The Botany of Iceland - 01.12.1930, Blaðsíða 130

120
H. M0LHOLM HANSKX
of 31, the H percentage is 53, the A percentage 44, the A 3 percen-
tage 24, and the E (3—2—1) percentage 35.
The Geranium belt represents the area where the characteristics
peculiar to geiri are most striking. If we pass from the marginal
zone through the Geranium belt to the bottom vegetation, or from
a snow patch with a southern exposure to one with a northern
exposure, the southern contingent is largest in the Geranium belt
or the snow patch with the southern exposure, while it again de-
creases in the bottom vegetation or on the northern slope. This
is especially due to a decrease in the southernmost E subgroups.
Looking for the cause of this change we find it in the unusu-
ally long period during which the formations in question are covered
with snow. In the case of the southern types of plants, which
obtain the most favourable life-conditions in the Geranium belt (that
is to say, the greatest possible protection from the winter cold and
the most favourable temperature in the period of vegetation), the
long-lasting snow-covering causes the vegetation period to become
too short for these plants.
The number and density of species which attain their highest
values, 36 and 15 respectively, in the mo, have the values 32 and
13 in the Geranium belt. This diminution is continued in the
bottom vegelation, so that here the number of species is only 21
and the density 10.
If these changes are continued, wilh the successive diminution
of the number of species and the quantity of southern species, as
a final result we may anticipate to find, at still greater altitudes
above the sea, the Salix herbacea and Sibbaldia formations described
by Helgi Jónsson and at still higher levels the Anthelia-vegetation.
At the level at which Arnarvatnsheiði is situated, in the Iower
zone of the mountain region, we have thus the following types of
vegetation.
I. Melar, conditioned by a slight snow-covering and a compara-
tively strong desiccation of the soil.
II. The Betula nana mo, where the snow-covering is deeper
and the moisture of the soil greater: while the melar vegetation is
peculiar to the denudation area of solifluction, the Betula nana mo
is peculiar to the accumulation area.