, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Surface characteristics affecting active layer formation in , 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 surface: bare , 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 change hypothesis proposed by Railton and Sparling (1973) according to which palsa forma- tion 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 and cracking of peat on palsas and were also observed. Initial hypotheses were that: 1. The active layer on palsas is thinner under vege- tated surfaces than under a bare peat surface, 2. the height and size of a palsa also affects the thick- ness 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 study area contains two palsa mires, Luovdijeäggi the coldest month is January (MMAT 16°C), the (12.6 ha in area) and Tsulloveijeäggi (8 ha), close to warmest July (13°C); the mean annual air tempera- Lake Ahkojavri, in western Utsjoki, northernmost ture is 2.0°C (Climatological Statistics in Finland, Finnish Lapland (about 69°35N 26°11E) (Fig. 1). 1991). The mean annual precipitation at Kevo is The mires are located about 350 m above sea level. 395 mm. The snow free season is from 20 May to There are 48 palsas on Luovdijeäggi and 28 palsas on the end of September. The growth season is about Tsulloveijeäggi. The height of the palsas varies between 110 days and the thermal sum (5°C degree.days) 0.4 m and 3.3 m. The palsas cover about 10 percent of varies between 400 and 900. the areas of the mires. Low mountains with deep river valleys characterize The nearest weather station is at Kevo, some the topography; the elevation of the region is mostly 40 km NEE of Lake Ahkojavri at 107 m a.s.l. There between 250 and 400 m a.s.l.

995 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. Figure 2. Active layer observations classified according Active layer depths were measured on horizontal to their thickness. 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 for each height class, surface type, block erosion type, 131 on Tsulloveijeäggi. The height of the Betula nana and abrasion type. shrubs was also measured in order to investigate The data collected from two mires were analysed whether their height affects active layer depth. together as a single set. The mean depth of active layer Each palsa was described according to the following on Luovdijeäggi was 53.9 cm and on Tsulloveijeäggi criteria: depth of active layer under various surface cov- 53.2 cm. These data support the view of Seppälä ers, palsa height and diameter, amount of block erosion (1983) that the depth of active layer varies little between and abrasion, and degree of cracking. Palsa height was palsa mires. The thinnest active layer was 29 cm and determined visually or with a staff. The diameter was the deepest 74 cm. Standard deviation was 9.0 cm. The measured using a measuring tape or by pacing. The average was 53.7 cm and median 54 cm (Fig. 2). direction of edges with block erosion was determined The cracking of palsas had no effect on the thaw with a compass with 5 degrees precision. The ori- depth; the active layer was equally deep in cracks and entation of the abrasion surfaces was also measured on the surrounding peat. (Seppälä in press). The intensity of abrasion was esti- mated on a scale of zero to three (zero no abrasion, 1 palsa surface one third abraded, 2 one to two 4.1 Active layer and surface characteristics thirds of palsa surface abraded, 3 two thirds of palsa surface abraded). Block erosion was assessed on The thinnest active layer (50.7 cm) was, according to a similar scale (0 no block erosion, 1 a little block the average and median, beneath dwarf birch (Betula erosion, 2 some block erosion, 3 palsa much col- nana) shrubs as expected and under moss cover lapsed). The vegetation on palsas was also determined. (53.6 cm) but the median (52 cm) was the same in both The data were analysed using Excel and SPSS sta- cases. tistical charting, plotting, and data analysis programs. The second smallest mean of active layer thickness Average, standard deviation, median minimum and was beneath unvegetated peat surfaces. The median of maximum were calculated for the data. The data were the second thinnest active layer was under Empetrum then stratified, initially into three classes, according to nigrum surface, which had almost the same mean depth palsa height, degree of block erosion and abrasion. (53.1 cm) as the moss surfaces (53.6 cm) (Table 1). The Statistical indices were calculated for the depths of the mean active layer thickness was greatest under the active layer under different surfaces separately. Height lichen surface (59 cm) with the median (60 cm) ranging classes included palsas with the height 1.5 m, from 44 cm to 74 cm. 1.5–2 m and 2 m. Because few palsas scored a zero A variance analysis (ANOVA) was carried out to value, palsas were then divided into 3 groups, which investigate the significance of these differences. The guaranteed enough palsas in each class for statistical deviation of the thickness of active layer is fairly analysis to yield significant results. The aim was to large, about 8 cm, according to the differing surface find the most important factor determining the thaw characteristics. The smallest deviation is on the depth of active layer on palsas. unvegetated peat surfaces and largest beneath the Betula shrubs (10.2 cm). The data are not normally distributed but almost bimodal. The explanation could 4 ANALYSIS AND RESULTS be a residual thawed layer. The previous summer the active layer had melted so deeply that the seasonal A frequency diagram (Fig. 2) shows the depth of the frost of the following winter did not penetrate to the active layer in the palsas studied for all data combined, permafrost table (cf. Salmi 1970).

996 Table 1. Mean thickness of active layers (in cm) Table 2. Spearman’s correlation coefficients of abrasion, according to the nature of the studied palsas. block erosion and heights of palsas. Thickness of Block Surface quality active layers Abrasion erosion Height Barren peat 52.1 Abrasion 1.000 0.487** 0.291* Empetrum nigrum 53.1 Block erosion 0.487** 1.000 0.558** Lichen 59.0 Height 0.291* 0.558** 1.000 Betula nana 50.7 Moss 53.6 Height 4.4 Active layer depth and palsa height 1.5 m 49.0 1.5–2 m 56.0 Mean and median active layer depths are smallest on 2 m 58.0 palsas lower than 1.5 m and thickest on palsas over 2 m Abrasion in height. On unvegetated peat surfaces the active layer 1 1/3 50.5 is thinnest on the lowest palsas but thickest on the pal- 2 2/3 54.6 sas 1.5–2 m high. The difference compared to the high- 3 2/3 56.2 est palsas is only a few millimetres. Because the Block erosion differences were minimal, variance analysis was done 0 no 49.8 to compare the mean values. Variation is great in every 2 some 55.1 palsa group regardless of height or surface characteris- 3 much 57.4 tics. The height of a palsa does not by itself explain the Total mean 53.7 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 Relationships between the depth of active layer and and palsas 1.5–2 m high were compared the difference the height of the palsa were investigated with regres- was statistically significant, but when the 1.5–2 m and sion analysis. This confirms that average active layer over 2 m high palsas were compared, no significant depth does not differ between surfaces of different difference was observed. This same phenomenon was characteristics. According to the results of the vari- noticed when comparing the different abrasion and ance analysis the hypothesis is rejected that palsa block-erosion amounts. The biggest difference between height is a significant control on active layer depth. 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 4.2 Active layer depth and block-erosion and block-erosion amount. Frequencies show that active layer depth increases The depth of the active layer increases as the amount with height, amount of block-erosion and abrasion, of block erosion increases on palsas as the insulating but this is to be expected since they correlate with peat cover is eroded by collapse. Mean active layer each other. depths are 49.8 cm, 55.1 cm and 57.4 cm in the three block-erosion classes. In palsas that are little col- lapsed the active layer is thinnest beneath the Betula 4.5 Relationships between palsa characteristics nana surface according to the median (45 cm), but according to the average (46.6 cm) it is thinnest under- The relationship between observed palsa characteris- neath moss-covered surfaces. On all kind of surfaces tics were investigated by Spearman rank correlation the active layer gets thicker as the block-erosion (Table 2). The relationship between palsa height and amount increases. 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 4.3 Active layer depth and abrasion similarly tested and yielded r 0.291. This shows a relationship that is almost statistically significant. The variance analyses indicates that active layer depth The relationship between surface abrasion and increases with greater abrasion, regardless of the sur- block-erosion gives r 0.487, and is statistically face character. The active layer is thinnest beneath the significant. A low palsa can be very abraded because Betula surface and thickest under the lichen surface. it is old and worn. Block erosion can be detected only The standard deviation is rather large and the distribu- from big palsas because the collapsed peat blocks are tion of data is bimodal. already sunk into the wet mire.

997 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. Abrasion/ Active Height block erosion layer (cm) Figure 3. Relationships between the height of palsas and I active layer thickness on different kinds of palsa surface. A 120 1–246–52 B 120–160 2 41–56 C 150–170 1, 2, 3 50–62 4.6 Regression analysis II D 180–250 2–348–64 Regression analysis was used to investigate whether E 200–230 2–349–64 palsa height controls the active layer depth. F 270–330 2–357–64 The regression model gives R2 0.19, which means that palsa height explains only 19% of the thickness of the active layer. The correlation coeffi- cient between palsa height active layer thickness is: below or equal to 170 cm high, with little or some r 0.43 (n 381, p 0.01). A polynomial regres- deflation and collapsed and their active layer is fairly sion model yielded R2 0.23. thin. Palsas in cluster two are over 180 cm high and It was observed that vegetation cover causes great little or much abraded and collapsed and their active variance in active layer thickness (Fig. 3). The thinnest layer is thicker than the palsas in cluster one (Table 3). active layer is below the Betula nana shrubs and A palsa which does not fit into these clusters is thickest on the lichen-covered surface. rather shallow (only 60 cm high) but its active layer is The effect of vegetation was eliminated by calculat- deeper than other palsas of the same height. Its abra- ing the mean active layer depths of the different sur- sion and collapsing rate is in class 1. It obviously is an face types for each palsa height class. The relationship old palsa melting from its bottom. between palsa height and active layer was then inves- Fairly low and much abraded palsas belong to the tigated. The linear regression model based on mean second cluster; they are classified according to palsa values shows that palsa height explains 60% of active height and active layer thickness. In the main cluster layer thickness (n 381). The polynomial trend line one there are three palsas that appear not to belong to gives R2 0.77 (Fig. 4). Palsas cannot be infinitely any of the sub clusters. high nor the active layer infinitely deep (Fig. 3). Even though the majority of old palsas at Luovdijeäggi mire melt there were found four new 4.7 Cluster analysis palsas with shallow active layers. They indicate that the climatic conditions are suitable for palsa forma- Because the variables were both quantitative and qual- tion. Their formation as well as the presence of largely itative, hierarchical cluster analysis was undertaken. abraded palsas mean strong winter storms and prob- The variables were palsa height, abrasion degree, ably changing wind conditions (Seppälä, in press). amount of block-erosion and surface cover (unvege- tated peat, Empetrum nigrum, lichen, moss and Betula nana). All the variables were measured for 37 palsas 5 DISCUSSION AND CONCLUSION and used in the cluster analysis. Two main clusters were identified; they divide into The thermal balance of the ground is a complex mat- 3 or 4 smaller clusters. Palsas in cluster one are all ter. This study concentrates only on the conditions

998 prevailing at the end of the thawing season. It is there- lichen-cover, which should decrease the thickness of fore important to remember that the conditions in active layer. They assumed that the vegetation succes- winter and spring also affect thawing. Snow cover, for sion from Sphagnum fuscum cover to Cladonia sp. example, has a strong influence on the temperature dominant vegetation (surface albedo) causes the forma- system in the ground and also the hydrological condi- tion of palsas. tions in spring. This study also does not support the theory that According to several studies an active layer beneath melting of palsas starts when the abraded dark surface a vegetation cover does not thaw so deeply as beneath of peat absorbs more radiation during the summer. bare ground. Removal or thinning of the insulating Cummings and Pollard (1990) claimed that the low peat cover normally increases the to cause albedo of unvegetated peat surface causes the thicker thermokarst (Brown 1970; Luthin & Guymon 1974; active layer. Low palsas have shallow active layers Smith 1975; Allard et al. 1996). It was presupposed regardless of the development stage. Generalizing it that the active layer would be thickest on an unvege- could be said that the active layer becomes thicker as tated surface, but was not found in the present study. abrasion, block erosion and the height of the palsa One reason for the fact that the mean active layer depth increase. It seems that palsa height is the controlling on the unvegetated surface is almost the same as on factor on active layer thickness in the Paistunturit area. Empetrum nigrum and moss surfaces could be evapo- ration during the springtime. Melt evaporates more quickly from a bare peat surface than from a veg- ACKNOWLEDGEMENTS etated surface because the vegetation holds moisture well. A dry peat layer protects permafrost below. Professor Derek Mottershead kindly revised the Other explanations for the relatively thin active layer English of the manuscript. Kirsti Lehto made the final could be that the unvegetated peat surface is free of drawing of the figures. Hilkka Ailio finished the lay- snow during winter. Cold can penetrate deeply into the out of the paper. Field investigations were financially peat. The dense or high vegetation collects snow but supported by Seth Sohlberg’s Delegation. also protects the permafrost from spring warming and causes a thinner active layer than sparse or low vegeta- tion (Smith 1975). The active layer was thinner under Betula nana shrubs even though the difference com- REFERENCES pared to other surfaces studied is not significant accord- ing to variance analysis. 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The growing conditions for Betula nana gorization of peat and mineral cored palsas in the are more favourable when the active layer is thicker. Schefferville area, Quebec. Collection nordicana N°54, The active layer was thickest under the lichen-covered Proceedings of the 5th Canadian Permafrost Con- ference, 95–102. Centre d`etudes nòrdiques, Quebec. surfaces regardless of the size of palsa or abrasion or Kershaw, G.P. & D. Gill (1979). Growth and decay of palsas block-erosion. The peat under the lichen cover is fairly and peat plateaus in the Macmillan Pass – Tsichu porous and provides a good insulator but on the other River area, Northwest Territories, Canada. Canadian hand porous peat infiltrates meltwater and rain easily Journal of Earth Sciences 16: 7, 1362–1374. and that enhances thawing. Also the low evaporation Luthin, J.N. & G.L. Guymon (1974). moisture- might explain the high depth of active layer under lichen vegetation-temperature relationships in central Alaska. cover. 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999 Proceedings of the Helsinki Symposium 1966, 143–153. Alaska, 1127–1132. National Academy Press, UNESCO, Paris. Washington D.C. Seppälä, M. (1976). Seasonal thawing of a palsa at Seppälä, M. Surface abrasion of palsas by wind action in Enontekiö, Finnish Lapland, in 1974. Biuletyn Finnish Lapland. Geomorphology (in press). Peryglacjalny 26, 17–24. Smith, M.W. (1975). Microclimatic influences on ground Seppälä, M. (1983). Seasonal thawing of palsas in Finnish temperatures and permafrost distribution, Mackenzie Lapland. In: Proceedings of the fourth international Delta, Northwest Territories. Canadian Journal of conference on permafrost, July 17–22, Fairbanks, Earth Sciences 12: 8, 1421–1438.

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