The Botany of Iceland - 01.12.1930, Side 120
110
H. M0LHOLM HANSEN
loped it is easily distinguished from the other types of vegetation
by the very large knolls of which the surface consists. These often
attain a height of 1 m. or more and 1 or several m in diameter.
The distance between the individual knolls is up to Vs m. The
jaðar vegetation belongs to moderately damp soil having a normal
snow-covering in the winter. The composition of the vegetation
can be shown to differ somewhat in the different localities, and
possibly several types of highland jaðar may with good reason be
established in the future, according to the degree and kind of moi-
sture of the soil, and the amount of shelter provided by the snow-
covering.
On an average, the density of species was c. 12, the number of
species 35, in the jaðar localities examined by me. Ch have become
much less dominant, the Ch percentage is only 20.5 and attains a
relative minimum with this value, whereas the H percentage attains
its maximum here, the average H percentage being' 51.3. The G
percentage which has risen steadily has reached a value of 27; it
is noteworlhy that the Th percentage attains a relative maximum
of 1.5 in the jaðar vegetation.
The peculiarities to be noted in the species group spectrum are
a diminution of the A percentage due in the main to a diminution
of the A 3 percentage, a slight rise in the E 3 percentage, and the
appearance of E 2 species in a quantity of 3.3 per cent.
The more conspicuous chamaephytes are Empetrum nigrum and
the Salix species Salix herbacea, glauca, and phglicifolia; Salix
phglicifolia is the characteristic dominant for jaðar. In more scat-
tered growth there occur Vaccinium uliginosum, Silene acaulis, Armeria
vulgáris, and Cerastium alpinum. For the rest it is H and G which
characterise the vegetation, notably Carex rigida, and in addition,
on the wettest soil, Deschampsia alpina. The more prominent species
are Polggonum viviparum, Thalictrum alpinum, Luzula spicata, Carex
rigida, Equisetum arvense, E. variegatum, Festuca rubra, F. ovina,
Deschampsia alpina, Poa alpina, Rumex acetosa, Galium Normanni,
Cardamine pratensis, Taraxacum officinale, Potentilla verna, Poa pra-
tensis, Calamagrostis neglecta, Viola palustris, and Saxifraga Hirculus.
A number of these species are characteristic of the jaðar.
A couple of types which I had not time to examine more
closely shall be brielly mentioned here. One is the moss mo which
St. Stefánsson has described from Grimstungnaheiði (1894, p. 201
(cf. p. 10)). This type of vegetation was also found on Arnarvatns-