Rit (Vísindafélag Íslendinga) - 01.06.1975, Blaðsíða 148

ly passes into heath, but at same time jaðar vegetation is found in
the wettest parts.” A similar process probably takes place on river
sandbars and on glacial moraines which are developing a vegeta-
tion cover, since the jaðar vegetation is confined to certain humi-
dity conditions, but may evolve either into grassfield or Salix-heath,
or conversely into mýri if the soil beccomes wet.
In accordance with the scope of this paper I will here deal ex-
clusively with the jaðar proper, that is the border area between
mire and the ground.
It is clear that the location and conditions of the jaðar may re-
sult in rather varied and changeable vegetation. It may be a matter
of doubt, therefore, wether it is justifiable to describe it in terms
of sociations or associations, or even to refer to it as an indepen-
dent vegetation type. Perhaps it would be more to the point to
classify it as mire or dry ground according to circumstances. Han-
sen (1930 pp. 52-53) expresses the same opinion, but is however
inclined to designate it as an independent vegetation type since
“it is a factor of some importance that it appears as a very charac-
teristic feature of the landscape in the highland tracts.” It is absolu-
tely correct that the jaðar is a kind of landscape formation, but it
is, however, far from being a characteristic feature of the high-
land regions, although this happened to be the case on Amarvatns-
heiði which was examined by Hansen. According to my experience
the jaðar is more clearly delimited in lowland areas than it is in
the highland.
The limits between jaðar and adjacent vegetation types are
often indistinct and the species composition of the jaðar must de-
pend largely on the contiguous mire and dry ground sociations;
this is probably the principal reason why individual jaðar socia-
tions are not clearly defined. On the basis of my experience, how-
ever, I believe the jaðar vegetation should be grouped into four
main divisions: Juncus jaðar which is, however, separated into two
subdivisions, characterised respectively by J. balticus or J. filifor-
mis. The former accompanies the mire where its border is con-
tiguous with flat grassfield, but the latter only occurs where a
mire tract is adjacent to a grassy hillside. Equisetum-jaðar mostly
occurs on the border of mýri and grassfield. Gramineous jaðar,
between mýri and grassfield, and finally shxub-jaðar, between flói
148