Jökull - 01.12.1983, Side 116

diameter of the largest inscribed circle of nearly
circular lichens was used. Several of the members of
the party carried out lichen measurements but the
majority of the measurements in Gljúfurárdalur
were made by R. Johnson and A. Tipper.
Because of the lack of dated surfaces in Gljúfur-
árdalur a calibration curve íbr lichen size against
age had to be constructed udlising measurements
obtained from lower ground in Skíðadalur. Al-
though growth rates can be affected by variations in
local climate (Ten Brink 1973) the differences bet-
ween Skíðadalur and Gljúfurárdalur are not con-
sidered sufíicient to manifestly affect the rate of
lichen growth. In Skíðadalur stones on the inside
walls of abandoned farmsteads were used to provide
lichen sizes on dated surfaces. In all six farmsteads
provided measurements, five from Skíðadalur
(Sveinsstaðir, Gljúfurárkot, Krosshóll, Hverhóll
and Holárkot) and one from the coast near Dalvík
(Sauðanes). In the construction ofthe curve lichen
observations from Sauðanes were omitted because
of the very different nature of the site to other farm-
steads, and measurements from Holárkot were also
excluded due to the low number of lichens found in
the abandoned footings.
Using the remaining four farmsteads growth
curves were constructed using measurements of the
largest lichens, the means of five largest lichens and
the means of ten largest lichens. Different workers
Years A D.
Fig. 2. Lichen growth curve for Skíðadalur
Mynd 2. Samband aldurs og þvermáls flétta í Skíðadal.
45
Mean size of 5 largest lichen diameters (mm)
Fig. 3. Relationship between the size ofthe largest
lichen and the mean size of the five largest lichens.
Mynd 3. Samband á milli þvermáls steerstu flétta og
meðalþvermáls fimm stærstu flétta.
use these different criteria; some follow Beschel
(1961) in using only the largest lichen whileothers
use averages (Stork 1963, Matthews 1974). ForSkíða-
dalur regression lines constructed by the “least
squares” method show that the mean of five largest
lichensgives the bestfitto thedata (Fig. 2) but there
is very little difference between this and using the
largest lichen. Moreover the small number ofpoints
on the line means that, perhaps not surprisingly, the
fits are extremely close (e.g. r=0-996, t= 16-008, sig.
at 0 001%). There is also a very closefit between the
'size ofthe largest lichenand the mean size ofthe five
largest lichens for each site (Fig. 3). Whichever line
is used they clearly illustrate that for the time period
covered a linear relationship can be assumed bet-
ween lichen diameter and age. Thus this period falls
within the “great period” of lichen growth first
defined by Beschel (1950). The rates oflichen growth
in this part of Northern Iceland are therefore
56-88mm/100 years for the laigest single lichen and
50-99mm/100 years using the mean of the five larg-
est lichens. These rates compare favourably with
the slightly higher rate of 63mm/100 years found at
Breiðamerkurjökull in Southern Iceland where
rainfall is higher (Gordon 1980 and pers. corara.),
and with approximate rates of65-70mm/100 years
114 JÖKULL 33. ÁR