Rit (Vísindafélag Íslendinga) - 01.06.1975, Side 13

basis upon wliich the classification of vegetation rests. A closer
exmination, however, reveals that this definition is in most respects
correlated to the botanical definition. In the mires there always
grow such species as either not at all occur or are very rare in
other plant communities, and the composition of species in the
plant sociations is such that they are fully identifiable in accordance
with the landscape, so that it is evident that the height and level of
the ground water is the determining factor in connection with its
fluctuation. H. Molhohn Hansen (1930) offers the same basis for
his classification of Icelandic mires.
Climate and landscape are the factors which determine the forma-
hon of mires. The more favourable either one of these factors is the
less will he required of the other. Thus, where the climate is dry
and therefore imfavourable for development of mires and peat, mires
tinll only form where the landscape is favourable, e.g. where the
land is completely level and without drainage. If the climate is
favourahle, i.e. where it is cool and precipitation is substantial, so
that evaporation is slight, mires can develop almost everywhere.
As stated above the water of the mires consists of two kinds, the
rain water, which is poor in nnnerals, and adfluvial water, which
ls rich in minerals suhstance. Those mires which receive only rain
Water usually become poor in minerals and rich in acid content.
Shortage of oxygen in the stagnant water makes it difficult for
oi'ganic matter to dissolve and peat formation is relatively substan-
tial and rapid. Such mires are called ombrogene and their vegetation
ombrotrophic. Pure omhrogene mires are non-existent in Iceland,
but some sociations in flói (level mire) vegetation are akin to
•ttossar in Scandinavia, and mossar are ombrogene mires. There is,
however, a significant difference, since Carices are not found in
them, but a mire without Carices hardly occurs in Iceland. The
relationship hetween the two is most evident in the Scirpus socia-
tions of the flói.
Those mires which receive adfluvial water are minerogene or
geogene and their vegetation is minerotrophic. Depending on the
topography the ground water of minerogene mires can be either
still or in motion. If it is still, the mire is topogene, but if the water
ls in motion the mire is soligene. Topogene mires are therefore flat
and almost horizontal, and are usually situated in plains, e.g. in
13