Jökull - 01.12.1990, Blaðsíða 78

INTERPRETATION OF THE POLLEN
DIAGRAM
Before any attempt is made to interpret the pollen
analytical data, one aspect of the question about the
origin of the Icelandic flora shall be briefly mentioned.
At the time of retreat of the glacier from the Kross-
hólsmýri site, Flateyjardalur was already ice-free since
several millennia. Thus the conditions for an estab-
lishment of a vegetation cover in the former bed of
Brettingstaðadalir- glacier were not the same as for
the pioneer vegetation proper, recovering several mil-
lennia earlier, i.e. after the long duration of the Weich-
selian glaciation. Because of this difference, the very
first signs of a pioneer vegetation, probably some kind
of herb communities (c/. Fredskild 1973), is not to
be expected in the deepest part of the Krosshólsmýri
sediment core. In Flateyjardalur the floral elements
already had got several millennia to immigrate and
establish themselves, perhaps from a nearby refugium
(Steindórsson, 1962 c/. Norðdahl 1991),perhaps from
some distant regions in Iceland, or even from remote
continents. A difference of this kind in origin and
or immigration routes is impossible to distinguish or
infer about from the Krosshólsmýri core alone. There-
fore this investigation can not answer questions about
the origin of the Icelandic flora, which might seem
controversial, as this is the oldest organic sediment of
Holocene age in Iceland hitherto radiocarbon dated, as
well as containing evidence about a local vegetational
succession.
KHM 1, Salix-Ericales-Cruciferae-Ranunculus-
Lycopodium Local Pollen Assemblage Zone. Esti-
mated age ca 9700-9400 B.P. Salix is alredy flourish-
ing and so is Empetrum nigrum and Ericaceae undiff.
too. The high Salix percentages may be attributed to
scrub, snowbed communities, or both (Faegri 1953).
This zone is showing the most diversity in pollen taxa,
probably indicating little competition and thus a vari-
ety of plant species, which later on were not able to
share place and live together as the vegetation cover
became more dense. Tofieldia and Chenopodiaceae
appear in this zone only, Plantago maritima, Erigeron,
and Saxifraga type are all represented here, but have
a sparse occurrence later on. Otherwise Ranunculus
and Cruciferae are the best represented herb group. A
very local origin of those groups is not to be precluded,
as both do include the limnic species, i.e. Ranunculus
reptans, R. hyperboreus, R. confervoides and Subu-
laria aquatica.
The few Betula pollen grains in this zone are all
rather small (Fig. 5), thus most probably derived from
B. nana. As the (dwarf-) shrub type of Betula is known
for dispersing its pollen badly and a percentage of 2-6
may indicate local stands (cf. Andrews et al., 1980) it
might have been locally present.
In the lake Myriophyllum alterniflorum, Hippuris
vulgaris and some species of Eu-Potamogeton were
flowering and Isoetes echinospora may have been
present. However as only one spore was found in
sample 3 and Isoetes does not reappear until in sample
12 a contamination can not be precluded. The green
algae Pediastrum boiyanum flourish, most certainly
there are several varieties, also P. duplex has arrived
to the lake.
A radiocarbon date at the beginning of the se-
quence (Lu-1433), and an interpolation of the two
dates from the sediment core, place this first Local
Pollen Assemblage Zone at the end of the Early Pre-
boreal and the beginning of the Late Preboreal Chrono-
zones (sensu Mangerud et al., 1974).
KHM 2a, Juniperus-Betula-Salix-Cyperaceae Lo-
cal Pollen Assemblage Subzone. Estimated age ca
9400-8400 B.P. Juniperus and Betula are both ex-
panding. The size frequency diagram (Fig. 5) shows
a positively skewed distribution, suggesting that tree
birch arrived in this subzone, maybe not until the latter
half of the subzone. Towards the end of the subzone
a Betula pollen percentage of 25 indicates that some
birch copses have developed, althoughthe dwarf birch
B. nana is locally present too. As mentioned before
the first pollen of Betula pubescens type was found in
sample 6. However until more work is done, espe-
cially on macrofossils so as to preclude effects of long
distance transport of pollen (c/. Fredskild 1973), the
precise age of tree birch in the flora of Flateyjardalur
is not known. Lycopodium annotinum is still the best
represented of the Lycopodiaceae and of the pteridio-
phytes as a whole, suggesting widespread heaths. As
yet the vegetation has the character of a dwarf shrub
74 JÖKULL, No. 40, 1990