The Botany of Iceland - 01.12.1945, Síða 60

402
STEINDÓR STEINDÓRSSON
Cerastium trigynum
Saxifraga stellaris
„ rivularis
Poa alþina Oxyria digyna
Polygonum viviþarum Veronica alþina
Deschampsia alþina
Comparing these lists with those given by J ó n s s o n, we shall see
that the species of Epilobium, Montia rivularis, and Catabrosa aquatica
listed byjónsson as the commonest species of the dý vegetation are
absent here. These species are either very rare or absent altogether
within my area of investigation, and they are no doubt all very rare
at altitudes above 500 m. Thus we may speak of two types of dý
vegetation: the lowland type dominated by Epilobium species, Montia,
and Catabrosa aquatica, and the alpina type, in which all these species
are lacking, while Cerastium trigynum, Deschampsia alpina, Erio-
phorum Scheuchzeri, and various Saxifraga are the commonest species.
d. The Jaðar Vegetation.
This formation was first described from Iceland by M 0 1 h o 1 m
H a n s e n. It is the transitional form between the mýri formation and
the communities of the dry soil, and has some characters of both. M 0 1-
h o 1 m H a n s e n (1. c. pp. 109-110) describes the jaðar vegetation on
Arnarvatnsheiði as follows: “In depressions between the moraine walls,
on the borders of the mýrar, in small damp hollows in the mó, and as
a fringe along the rivers the jaðar vegetation is met with. When typical-
ly developed it is easily distinguished from the other types of vegetation
by the very large knolls of which the surface consists. These often
attain a height of 1 m or more and 1 or several m in diameter. The
distance between the individual knolls is up to /2 m. The jaðar vegeta-
tion belongs to moderately damp soil having a normal snow-covering
in the winter.”
In the highland regions visited by me I failed to find this typical
jaðar described by Molholm Hansen from Arnarvatnsheiði; this
does not agree with M 0 1 h o 1 m H a n s e n’s contention that the jaðar
“appears as a very characteristic feature of the landscape in the high-
land tracts” (1. c. p. 53). However, different types of this formation
may doubtless occur. The only feature which the jaðar described by me
has in common with that of Arnarvatnsheiði as regards the conditions
of the soil and the habitat factors, is that the soil is moderately damp
and the snow-covering normal. The formation described by me here as