Rit (Vísindafélag Íslendinga) - 01.06.1975, Blaðsíða 12

which are never under water and on the other hand those which
are continuously flooded. The intermediate types, however, are
more difficult to contend with. This is not to be denied, hut yet it
seems to be that the intermediate types may be classified in the
same manner, i.e. that changing, seasonal water levels cause spe-
cial vegetation forms, although this may be difficult to determine
on the basis of phanerogams exclusively.
Although the ground water level is an important factor in the
development of the mire vegetation, it does not tell the whole story.
The water in the mires is traceable to two sources, on the one hand
pure rain water, which falls directly on to the mire surface itself,
and on the other to adfluvial water. The nature of these two kinds
of water differs and the ratios are subject to seasonal change. Rain
water is poor in minerals and hence it affords no nourishment to
the mire. Adfluvial water which has been strained during a longer
or shorter course through the soil contains varying amounts of
nutritional materials, minerals and even nitrogen, which the water
has dissolved in its course. As far as Icelandic mires are concerned,
it should also be remembered, that they are enriched to a varying
degree in minerals by wind-blown deposits, which results in in-
creased mineral content of the soil, but it is dissolved in the ground-
water regardless of its origins. Thus it is possible that the character
difference between rain water and adfluvial water is not as im-
portant since the mire receives minerals by other means than
water alone. This could for example be the reason for the prevalence
of Sphagnmn were blown deposits are scarce as will be shown later.
All these factors considered, it is apparent that there are in
fact three definitions of mire which emerge in writings: a geological
definition, where the peat formation is exclusively taken into ac-
count, a botanical definition, where the vegetation alone is con-
sidered, i.e. the wet ground is a community of hygrophile plants,
and finally a topographical definition, i.e. one which relies on the
local geography and accordingly on the possibility of adfluvial
water. Here the topographical definition, which is both clear and
simple, will mainly be adhered to. The level of the ground water
and its fluctuation jointly determine the type of vegetation, both
of which are in tum determined by the landscape. The fluctua-
tion of the ground water therefore constitutes in every respect the
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