The Botany of Iceland - 01.12.1930, Blaðsíða 136

126
H. M0LHOLM HANSEN
the latter 45. On Arnavatnsheiði, at a level of c. 500 m., the same
formation shows a Ch percentage of 52.
Taking the mosathembur vegetation of Lýngdalsheiði from
altitudes of c. 250 m., 332 m., and 400 m, we get the following
scale for the Ch percentage: 10—^-55—>68, with a corresponding rise
of the A percentage: 75—90—» 100.
Thus the influence of the cold on the Ch and A spe-
cies is beyond a 11 doubt.
Table 26 shows the distribution of the life-forms in relation to
the scale of moisture. A remarkable circumstance appears in con-
nection with the cham aephytes. From a comparative minimum
in zone IV (the jaðar vegetation) the Ch percentage increases in
value, upwards as well as downwards. This applies equally to
the Bjerk series, the Lýngdalsheiði series, and the highland series.
In the Lækjamót series the increase does not appear in the lower
part of the scale for the halla mýri series, whereas the rule seems
to apply to the fór mýri series here.
It is difficult to find any plausible reason whj' Ch should thrive
best in the mo and the mýri, and badly in the intermediate jaðar.
Anyhow, it is a fact that the environment in the jaðar is unfavour-
able to Ch. Species such as Empetrum nigrúm, Vaccinium uligi-
nosum, and Betula nana occur with a higher F.-percentage above
as well as below the jaðar.
Possibly the cause may be found in the annual variations in
the level of the water. It is of minor importance to Ch whether
the moisture is great or small, if only it is the same all the time.
If great variations take place, as is the case in jaðar where the
plants grow now on damp, now on dry soil, Ch decline.
On the heaths of Jutland it may often be observed how great
variations in the water level tend to kill the chamaephytes, and at
the level of moisture corresponding to the jaðar, viz. the edge of
the bog, the following facts may be observed. In the middle of
the heath where the variations are only small, the chamaephytes
play a prominent part in the composition of the vegetation, near the
valley of the river where the variations are greater, the chamae-
phytes disappear. In a series of investigations on this zone of
moisture at Norholm Heath the former formation showed an average
Ch percentage of 55, while the latler formation had only a Ch per-
centage of 4.