The Botany of Iceland - 01.12.1945, Side 127

THE VEGETATION OF CENTRAL ICELAND
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advance of the glacier, or about the year 1900. In this connection it
should be borne in mind that the first decades after the advance of
the glacier were rather cold, while thc most recent decades have been
fairly warm, so the glacier has, no doubt, receded at a much more rapid
rate during recent years. Thus it is beyond doubt that the vegetation
found on the ground moraine is still growing and that pioneer associa-
tions still occur there. It would be more appropriate to speak of a
vegetation than of associations in this connection, for at such an early
stage of the vegetation, associations in the same sense as in the othcr
formations can hardly have developed. The terminal moraine itself
forms a special division. It was probably invaded by the first plants
as soon as the glacier stopped, and since its bottom contains a greal
deal of humus, it was covered with vegetation much more rapidly than
the ground moraine. At present the greater part of it is entirely covered
with vegetation, but still thc vegetation exhibits some zonation. The
north-facing slope of the terminal moraine, that is to say, the slope
turned away from the glacier, supports a rather scattered vegetation
dominated by Salix herbacea. On the ridge and on the upper part of
the south-facing slope a shrub heath is found with dominant Salix
glauca and S. herbacea, however, the greater part of this slope is
covered'by such a vigorous and luxuriant vegetation that it reminds
one of a well-manured home-field. Gramineae dominate physiognomic-
ally, and the whole character of the vegetation is more like the most
luxuriant patches met with in the lowland than a high-alpine form of
vegetation. Analysis 7 was made in this belt. Dominant species are
Poa alpina, P. pratensis, Festuca rubra, Rumex Acetosa, Phleum al-
pinum, and Equisetum arvense, the three first-mentioned species having
by far the greatest degree of covering. As shown by the table, the
nurnber of species and the density are great, and the biological spectrum
shows a particularly high E percentage, which is in sharp contrast to
other formations at similar levels. Furthermore the high H percentage
and the low Ch percentage are remarkable. On the lowe part of the
slope Equisetum arvense dominates together with Taraxacum croceum,
while the Gramineae are somewhat reduced in number. The vegetation
is more open here, and the Salices become more conspicuous. An un-
common thing in the highland is that large groups of mushrooms
occur here and there in this formation. What conditions have given
rise to this unusual vegetation in this place? First the shelter and the
thick snow-covering found on the south side of the moraine ridge
must be taken into consideration. But it is not sufficient, for if so, only