The Botany of Iceland - 01.12.1930, Blaðsíða 160
150
H. M0LHOLM HANSEN
the same if the species numbers alone are employed instead of the
frequency numbers.
The occurrence and quantitative distribution of the species
groups in the Icelandic types of vegetation is determined throughout
by the temperature conditions prevalent in the locality. The pro-
longed low temperature prevalent in the highlands and
the north country but especially in localities where the
snow is blown away in the winter, promotes the growth
of northern but restricts the growth of southern species,
while a prolonged high temperature, as it occurs in the
lowlands and the south but especially wherever the
ground is covered withadeepandconstant layerofsnow,
restricts the growth of northern but promotes the growth
of southern species. This is abundantly confirmed by
the vegetation around the hot springs.
We are thus fully justified in regarding the species
groups as indicators of environment, and the species
group spectra will then prove an important guide in a
more precise analysis of environment. Under the treatment
of the distribution of the species groups in the Icelandic scale of
moisture, the spectra furnished important holds for an examination
of the physical conditions. The legitimacy of the above-stated con-
siderations is further confirmed by the distribution of the species
groups in the scale of snow-covering, in the types of vegetation of
the different parts of the country and the altitudinal zones, and by
the vegetation around the hot springs.
In two areas, partly in the geiri and parily in the flói, on soil
covered respectively with snow and with water, the E species are
unusually abundant. In both places the winter temperature must be
supposed to be almost the same, at or below zero. When the snow
has melted in the geiri, the heat that is left will directly beneíit the
plants. In the flói, on the other hand, a great deal of the heat is
latent in the water whicli still covers the vegetation, consequently
the result will be a relatively low summer temperature. The flói
is thuswarm in the winter but cold in the summer, while
the geiri is warm both in the winter and the summer.
The result will be that in both places the species group spectrum
is characterised by a high E percentage; on the water-covered soil
it is 72, on the snow-covered soil 66. The difference in the summer