The Botany of Iceland - 01.12.1945, Síða 149

THE VEGETATION OF CENTRAL ICELAND 491
“ i) Flá is only found where the land is level and where the ground
is comparatively free from snow. Flá will never occur where the
snow-covering is considerable, even though other conditions seem
to be present.
2) The higher the flá is situated above sea-level, the larger are the
rústs; in all flás at the same level the rústs are of almost the same
size.
3) The damper the flá is, the more scattered are the rústs.
4) Ice is found in the rústs all the summer....
5) Rústs and knolls are of the same nature. Where flá and mó meet,
they pass into each other without any sharp limits, and all kinds
of intermediate forms between rústs and knolls are found. Flás
are only found where the mean temperature of the year is below
o°C., and they are the more well developed the lower the annual
temperature is.”
As far as my observations go, they lend support to all the main
points of these views. However, some remarks will be in place. As to
point 1 it may be urged that the investigated flá in Kringilsárrani was
found on steeply sloping ground, so the land need not be quite level
in order that the flá may develop, though this is the rule. As regards
the snow-covering, there can hardly be any doubt that the snow-cover-
ing of the flá is always inconsiderable, the tops of the rústs being large-
ly free from snow in the winter.
As to point 2. All the flás investigated by me are found at almost
the same altitude, still the rústs in them are of different size. Thus
quite probably it is not the altitude above the sea alone which is the
determining factor as to their size. I wish to add, however, that in the
flá in Eystri Pollar, the flá of all those investigated by me which is
situated at the highest level, the rústs are beyond all comparison the
largest. In this connection I would point out that Thoroddsen
(1914, pp. 263-264) says that he has observed the largest rústs near
Úlfsvatn on Arnarvatnsheiði at c. 460 m altitude and on Snæfellsöræfi
at c. 690 m altitude. This seems to be directly contrary to Hannesson’s
contention; I hold the view, however, that if the other factors are
alike, the rústs are the larger the higher the flá is situated.
Point 3 is in full agreement with my observations. A flá will not
develop where the ground is particularly damp, but not in the driest
parts of the mýri either. However, here and there we may meet with
occasional rústs at large intervals in the wettest flói. In this connection
I think that the formation of rústs in Eyvindarver is noteworthy, for
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