The Botany of Iceland - 01.12.1930, Side 161

STUDIES ON THE VEGETATION OF ICELAND
151
temperature appears especially in the E group spectrum in that the
lower E sub-groups occur abundantly on snow-covered soil, but
are entirely absent from water-covered soil. The E 2 and E 1 species
are such as require a high temperature to be able to thrive, and
this requirement is only satisfied in geiri and forest.
The A group spectrum shows a peculiar difference between the
two types of vegetation. The A percentage is 34 in the geiri, 28 in
the flói. In the flói 20 p. c. of these species are A 1 species, and
only 8 p. c. are A 3 and A 2 species, while in the geiri tlie A 1
percentage is only 4, and the sum total of the A 3 and A 2 per-
centages is no less than 30.
The high A 3 percentage in the snow-covered vegetation and
the low A 3 percentage in the water-covered vegetation must be
assumed to be a consequence of difference of stability in the covering
medium. In the flói the water is always present. hence the vegeta-
tion is never exposed to severe cold. Thus a condition necessary
for the growth of A 3 species is not present.
Compared with the water in the llói, the snow in the geiri is
less stable. A hard frost may set in before the first snowfall, and
frost in the night may affect the vegetation after the snow has
melted. This provides a possibility for the growth of the A 3
species, lience compared with water-covered soil the A 3 percentage
is high, but compared witli snow-bare soil or a vegetation with a
normal snow-covering, it is comparatively low. The A 3 percentage
is higher in geiri than in forest, respectively 23 and 14, which further
confirms the abov.e considerations.
Thus the difference between the species group spectra
for water-covered and snow-covered vegetation is as fol-
lows. The species in tlie species group spectrum of the
water-covered vegetation are largely concentrated in the
central part of the spectrum with a marked dominance
of E species, corresponding to a favourable and fairly
stable winter temperature, and only a slight difference
between the winter and summer temperatures. The spe-
cies in the species group spectrum of snow-covered soil,
on tlie other hand, though also showing a preponderance
of E species, are distributed over the entire scale in con-
sequence of a relatively high winter and summer tempe-
ratúre, and greater instability in the winter.
As previously mentioned, the species group spectrum for the