The Botany of Iceland - 01.12.1945, Side 117

THE VEGETATION OF CENTRAL ICELAND
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s e n here describes similar sandy areas to those treated in the present
paper, but where he says (1. c. p. 332) : “Extensive tracts are covered
with different Salices, especially in Fjallasveit”, we are no longer con-
cerned with the sand vegetation but with the shrub heath.
(Table XX. A-B).
If we consider the sand vegetation as a whole and its biological
spectra, it is remarkable that the southern species constitute the great
majority. Thus the E-groups are highly predominant in five of the
eight analyses. This contrasts sharply with the conditions in most other
formations of the highland, especially the formations of the fcll-field
series. The reason for this may perhaps be found in the circumstance
that no other than xerophilous species can, of course, thrive in the dry
and unstable sandy soil. The ability of the plants to resist desiccation
is the factor that determines which species may thrive in the dry, sandy
tracts. On the other hand sand is a warm soil. The basaltic sand is
black and is consequently strongly heated, though the temperature
fluctuations are greater there than in any other kind of soil. But owing
to this strong heating of the sand the soil will often be free of snow
rather early in the spring, which means that the growing plants will
have a longer vegetation period than in other formations.
The chief associations of the sand formation are:
1. Elymus arenarius—Festuca rubra-Ass. (Tab. XX. A-B, 1).
The two character species dominate entirely. The analysis was
made in Arnardalur, where the bottom of the valley is surrounded by
a belt of low dunes largely covered with this association. However,
Elymus is here of relatively low growth and does not seem to have
favourable life conditions. Such patches of Elymus are found here and
there in the regions investigated, but they are nearly always of small
extent. Its accompanying species are nearly always the same as here.
The association is very frequent on Brúaröræfi, right up to Hvanna-
lindir; but it is most frequently met with near Arnardalur, between
this valley and Jökulsá. Where Elymus is least developed, no dunes
will form. A comparison of the Elymus vegetation described here with
for instance the growths of Elymus near Grímsstaðir á Fjöllum, will
show that it is much behind these in development, probably on account
of the altitudinal difference between the two places. In my opinion
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