The Botany of Iceland - 01.12.1930, Side 165

STUDIES ON THE VEGKTATION OF ICELAND
155
ciable cliange, are indifferent. Passing upwards from the level of
the sea to the snow-line in a mountain district, we íind the same
changes.
In Iceland the same holds good for the variation according to
altitude: The H percentage remains unchanged throughout the al-
titudinal zones, the arctic life-form Ch increases rapidly, wliile the
southern life-forms, Pt, Pli, G, HH, and Th, decrease in tlie same
proportion.
These changes in the life-form spectra are connected with, or
caused by, a decrease in the warm temperature of the summer and
an increase in the cold temperature of the winter. The summer
temperature grows lower and the summer of sliorter duration, wliile,
on the other liand, the winter grows longer and more severe tlie
further northward we go.
If we calculate the biological spectra for the Icelandic types of
vegetation and compare the spectra for the types on snow-bare soil,
on soil with a normal snow-covering, and on soil with a constant
snow-covering, we get the same variations as above on passing from
arctic towards temperate regions or from the snow-line towards
the level of the sea.
The result will be the same whetlier we base our calculations
on the number of species or on the number of individuals, i. e. on
the frequency sum.
There is no reason to believe but that, in this case loo, it is
the temperature conditions which determine the occurrence and
development of the life-forms. The temperature conditions favourable
to the artic species are due to the fact that the snow is blown away
so that the cold can act with its full force on tlie vegetation with
the result that southern tj'pes cannot survive, while arctic types
thrive. Where the snow-covering is deep and constant, the case is
reversed; here the environment will be unfavourable to' arctic but
favourable to temperate tj'pes because the snow-covering will prevent
the extreme variations in temperature from reaching the vegetation.
Hence, under conditions where the external factors
are not easily obServable, the individual life-fprms may
with full justice be employed, as above, as indicators of
environment. Thus, in Iceland, rnany Ch will indicate
severe cold, many Ph, K, and Th relatively favourable
temperature conditions.
From a scientiíic point of view, as a means of checking life-