The Botany of Iceland - 01.12.1930, Page 143

STUDIES ON THE VEGETATION OE ICELAND
133
any rate the case within the sub-groups of each of the two larger
groups, and this justifies a linear grouping like the one employed.
If now we regard the spectra of the scale of nioisture as a whole,
as a series-spectrum, it sliows a pronounced tendency to form a
wedge downwards, produced bjr a decrease in quantity of the sub-
groups of both main groups from without inwards. This wedge
shape appears in all the series.
The wedge shape of the series-spectrum must be put down to
the different temperature conditions in the different sections of
the scale.
Thus the variations in temperature in the upper part of the
scale will be greater than in the lower part since the specific heat
of the water will act as a buffer here against changes of tempera-
ture. To this must be added the fact that considerably more water
will evaporate from the damper than from the drier areas, and
since the temperature at which water evaporates, as well as its
specific heat, is very high, this will in practice mean a slower and
slighter heating of a moist than of a dry area. In the summer,
therefore, the temperature will be lower in the former than in the
latter area. In the winter the reverse will be the case. If, in addi-
tion, the temperature drops below zero, considerable anrounts of
heat will be liberated within the moist areas, viz. the heat which
has become latent by the thawing of the ice, the elfect of which
'will be that a damp soil will freeze slower and not to such depths
as a dry soil.
In the case of moderately moist soil, one more circumstance
must be noted. This zone is relatively dry in the summer, and
relatively moist in the winter, which gives it more or less the
advantages of dry soil in the summer and of wet soil in tlie winter.
All in all this area will have more favourable temperature conditions
than the areas above and belowr.
The interaction of temperature conditions and vegetation in the
various zones of moisture will thus be as followrs: —
A. Melar (Zone I) is in pronounced degree cold in the winter
as well as warm in the summer. Hence plants which require much
cold (Ch and A 3 species) and much heat (E 2 +1 species) tlirive
well here. In contrast to the other zones of moisture it is, however,
bare of snow in the winter. This will further encourage tlie A 3
species, while the E2 + 1 species will decrease in quantity.
B. The Mo (Zones II—III) is likewise cold in winter and warm