Jökull - 01.12.1969, Side 157

Fig. 9. Average yield of hay in 100 kg per
hectare (1951 — 1968) in two districts in both the
southern and northern parts of Iceland.
annual temperature and the growth have been
below average. In the graph (Fig. 11) showing
the average yield of hay in 100 kilos per
liectare for the whole of Iceland, a drop in the
yield is thus clearly discernible during the sea
ice year of 1967.
Low temperatures during the growing months
do not merely retarcl growth. Hard frosts in
late winter cause, for example, great damage
to grass by winter killing, thus substantially
reducing the area of the hayfields that ought
otherwise to produce yields. Such damage bv
winter killing has, in fact, been most severe
crop is measured in the barns and recorded
in 100 kg or m3. Also available are measure-
ments of size of hayfields over the whole
country. It is tlius possible to calculate the
annual yield for a given area. When the hay
stocks are calculated in this way there may be
errors involved. However, it is evident that
there are big fluctuations in the average yield
per hectare. It is noteworthv (Fig. 9) that the
yield decreases in the later years, wlien more
drift ice has been notecl than in previous
years and it falls more rapidly in tlie northern
districts. An increase in drift ice lowers the
annual temperature or coincides with a low-
er temperature. If the yield of individual
districts is compared with the annual mean
temperature of a nearby meteorological sta-
tion, it should be possible to obtain an estimate
of the effect of temperature on the hay yield.
Fig. 10 shows in this way the correlation be-
tween yield ancl temperature in individual
years in two southern and two northern
districts for the periocl 1950 to 1968. The
comparison sliows that both the yielcl and the
temperature are considerably lower in the
northern districts, but it is also seen that the
difference in yield amounts to about 10 hay-
loads of 100 kg (= 1 ton) per hectare in both
areas if there is a difference of one degree in
the mean temperature. Tliere have been few
major ice years during the 20th century. Never-
theless, the grass growtli has varied witli fluc-
tuations in temperature, which have not been
dependent on any visible sea ice. But during
the few ice years that have occurred, both the
Fig. 10. Comparison of average hay yielcl in
100 kg per liectare and rnean annual tempera-
tures in °C of northern and southern districts
in Iceland.
JÖKULL 19. ÁR 1 53