The Botany of Iceland - 01.12.1945, Qupperneq 135

THE VEGETATION OF CENTRAL ICELAND
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or four formations interwoven like a mosaic. But this formation mosaic
forms a unit which only develops when certain conditions are present,
so it must necessarily be treated en bloc in spite of its multiplicity.
Thus it is quite justifiable that Nielsen (1933, p. 242) says: “It
is doubtful whether as regards the vegetation the “flá” is to be consid-
ered as a unity, as its principal peculiarity lies just in an alternation
between swamp-formation of the “flói-mýri” type and large hummocks
whose sides bear a chamæphyte vegetation; in addition there is
still another type of vegetation, found on the top of these hummocks
and, at any rate in certain parts of the highlands, characterized by
Cetraria islancLica”.
The flá is mainly found on level terrain with a multitude of lake-
lets or pools surrounded by level flói or mýri areas chiefly covered with
Eriophorum. Among these areas of flói there occur large knolls or
hills, called “rústir” in Icelandic, which means ruins, or they are
called “dys” (tumuli). I here use the name “rúst”. These hills are of
different size; in the flá areas investigated by me their height varied
from c. 0.75 to c. 2 m, they were up to 15 m in length and up to
10-12 m broad; but often they are much smaller, their length being
2-5 m and their breadth much less. They vary in shape, also, but most
frequently they are oblong, sometimes nearly oval, or perhaps round.
At times they are curved, with a pool in the curvature. Fig. 15 gives
an idea of the shape of the rústs. Sometimes the highest rústs are con-
nected by a lower tufted ridge. The margins of these rústs are likewise
of different shape, somctimes they slope evenly so that the whole rúst
is flattened, in other cases they are steep, the rúst being sharply
delimited from its surroundings. Often, especially when pools occur
quite close to the rúst, the side turning towards the pool is devoid of
vegetation, and the water has, as it were, cut its way into the rúst.
These degeneration slopes in the flá are especially frequent on Snæ-
fellsöræfi.
As already mentioned, the vegetation of the flá is composed of
several formations. The lakes or pools themselves are devoid of vegeta-
tion or support a very poor aquatic vegetation except where Carex
rostrata forms growths near the margins of the pools. Nearest the open
water there is a zone of flói, in which Eriophorum polystaichyum is
always the dominant species, often it is mixed with Calamagrostis
neglecta, and wherc the soil is very sandy, E. Scheuchzeri may be
dominant. In this zone the vegetation is, as a rule, vigorous and does
not differ essentially from the typical highland flói. Next to this belt