Náttúrufræðingurinn - 2016, Síða 26

Náttúrufræðingurinn
26
tant species in Icelandic vegetation.
Woodland birch in Iceland is low-grow-
ing and often polycormic, probably as a
consequence of hybridisation between
the two species. The species have differ-
ent chromosome numbers and therefore
their hybrids produce mostly abnormal
gametes. But a few healthy ones are suf-
ficient to backcross to the parental spe-
cies, allowing gene flow between them,
the process known as introgressive hy-
bridisation or introgression. Abnormal
Betula pollen grains in peat or sedi-
ments thus indicate past hybridisation.
In this study we searched for such pol-
len in order to find hybridisation peri-
ods during the Holocene. Samples from
peat were collected in three locations in
Iceland: Grímsnes, Eyjafjörður and
Þistilfjörður. Betula pollen grains were
measured and the species proportions
calculated from size. Abnormal Betula
pollen grains were counted separately.
In all three places, periods of elevated
proportions of abnormal Betula pollen
were detected. By comparison to climate
data from the Greenland Ice Core
Project, the effect of climate on birch
woodlands could be seen. The hybridi-
sation periods were found to be con-
nected to the advance of woodland-
forming downy birch over dwarf birch
habitat under warming climate. Such
hybridisation may have taken place in
most parts of northern Europe when
woodland expanded in the beginning of
the Holocene. In Iceland the climate
stayed near the lower limits of birch
woodland tolerance for most of the
Holocene, repeatedly creating condi-
tions that facilitated hybridisation. With
the warming of climate in the last few
decades a new wave of birch hybridisa-
tion has started.
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