Jökull - 31.12.2001, Blaðsíða 23

Dating of the Fláajökull moraine ridges
Diameters of Rhizocarpon geographicum agg. thalli measured in 2000
80 i
moraine ridges and depressions between ridges
ddifference between single largest thallus and mean of 5 largest thalli
Dmean of 5 largest thalli
Figure 4. Mean diameters of 5 largest Rhizocarpon geographicum agg. thalli (measured in year 2000) out of
100 thalli on each of the moraine ridges and depressions between the ridges. Note the large size of lichens in
depressions IV/V and III/IV. - Meðalþvermál 5 stærstu fléttanna afgerðinni Rhizocarpon geographicum agg.
Skyggða svœðið sýnir frávik (mm) þeirrafrá meðaltalinu.
(1983), Snorrason (1984), and Evans et al. (1999) are
not in agreement regarding the growth rate of the Rhi-
zocarpon agg. lichen under the climatic conditions
of southeast Iceland. These three glaciers formed a
joint piedmont glacier lobe during 1860-1870 (Evans
et al. 1999). Wójcik (1976) argues that Heinabergs-
jökull and Fláajökull were in contact up to the 1930s
but this is not supported by the findings of Evans
et al. (1999). Given the close proximity of these
glacier forelands growth conditions for lichens should
be nearly the same.
Jaksch (1975) also used lichenometry but pro-
vided measurements of Rhizocarpon geographicum
only for the oldest moraine ridge of Fláajökull. Thalli
diameters up to 5 cm were dated to 1890 (measured in
1972), but he provided no rate of growth nor coloni-
sation lag time in his work. However, Jaksch did dis-
cuss obstacles for lichen colonisation, particularly the
influence of solifluction and lack of large boulders.
A constant (linear) lichen growth rate does not
seem to be justified as shown by Thompson and
Jones (1986) who dated proglacial river terraces in
south-east Iceland. Using a changing growth rate
for Rhizocarpon agg. they obtained a growth rate of
0,725 mmyr-1 for lichens younger than 50-60 years,
slowing down to 0,585 mmyU1 for older lichens.
A non-linear curve was also obtained by Bradwell
(2001) for the last 270 years including an almost
linear relationship on moraines younger than c. 100
years. Differences in field measurements pertaining
to the shape of the thallus, its axis, type of rock on
which it grows and extent of survey area are most
likely the cause of variations between different results.
For example Evans et al. (1999) used the mean of the
5 largest thalli (their longest axis) whereas Bradwell
(2001) used only the largest thalli in each field (the
longest axis regardless of shape).
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