Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
Surface characteristics affecting active layer formation in palsas, Finnish Lapland
M. Rönkkö & M. Seppälä
Department of Geography, University of Helsinki, Finland
ABSTRACT: Active layer thickness was measured in palsas on two large mires close to Lake Ahkojavri in
northernmost Finland in July and August, 1999. 381 measurements were made on five types of palsa surface: bare
peat, Empetrum nigrum ssp. hermaphroditum, Betula nana, moss and lichen covered surfaces. Palsa height was
measured, and also the proportion of abraded surfaces and collapsed edges. Active layer thickness ranged from 29
to 74 cm. Statistical analysis showed that the active layer thickness correlated with the height of palsas, with their
degree of erosion and collapse. The active layer was thickest on lichen-covered surfaces (mean 59 cm) and
thinnest under Betula nana (mean 51 cm). A surprising result was that the bare peat surfaces (mean active layer
thickness 52 cm) did not increase the thawing of the active layer. These observations are in clear disagreement
with the vegetation change hypothesis proposed by Railton and Sparling (1973) according to which palsa formation depends on changes in surface albedo. Many of the studied palsas are strongly eroded but four new palsas
(60–80 cm in height) were also found on the same mires. The present development stage of the palsas shows that
winter wind activity is becoming stronger, causing surface abrasion and forming new palsas. In winter
1998–1999 the former active layer on some palsas had not frozen totally. An unfrozen layer was observed between
the permafrost layer and the thawing seasonal frost layer.
1 INTRODUCTION
The aim of the study is to measure the active layer
thawing depth and factors affecting it in the Paistunturit
fell area, Utsjoki, Finnish Lapland. The characteristics
observed are palsa height, vegetation cover, abrasion
degree and the amount of block erosion. The amounts
of thermokarst and cracking of peat on palsas and were
also observed. Initial hypotheses were that:
1. The active layer on palsas is thinner under vegetated surfaces than under a bare peat surface,
2. the height and size of a palsa also affects the thickness of the active layer (the bigger the palsa, the
thicker the active layer),
3. the much abraded and collapsing palsas have a
thicker active layer than uneroded palsas.
Figure 1. Location of the studied palsa mires in Finnish
Lapland.
2 STUDY AREA
the coldest month is January (MMAT 16°C), the
warmest July (13°C); the mean annual air temperature is 2.0°C (Climatological Statistics in Finland,
1991). The mean annual precipitation at Kevo is
395 mm. The snow free season is from 20 May to
the end of September. The growth season is about
110 days and the thermal sum (5°C degree.days)
varies between 400 and 900.
Low mountains with deep river valleys characterize
the topography; the elevation of the region is mostly
between 250 and 400 m a.s.l.
The study area contains two palsa mires, Luovdijeäggi
(12.6 ha in area) and Tsulloveijeäggi (8 ha), close to
Lake Ahkojavri, in western Utsjoki, northernmost
Finnish Lapland (about 69°35N 26°11E) (Fig. 1).
The mires are located about 350 m above sea level.
There are 48 palsas on Luovdijeäggi and 28 palsas on
Tsulloveijeäggi. The height of the palsas varies between
0.4 m and 3.3 m. The palsas cover about 10 percent of
the areas of the mires.
The nearest weather station is at Kevo, some
40 km NEE of Lake Ahkojavri at 107 m a.s.l. There
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3 METHODS
Field measurements were made from 26 July to 17
August 1999. According to former studies (Seppälä
1976, 1983) the active layer on palsas does not
increase much after the end of July.
The depth of the active layer was measured with a
metal rod (1 m long, 1 cm in diameter), which was
pushed through the active layer to the frost table. The
measurements were done with 1 cm precision.
Active layer depths were measured on horizontal
surfaces with different covers: Empetrum nigrum ssp.
hermaphroditum, Betula nana, light lichen cover, moss
and bare unvegetated peat (wind abraded surface)
at 381 points, of which 250 were on Luovdijeäggi and
131 on Tsulloveijeäggi. The height of the Betula nana
shrubs was also measured in order to investigate
whether their height affects active layer depth.
Each palsa was described according to the following
criteria: depth of active layer under various surface covers, palsa height and diameter, amount of block erosion
and abrasion, and degree of cracking. Palsa height was
determined visually or with a staff. The diameter was
measured using a measuring tape or by pacing. The
direction of edges with block erosion was determined
with a compass with 5 degrees precision. The orientation of the abrasion surfaces was also measured
(Seppälä in press). The intensity of abrasion was estimated on a scale of zero to three (zero no abrasion,
1 palsa surface one third abraded, 2 one to two
thirds of palsa surface abraded, 3 two thirds of
palsa surface abraded). Block erosion was assessed on
a similar scale (0 no block erosion, 1 a little block
erosion, 2 some block erosion, 3 palsa much collapsed). The vegetation on palsas was also determined.
The data were analysed using Excel and SPSS statistical charting, plotting, and data analysis programs.
Average, standard deviation, median minimum and
maximum were calculated for the data. The data were
then stratified, initially into three classes, according to
palsa height, degree of block erosion and abrasion.
Statistical indices were calculated for the depths of the
active layer under different surfaces separately. Height
classes included palsas with the height 1.5 m,
1.5–2 m and 2 m. Because few palsas scored a zero
value, palsas were then divided into 3 groups, which
guaranteed enough palsas in each class for statistical
analysis to yield significant results. The aim was to
find the most important factor determining the thaw
depth of active layer on palsas.
Figure 2. Active layer observations classified according
to their thickness.
for each height class, surface type, block erosion type,
and abrasion type.
The data collected from two mires were analysed
together as a single set. The mean depth of active layer
on Luovdijeäggi was 53.9 cm and on Tsulloveijeäggi
53.2 cm. These data support the view of Seppälä
(1983) that the depth of active layer varies little between
palsa mires. The thinnest active layer was 29 cm and
the deepest 74 cm. Standard deviation was 9.0 cm. The
average was 53.7 cm and median 54 cm (Fig. 2).
The cracking of palsas had no effect on the thaw
depth; the active layer was equally deep in cracks and
on the surrounding peat.
4.1
Active layer and surface characteristics
The thinnest active layer (50.7 cm) was, according to
the average and median, beneath dwarf birch (Betula
nana) shrubs as expected and under moss cover
(53.6 cm) but the median (52 cm) was the same in both
cases.
The second smallest mean of active layer thickness
was beneath unvegetated peat surfaces. The median of
the second thinnest active layer was under Empetrum
nigrum surface, which had almost the same mean depth
(53.1 cm) as the moss surfaces (53.6 cm) (Table 1). The
mean active layer thickness was greatest under the
lichen surface (59 cm) with the median (60 cm) ranging
from 44 cm to 74 cm.
A variance analysis (ANOVA) was carried out to
investigate the significance of these differences. The
deviation of the thickness of active layer is fairly
large, about 8 cm, according to the differing surface
characteristics. The smallest deviation is on the
unvegetated peat surfaces and largest beneath the
Betula shrubs (10.2 cm). The data are not normally
distributed but almost bimodal. The explanation could
be a residual thawed layer. The previous summer the
active layer had melted so deeply that the seasonal
frost of the following winter did not penetrate to the
permafrost table (cf. Salmi 1970).
4 ANALYSIS AND RESULTS
A frequency diagram (Fig. 2) shows the depth of the
active layer in the palsas studied for all data combined,
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Table 2. Spearman’s correlation coefficients of abrasion,
block erosion and heights of palsas.
Table 1. Mean thickness of active layers (in cm)
according to the nature of the studied palsas.
Surface quality
Barren peat
Empetrum nigrum
Lichen
Betula nana
Moss
Height
1.5 m
1.5–2 m
2 m
Abrasion
1 1/3
2 2/3
3 2/3
Block erosion
0 no
2 some
3 much
Total mean
Thickness of
active layers
Abrasion
Block erosion
Height
52.1
53.1
59.0
50.7
53.6
4.4
49.0
56.0
58.0
50.5
54.6
56.2
49.8
55.1
57.4
53.7
Active layer depth and block-erosion
The depth of the active layer increases as the amount
of block erosion increases on palsas as the insulating
peat cover is eroded by collapse. Mean active layer
depths are 49.8 cm, 55.1 cm and 57.4 cm in the three
block-erosion classes. In palsas that are little collapsed the active layer is thinnest beneath the Betula
nana surface according to the median (45 cm), but
according to the average (46.6 cm) it is thinnest underneath moss-covered surfaces. On all kind of surfaces
the active layer gets thicker as the block-erosion
amount increases.
4.3
Block
erosion
Height
1.000
0.487**
0.291*
0.487**
1.000
0.558**
0.291*
0.558**
1.000
Active layer depth and palsa height
Mean and median active layer depths are smallest on
palsas lower than 1.5 m and thickest on palsas over 2 m
in height. On unvegetated peat surfaces the active layer
is thinnest on the lowest palsas but thickest on the palsas 1.5–2 m high. The difference compared to the highest palsas is only a few millimetres. Because the
differences were minimal, variance analysis was done
to compare the mean values. Variation is great in every
palsa group regardless of height or surface characteristics. The height of a palsa does not by itself explain the
depth of active layer. According to variance analysis
there is a difference between the averages when all the
height classes are compared. When the lowest palsas
and palsas 1.5–2 m high were compared the difference
was statistically significant, but when the 1.5–2 m and
over 2 m high palsas were compared, no significant
difference was observed. This same phenomenon was
noticed when comparing the different abrasion and
block-erosion amounts. The biggest difference between
the active layer depths was between the classes 1 and
the other two classes; classes 2 and 3 are more similar.
Class 1 differs most from the others in height, abrasion
and block-erosion amount.
Frequencies show that active layer depth increases
with height, amount of block-erosion and abrasion,
but this is to be expected since they correlate with
each other.
Relationships between the depth of active layer and
the height of the palsa were investigated with regression analysis. This confirms that average active layer
depth does not differ between surfaces of different
characteristics. According to the results of the variance analysis the hypothesis is rejected that palsa
height is a significant control on active layer depth.
4.2
Abrasion
4.5
Relationships between palsa characteristics
The relationship between observed palsa characteristics were investigated by Spearman rank correlation
(Table 2). The relationship between palsa height and
block erosion yielded r 0.558. Thus, the higher
the palsa, the more collapsed it is likely to be. The
relationship between palsa height and abrasion was
similarly tested and yielded r 0.291. This shows a
relationship that is almost statistically significant.
The relationship between surface abrasion and
block-erosion gives r 0.487, and is statistically
significant. A low palsa can be very abraded because
it is old and worn. Block erosion can be detected only
from big palsas because the collapsed peat blocks are
already sunk into the wet mire.
Active layer depth and abrasion
The variance analyses indicates that active layer depth
increases with greater abrasion, regardless of the surface character. The active layer is thinnest beneath the
Betula surface and thickest under the lichen surface.
The standard deviation is rather large and the distribution of data is bimodal.
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Figure 4. Regression between the means of active layer
thicknesses and of heights of palsas. Linear regression with
hatched line.
Table 3. Palsa classes identified by cluster analysis.
Figure 3. Relationships between the height of palsas and
active layer thickness on different kinds of palsa surface.
4.6
I
A
B
C
II
D
E
F
Regression analysis
Regression analysis was used to investigate whether
palsa height controls the active layer depth.
The regression model gives R2 0.19, which
means that palsa height explains only 19% of the
thickness of the active layer. The correlation coefficient between palsa height active layer thickness is:
r 0.43 (n 381, p 0.01). A polynomial regression model yielded R2 0.23.
It was observed that vegetation cover causes great
variance in active layer thickness (Fig. 3). The thinnest
active layer is below the Betula nana shrubs and
thickest on the lichen-covered surface.
The effect of vegetation was eliminated by calculating the mean active layer depths of the different surface types for each palsa height class. The relationship
between palsa height and active layer was then investigated. The linear regression model based on mean
values shows that palsa height explains 60% of active
layer thickness (n 381). The polynomial trend line
gives R2 0.77 (Fig. 4). Palsas cannot be infinitely
high nor the active layer infinitely deep (Fig. 3).
4.7
Height
Abrasion/
block erosion
Active
layer (cm)
120
120–160
150–170
1–2
2
1, 2, 3
46–52
41–56
50–62
2–3
2–3
2–3
48–64
49–64
57–64
180–250
200–230
270–330
below or equal to 170 cm high, with little or some
deflation and collapsed and their active layer is fairly
thin. Palsas in cluster two are over 180 cm high and
little or much abraded and collapsed and their active
layer is thicker than the palsas in cluster one (Table 3).
A palsa which does not fit into these clusters is
rather shallow (only 60 cm high) but its active layer is
deeper than other palsas of the same height. Its abrasion and collapsing rate is in class 1. It obviously is an
old palsa melting from its bottom.
Fairly low and much abraded palsas belong to the
second cluster; they are classified according to palsa
height and active layer thickness. In the main cluster
one there are three palsas that appear not to belong to
any of the sub clusters.
Even though the majority of old palsas at
Luovdijeäggi mire melt there were found four new
palsas with shallow active layers. They indicate that
the climatic conditions are suitable for palsa formation. Their formation as well as the presence of largely
abraded palsas mean strong winter storms and probably changing wind conditions (Seppälä, in press).
Cluster analysis
Because the variables were both quantitative and qualitative, hierarchical cluster analysis was undertaken.
The variables were palsa height, abrasion degree,
amount of block-erosion and surface cover (unvegetated peat, Empetrum nigrum, lichen, moss and Betula
nana). All the variables were measured for 37 palsas
and used in the cluster analysis.
Two main clusters were identified; they divide into
3 or 4 smaller clusters. Palsas in cluster one are all
5 DISCUSSION AND CONCLUSION
The thermal balance of the ground is a complex matter. This study concentrates only on the conditions
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lichen-cover, which should decrease the thickness of
active layer. They assumed that the vegetation succession from Sphagnum fuscum cover to Cladonia sp.
dominant vegetation (surface albedo) causes the formation of palsas.
This study also does not support the theory that
melting of palsas starts when the abraded dark surface
of peat absorbs more radiation during the summer.
Cummings and Pollard (1990) claimed that the low
albedo of unvegetated peat surface causes the thicker
active layer. Low palsas have shallow active layers
regardless of the development stage. Generalizing it
could be said that the active layer becomes thicker as
abrasion, block erosion and the height of the palsa
increase. It seems that palsa height is the controlling
factor on active layer thickness in the Paistunturit area.
prevailing at the end of the thawing season. It is therefore important to remember that the conditions in
winter and spring also affect thawing. Snow cover, for
example, has a strong influence on the temperature
system in the ground and also the hydrological conditions in spring.
According to several studies an active layer beneath
a vegetation cover does not thaw so deeply as beneath
bare ground. Removal or thinning of the insulating
peat cover normally increases the thaw depth to cause
thermokarst (Brown 1970; Luthin & Guymon 1974;
Smith 1975; Allard et al. 1996). It was presupposed
that the active layer would be thickest on an unvegetated surface, but was not found in the present study.
One reason for the fact that the mean active layer depth
on the unvegetated surface is almost the same as on
Empetrum nigrum and moss surfaces could be evaporation during the springtime. Melt water evaporates
more quickly from a bare peat surface than from a vegetated surface because the vegetation holds moisture
well. A dry peat layer protects permafrost below.
Other explanations for the relatively thin active layer
could be that the unvegetated peat surface is free of
snow during winter. Cold can penetrate deeply into the
peat. The dense or high vegetation collects snow but
also protects the permafrost from spring warming and
causes a thinner active layer than sparse or low vegetation (Smith 1975). The active layer was thinner under
Betula nana shrubs even though the difference compared to other surfaces studied is not significant according to variance analysis. There is more snow on the
high vegetation and therefore the heat loss is less compared to other vegetation or bare ground (Kershaw &
Gill 1979). This could explain why the active layer
beneath the Betula nana bush is not notably thinner
than on the lower vegetation or unvegetated peat surface. Betula nana shrubs transpire more during the
summer so peat underneath it dries and protects the
permafrost. The ground also remains cooler due to
evaporation. The growing conditions for Betula nana
are more favourable when the active layer is thicker.
The active layer was thickest under the lichen-covered
surfaces regardless of the size of palsa or abrasion or
block-erosion. The peat under the lichen cover is fairly
porous and provides a good insulator but on the other
hand porous peat infiltrates meltwater and rain easily
and that enhances thawing. Also the low evaporation
might explain the high depth of active layer under lichen
cover. According to Rouse et al. (1977) evaporation is
least on the lichen heaths, so the ground cools less than
other vegetated surfaces and peat. It also remains moist
and its insulating effect is therefore weak. According to
variance analysis lichen affects the thaw depths a little
more than other surface types. This study does not support the views of Railton and Sparling (1973) that palsas
evolve because of the high albedo under a light coloured
ACKNOWLEDGEMENTS
Professor Derek Mottershead kindly revised the
English of the manuscript. Kirsti Lehto made the final
drawing of the figures. Hilkka Ailio finished the layout of the paper. Field investigations were financially
supported by Seth Sohlberg’s Delegation.
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