The Botany of Iceland - 01.12.1945, Blaðsíða 65

THE VEGETATION OF CENTRAL ICELAND
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level. In Arnardalur, where the association occurs, also, it is only found
in small patches, mostly in connection with the jaðar association 4.
If we consider the distribution of the jaðar associations in the in-
dividual parts of the highland, we shall see that the Juncus balticus
jaðar is almost exclusively found on Brúaröræfi and Síðumannaafrjet-
tur, whereas the Calamagrostis jaðar occurs chiefly on Landmanna-
afrjettur. In the last-mentioned area Juncus balticus is a rare species.
On Snæfellsöræfi the jaðar is but slightly distributed; possibly the
river banks there might be referred to the jaðar, but its species composi-
tion shows such a close relationship to the Salix heath that I have pre-
ferred to refer it to that association. The relation of the Juncus and
Calamagrostis jaðar to moisture and snow-covering respectively seems
to be that the Juncus jaðar is found in the driest places, where the
snow-covering is probably thinnest, while the Calwmagroslis jaðar pre-
fers the lowest-lying, wettest places. This appears very clearly from
the relative position of the associations where they occur in company,
for instance in Arnardalur.
It will be appropriate to make some remarks on the occurrence of
Calamagrostis neglecta in different formations and under highly variable
conditions as regards the snow-covering of the ground, the degree of
moisture, etc. It would seem that when the soil is only sandy, and
especially if fresh quantities of sand are constantly added, either fluvial
sand or blown sand, Calamagrostis may thrive as long as the soil is
not too dry. On damp sandy flats, which are being overgrown, the
succession of the associations is often very easy to follow. The pioneer
association is the Eriophorum Scheuchzeri association. This species oc-
curs either alone or possibly mixed with Calamagrostis, which may
sometimes be subdominant. At this stage the moisture is so great that
the surface is constantly flooded, that is to say, it is a flói formation.
Where the sand is not so wet that it is flooded, the initial stage is
generally a Calamagrostis—Equisetum arycníc-association, often strongly
mixed with Eriophorum Scheuchzeri. As the ground, owing to constant
supplies of fresh sand, gradually rises more and more above the surface
of the water and becomes drier, Calamagrostis takes the lead, while
Eriophorum Scheuchzeri gradually disappears, and a number of new
species appear. Thus at first an association rich in Calamagrostis ne-
glecta and Eriophorum Scheuchzeri will be met with (cf. Tab. IV, 1-4).