Fróðskaparrit - 01.01.1998, Page 248

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PALYNOLOGICAL AND VEGETATIONAL CHANGES ASSOCIATED WITH
THE DEPOSITION OF SAKSUNARVATN ASH IN THE FAROEISLANDS
OVARU HOYDALAR: percentages
Fig. 9. Pollen and spore diagram of selected taxafrom Upper Hoydalur. Percentage values are expressed as TLP
minus alien AP. Total identifiable palynomorph concentrations exclude alien AP.
Mynd 9. Flogsáð- og grókomstrikumynd av úrvaldum taxa úr Ovaru Hoydølum. Virði í prosentum eru lýst sum
TLP minus fremmant AP. I samanløgdu palynomorfkonsentratiónunum, sum kunnu eyðmerkjast, er fremmant AP
ikki tikið við.
selago may have benefited from their
growth in drier grasslands, at least in terms
of their percentage contributions to the
pollen and spore rain if not vegetationally.
The fall in Betula, if related to ash-fall
(birch values were already on a downward
path through the profile), may be due to the
direct impact of tephra upon birch commu-
nities (cf. Blackford et al. 1992), but the
Saksunarvatn ash, with a Si02 content of
around 50 % is not especially acid. With re-
gard to the falls in Betula pollen well before
Saksunarvatn ash times at all of his sites,
Jóhansen (1982; 1985) evokes a change
from subarctic to an oceanic climate in ex-
planation. This, however, is for a decline in
supposed Betula nana, not in tree birch
species. Pollen size measurements at
Hagamýra favour tree birch, but if there
was increased oceanicity around the Faroes
during the period covered by the Hagamýra
pollen diagrams, then there is no reason
why this would not have had a deleterious
effect upon, for instance, B. pubescens.
Ovaru Hoydalar
The ash layer at this site is of denser ap-
pearance at its base than at the top, which
reinforces the notion that only the base of
the ash layer derives from aerial input,
while the rest results from catchment runoff
into the former lake basin. The pollen pro-
file (Fig. 9) covers 234-266 cm and pre-
sents a more muted picture than was the
case at Hagamýra. Poaceae increases at the
start of tephra deposition and Lamiaceae
falls; Cyperaceae appears to have been de-
clining prior to ash deposition. At the top of