The Botany of Iceland - 01.12.1945, Page 61

THE VEGETATION OF CENTRAL ICELAND
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the jaðar differs from both the mýri and the heath vegetation. It should
then according to its position and other characteristics most naturally
be referred to the jaðar.
The jaðar formation as a rule occurs in the outer part of the mýri,
on somewhat higher-lying ground. The surface is mostly knolly, but
the knolls are generally small, of the same size or slightly larger than
those of the mýri. In the highland tracts investigated by me the knolls
are altogether small, the “rústir” of the “flá” excepted. The most
conspicuous species of the jaðar formation are Juncus balticus and
Calamagrostis neglecta, less conspicuous are Salix glauca, S. herbacea,
Deschalmpsia alpina, and Festuca rubra; the two last-mentioned species
are hardly found in the mýri vegetation.
(Tables VII. A-B).
The biological spectra show that the jaðar vegetation surpasses the
mýri vegetation as regards number and density of species, but in this
respect the jaðar described here ranges far behind the jaðar of Arnar-
vatnsheiði (Molholm H a ns e n 1930, p. 107). Of the species
groups the arctic plants preponderate, but they are distributed over all
three groups, whereas the mýri contained almost exclusively plants of
group A3. The H percentage is, as a rule, highest among the life-forms,
though in some few associations it is surpassed by the G percentage.
The Ch percentage is generally high, as a rule higher than in the mýri.
Accordingly the jaðar vegetation might be characterised as a hemi-
cryptophyte vegetation with a high chamaephyte percentage. I assume
that the absence of the typical form of jaðar, i. e. a high knolly ridge
at the boundary between the mýri and the mó, is largely due to the fact
that the country is too level. The difference in height between the mýri
formation and the nearby heath formation is too inconsiderable. In all
probability the formation of jaðar in the lowland is often a kind of soli-
fluction phenomenon. The high row of knolls is formed around the mó
because the mó, which is higher than the adjoining mýri, is flattened
out by its own weight, but near its borders this levelling movement
is counteracted, with the result that the knolly ridge is formed. As
pointed out above, the jaðar vegetation described here is found on
moderately damp soil, mostly in a zone between the mýri and the
heath formation. Its surface is most frequently without inclination, as
a rule somewhat knolly, but the knolls are of small size. The number of
species is moderately high, and the arctic species are in the majority.
H is the life-form most abundantly represented, but the Ch percentage