Fróðskaparrit - 01.01.1998, Síða 153

UMSKIFTI í HOLOSENA VEÐURLAGNUM í NORÐURATLANTSØKINUM,
SUM TAÐ SÆST AV TORVMÝRUROYNDUM
159
ies. One of the most important considera-
tions is that of human impact on the mire
surface. Vegetation changes can be caused
by the deliberate burning of bog vegetation,
by grazing of domesticated animals or
herded wild animals, by drainage of the
mire either deliberately in order to improve
grazing or access, or inadvertently through
cutting peat for fuel. People can also alter
the hydrological balance of a mire by re-
moving or planting trees. The presence of
human influence can be to some extent
monitored in the palaeoecological record
by analysing the pollen and charcoal con-
tent of the peat at times of change (cf. Bar-
ber, 1981; Chambers et al., 1997). In their
review of peat profiles from Norway,
Nilssen and Vorren (1991) noted a connec-
tion between increased indicators of an-
thropogenic impact and changes in the peat
humification curve, and suggested that
changes in land use may have been a result
of climatic change. This suggestion can be
(speculatively) developed as follows. The
following sequence of events is possible:
A climate change causes the water table
to fall, and the mire surface to become
drier
Vegetation changes to an assemblage
more suited to drier conditions
Grazing and/or hay value of the vegeta-
tion increases, and accessibility to the
bog for people and animals increases
Land use practices change to take advan-
tage of the new conditions, and vegeta-
tion is changed further by the impacts
of livestock.
A change to a climatic regime that caused
the peat surface to become wetter could
cause the following:
Climate change causes the water table to
rise, and the mire surface to become
wetter
Vegetation changes to an assemblage
more suited to wetter conditions
Grazing and hay values decrease and the
land becomes less accessible
Vegetation changes further due to re-
duced grazing pressure.
There are other limiting factors too- influ-
ences that to a greater or lesser extent un-
dermine the basic assumptions listed
above. One such factor is that of bog-
bursts, catastrophic mass-movement events
that remove material from the mire, and
lower the water-table over a wider area of
the bog. Bog-bursts or flows are known
from raised bogs and from blanket mire at a
variety of scales, but at a frequency that
suggests they must have been a factor
throughout the history of most mires areas
(Alexander et al., 1986; Carling, 1986;
Warburton and Higgitt, 1998). Historical
accounts suggest that bog bursts follow pe-
riods of heavy rain. This raises the possibil-
ity that an event that, in the palaeoenviron-
mental record, would appear to be a lower-
ing of the water table (and therefore inter-
preted as a change to drier climatic condi-
tions) could in fact result from a short-term
water surplus. There is a clear need, there-
fore, for careful palaeoecological recon-
structions at a high resolution, using pollen
indicators of human impact and, where
possible, more than one profile should be
used.