher, 1937)
and from low altitudes (Koenigia islandica-
Ranunculus ellipticum community: Hobbs
and Averis, 1991). Koenigia islandica has
a circumpolar distribution, although the