Non-Sorted Circle Occurrence

Non-sorted circle occurrence

A study of landscape positions where non-sorted circles form

A.J. (Anne) Hoek van Dijke June 2016

Non-sorted circle occurrence

A study of landscape positions where non-sorted circles form

A.J. (Anne) Hoek van Dijke June 2016

Supervisors: Dr. Ir. A.J.A.M Temme Dr. Ir. Y. van der Velde

Soil Geography and Landscape Group Wageningen University

This MSc report may not be copied in whole or parts without the written permissions of the author and the chair group.

Table of contents

1 Introduction 2 1.1 Introduction to non-sorted circle forming processes 2 1.2 Purpose and research questions 4

2 Methods 5 2.1 Study area 5 2.2 Materials and methods 5 2.2.1 Fieldwork 5 2.2.2 DEM analysis 6 2.2.3 Data analysis 7

3 Results 9 3.1 Fieldwork 9 3.2 DEM analysis 10 3.3 Activity analysis 12

4 Discussion 14 4.1 What are characteristics of non-sorted landforms in the Abisko area? 14 4.2 On what landscape position are non-sorted circles present and absent? 14 4.3 How does the activity of non-sorted circles change with landscape properties? 16 4.4 What are the eﬀects of a changing climate? 16

5 Conclusions 19

Bibliography 20

Appendix A 23

Abstract

Non-sorted circles are common frost features in periglacial environments. They play an important role in the storage of soil carbon and vegetation, nutrient and hydrology pattern. Global climate change is expected to alter non-sorted circle occurrence and the activity, however the direction and magnitude of the changes is still unclear. The purpose of this study was to determine landscape characteristics that drive non-sorted circle occurrence and activity. First field work was conducted in Abisko, northern Sweden, and second analysis were done using a Digital Elevation Model (DEM). In the field, characteristics of non-sorted circles were measured and analysed in relation to local altitude and slope. Using a DEM, the occurrence and activity of non-sorted circles was studied with respect to different landscape properties. This study shows that non-sorted circle occurrence is highest above the tree line, but limited at altitudes above 1000 m, where slopes are high and fine materials and soil moisture are limited. On a larger scale, concave topography increases non-sorted circle occurrence, but within the valleys, small ridges are favourable positions for non-sorted circle formation. Hypothesized is that non-sorted circles are favoured by high soil moisture and presence of fine materials, which is found in valley positions. A higher snow depth and longer snow cover in the lowest positions limits non-sorted circle formation. In the study area, few active circles were present. Under the current conditions, soil moisture is found to be the most important driver for non-sorted circle activity. The predicted higher precipitation for the 21st century could potentially increase activity, but higher air temperature and the effect it has on vegetation and active layer depth will decrease activity. 1| Introduction

With the current changes in the global climate high north- and Klaminder, 2012). Because of the large potential of ern latitude areas are expected and observed to warm subarctic soils to store carbon and the influence that pat- more than other regions (Meehl et al., 2007). Air tem- terned ground has on soils and the subarctic ecosystem perature warming is found to be correlated to warming it is important to study the effect of climate warming on of soils (Helama et al., 2011) and climate warming will these subarctic ecosystems therefore influence the permafrost and ice lens growth. Non-sorted circles, which are also known as mud boils, Large amounts of carbon are stored in the arctic soils frost boils, tundra craters or frost scars are a common (Hugelius et al., 2014) and thickening of the active layers form of patterned ground in areas where the July tem- could facilitate the breakdown of soil organic matter and perature is 2 − 12oC (Walker, 2003). Non-sorted circles release large amounts of the greenhouse gasses methane show as round and convex non-vegetated areas. They are and carbon dioxide (Davidson and Janssens, 2006; Mas- approximately 1 − 3 m in diameter and have little veg- tepanoc et al., 2008; Schuur et al., 2015). Past cryotur- etation on them, but they are surrounded by vegetation bation (all movements of soil due to repeated freeze-thaw (Daanen et al., 2008) (fig. 1.1). Non-sorted circles are processes) led to the burial of the major part of this or- found in locations with silty soils (Walker, 2003) and im- ganic matter (Bockheim, 2007) and climate has a major peded drainage (Drew and Tedrow, 1962). On steeper influence on the activity of cryoturbation (Kaiser et al., slopes (5 − 12o), non-sorted stripes may be found (Åker- 2007). Long-term cryoturbation can lead to the forma- man, 1980). Stripes are elongated non-vegetated areas, tion of patterned ground (Schaetzl and Thompson, 2015). which are surrounded by vegetation (fig. 1.2). The veg- Patterned ground is found in the form of circles, stripes, etated area closely surrounding circles and stripes is in nets, polygons and steps and these forms may be sorted or this study called the inter-circle zone, after Walker et al. non-sorted, where sorted forms are outlined by a coarse (2004). The subduction zone is the area in between the stony material (Allaby, 2004). Patterned ground is im- circle and inter-circle area (fig. 1.3). portant in determining soil carbon and vegetation pattern and is important for the solute, heat and nutrient transport in te soil (Bockheim, 2007; Frost et al., 2013; Makoto 1.1 Introduction to non-sorted circle forming processes

Many different mechanisms have been proposed to be important in non-sorted circle formation. Van Vliet-Lanoë (1991) reviewed these mechanisms and concluded that differential frost heave is the main process in the formation of non-sorted circles, although other processes may help. Some important frost and slope related processes involved in the formation of non-sorted circles will be introduced first.

Ice Lens Formation and differential frost heave Cooling of the surface can result in the formation of ice lenses in the soil. Water expands when it freezes and this transition of liquid to frozen water results in a volume ex- Figure 1.1: A non-sorted circle in Abisko, northern Sweden. pansion of approximately 9%, but larger heaving of soils is observed. Taber (1928) was the first to demonstrate that if water freezes, the surroundings get dryer and cryostatic suction pulls unfrozen water from below or the vegetated inter-circle area towards the freezing front. This water freezes onto the ice particles and thick ice lenses form. Because of a difference in insulation and wind chill, ice lens formation is larger on non-sorted circles than on the Figure 1.2: Non-sorted stripes on the slope of the mountain surrounding inter-circle areas (Romanovsky et al., 2008; Suorooaivi, northern Sweden. Walker et al., 2004). The difference in heave between 1.1. Introduction to non-sorted circle forming processes | 3 the circle and inter-circle area is called differential frost and settle vertical when thawing (Hargitai and Johns- heave. son, 2014). Solifluction can operate on gradients as low The formation of ice lenses requires (among others) as one degree and the annual rates of downslope move- repeated freeze-thaw cycles and water available for freez- ment vary between ca. 0.5 and 10 cm a−1 (Matsuoka, ing. Impeded drainage, e.g. a permafrost or bedrock 2004). Important factors that determine solifluction are layer, increases water availability. A high capillary action climate (Matsuoka, 2004), soil moisture, slope gradient and high water permeability allows water to move towards (increasing rates with inclination), grain size distribution the freezing front. In silty soils, these conditions are met and vegetation cover (Matsuoka, 2001). Larger stones or and therefore ice lenses preferably form in silty soils. Cold vegetation can stop soil movement and form the end of a temperatures to allow for freezing are found in locations solifluction lobe (Matsuoka, 2004). with little isolation; no plant cover, no organic matter and no thick isolating snow layer (Zhang, 2005). Initiation and self organization The development of non-sorted circles and stripes can Vertical and radial movements in non-sorted circles be explained with frost and slope processes. The initia- The soil in non-sorted circles moves circulatory (fig. tion of non-sorted circle and pattern formation is less well 1.3). The ice lenses heave the soil and during periods of known. The regularity (equal distances and sizes) sug- thaw, the heaved soil in the centre moves to the sides gest a better organization than would follow from ran- by frost creep and erosion (e.g. Klaminder et al., 2014; dom initiation. Nicholson (1976) suggested that the ran- Klaus, 2012). The open spaces formed by the melting dom heaving of one non-sorted circle might trigger the ice lenses are filled up with soil from below. In the sub- formation of non-sorted circles in the surrounding areas, duction zone soil is transported downwards (Klaminder by changing the vegetation, soil moisture or by elevation et al., 2014). This subduction hypothesis is supported by differences. E.g. cryosuction depletes the soil surround- evidence of buried organic material in the margins of non- ing a circle and thereby influences the surrounding soil sorted circles (Becher et al., 2013; Boike et al., 2008). and non-sorted circle formation (Fowler and Krantz, 1994; Vertical sorting of the soil takes place in non-sorted Peterson, 2011). This process could explain regular pat- circles, because larger stones are brought to the sur- tern formation. When a non-sorted circle field has devel- face faster than finer materials by frost pull and frost oped, with bare centres and vegetated inter-circle areas, push (Derbyshire et al., 1979). For the first process, ice the pattern will maintain itself. The circle centre differs lenses form under stones, (because stones have a higher from the inter-circle areas in (soil) temperature, hydrol- thermal diffusivity than the surrounding soil (Bowley and ogy, thaw depth and nitrogen and carbon content (Boike Burghardt, 1971)) and force the stones up. By frost pull, et al., 2008). The circle centres are the higher locations the soil above a stone is lifted during freezing and the in the landscape, where more heat escapes and are there- stone is pulled up when the adfreeze force around it is fore more susceptible to freezing and ice lens formation greater than the forces holding it to its place (Kaplar, 1965).The vertical sorting and radial soil movement can lead to the formation of sorted circles, (a circle of fine material is bordered by stones), if the sorting takes place for longer periods (Hjort, 2006; Hopkins and Sigafoos, 1950; Nicholson, 1976; Van Vliet-Lanoë, 2014).

Slope Processes On steeper slopes, non-sorted stripes are found which form by solifluction (King, 1971). The direction of the stripes reflects the direction of the greatest slope (Åk- erman, 1980; King, 1971). Different definitions of solifluction exist, the definition of the encyclopedia of geomorphology is used: "slow downslope movement of soil mass usually associated with freeze thaw cycles and frost Figure 1.3: Differential frost heave and radial soil movements in non-sorted circles. The inter-circle area is insulated by heave" (Matsuoka, 2004, p. 984). Solifluction includes vegetation and organic matter, while the circle centre is not. gelifluction and frost creep, where the first is a slow Therefore, the circle centre heaves by ice lens formation. Soil downslope flow of unfrozen earth on a frozen layer (per- material moves down the slope by solifluction and buries or- mafrost) and the latter is the movement of individual par- ganic material in the subduction zone. The voids left by the ticles which heave normal to the slope by frost heave molten ice lenses are filled from below with soil material. 4 | Chapter 1. Introduction

(Peterson and Krantz, 2003). Also vegetation contributes be tested: altitude, slope, curvature, Topographic Po- to the self-organization of non-sorted circles. Vegetation sition Index, Topographic Wetness Index, northness and grows in the inter-circle areas where nutrient availability eastness. is higher and they insulate the surface, which decreases The subquestions for this study are: ice lens formation in the inter-circle area. Plants also • What are characteristics of non-sorted landforms in promote nitrogen input in the soil which favours plant the Abisko area? growth. Conversely, plants do not grow in the circle cen- • On what landscape position are non-sorted circles tres, because these areas are unstable and the plant roots present and absent? experience frost damage (Jonasson, 1986). Walker et al. (2004) made a conceptual model of non- • How does the activity of non-sorted circles change sorted circle ecosystems, which consists of the circle and with landscape properties? inter-circle area and ice lenses, vegetation and the soil. In cold arctic systems, the biological processes are weak and Hypothesis physical processes dominate in the formation and main- Prerequisites for the occurrence of frost heave are taining of non-sorted circles. In a subarctic climate, the freezing temperatures, a high water availability and the vegetation processes dominate (Walker et al., 2004). presence of fine materials. Precipitation in the form of rain increases frost heave and a shallow bedrock or per- Activity of non-sorted circles mafrost can impede drainage and therefore enhance non- Non-sorted circles may be active or inactive and the sorted circle formation. The temperature is influenced by activity is dependent on climate (Hopkins and Sigafoos, the presence of snow and vegetation, which insulates the 1950). High amount of carbon buried by cryoturbation soil and also sunshine and wind influence the soil temper- in non-sorted circles was for example found to occur dur- ature. A difference in frost heave between the circle and ing periods of climate warming when the permafrost was inter-circle area is required for differential frost heave to thawing (Becher et al., 2013; Bockheim, 2007). This may occur. This requires the presence of vegetation. Regard- indicate that cryoturbation will increase as a consequence ing the landscape properties that will fill be tested and of climate warming (Bockheim, 2007; Henry, 2008). In there relation with presence and absence of non-sorted Canada, frost heave was studied along an arctic to sub- circles, it is hypothesized that: arctic transect (Walker et al., 2008) and in Abisko, differ- • Since temperature decreases with altitude, non- ential frost heave was studied along an elevation gradient sorted circle occurrence increases with altitude. (Klaus et al., 2013). Both studies concluded that differ- • The limited availability of soil moisture and fine ma- ential frost heave is highest in the middle, where the dif- terials at high altitude limits non-sorted circle forma- ference in vegetation between the centres and inter-circle tion. area is largest. A northward shift of vegetation as a result • Non-sorted circles are found in areas with a low slope, of climate warming may thus increase non-sorted circle because at steeper slope angles, solifluction will lead activity in unvegetated arctic areas if the inter-circle ar- to the formation of stripes. eas gets vegetated. In the Abisko area, a net overgrowth of non-sorted circles by vegetation was found in the last • More non-sorted circles are found in depressions, be- fifty years, which indicates that the circles became less ac- cause these are the locations where fine materials tive (Becher et al., 2013). This conclusion was supported and soil moisture collect. by the fact that no buried organic layers were found from • More non-sorted circles are found at the north facing the last decades. This overgrowth of vegetation may be slope. At south facing slopes, solar radiation is higher the effect of less intense frost heave, or the inactivity is which raises the air and soil temperature and thereby the result of an increase in vegetation, since roots can limits non-sorted circle formation. stabilize the soil (Raynolds et al., 2008). In short, this study consisted of two parts. First field work was conducted in the Abisko area and second, anal- 1.2 Purpose and research questions ysis were done using a digital elevation model. In the field, several aspects of non-sorted landforms (circles and To be able to predict the effects of climate warming and stripes) were measured and these data were statistically a changing precipitation on non-sorted circle occurrence analysed. Using a digital elevation model, presence, ab- and activity it is important to study the drivers for non- sence and activity of non-sorted circles in relation to dif- sorted circle occurrence and activity. The purpose of this ferent landscape aspects were tested. The methods, re- study is to determine landscape properties where non- sults and discussion are presented in the following chap- sorted circles occur. Seven different terrain aspects will ters. 2| Methods

2.1 Study area

This study was carried out in Swedish Lapland. The study area is approximately 15 by 7 km, with the Abisko field station in the northwest corner of the study area (68o210N, 18o480E). The study area is bordered by the valleys of the rivers Bessesjohka and Nissonjohka and at the north by Lake Torneträsk (fig. 2.1). Within the area, the elevation ranges from 340 to 1740 m. The average yearly temperature measured at the Abisko Scientific Research Station (for the period 1980- 2014) is −0.4oC and is increasing since 1980. The average January and July temperature is respectively −9.8oC. Figure 2.1: The study area in north-Sweden and 9.8oC. The area is relatively dry, because it lies in the rain shadow of the mountains in the west. The average precipitation is 313 mm/year of which 19% falls in In the field, the x- and y-coordinate and elevation July, the wettest month. A snow layer is present from ap- was determined for every landform using a gps and the proximately October to May and the average snow depth slope was measured using an inclinometer. The length (in is 34 cm, but average yearly snow depth is below aver- the downslope direction) and width (perpendicular to the age since 2000 (fig. 4.2). (Abisko Scientific Research length) of each landform was measured. The height of the Station, unpublished data) The snow layer is thickest in top of the non-sorted landforms relative to the vegetation March, on average 54 cm. In the study area, the per- was estimated visually in the following classes: < −20 mafrost is discontinuous (Brown et al., 1998, revised in cm, −20 − 0 cm, 0 − 10 cm, 10 − 20 cm, > 20 cm. The 2001) with active layers of approximately 60−90 cm deep surface stoniness (the percentage surface coverage of the which became thicker during the past decades (Åkerman non-sorted landforms with stones) was estimated visually and Johansson, 2008). for boulders (>25 cm), stones (7.5 - 25 cm) and pebbles (0.2 - 7.5 cm). The surface coverage by vegetation was 2.2 Materials and methods estimated visually, where a distinction was made between all vegetation and green vegetation, where the latter included grasses, forbs and shrubs. Last, it was noted down 2.2.1 Fieldwork if lichens, mosses, forbs, grasses, deciduous and evergreen The fieldwork was conducted in July 2015. In the study shrubs were present in the circle and/or inter-circle area. area, six different non-sorted circle fields and two different non-sorted stripe fields were studied (fig. 2.2). In the Soil moisture field, for every landform it was noted down whether it was a circle or stripe and if it was active or not. With land- Soil moisture of the circle centre and inter-circle area forms, both circles and stripes are meant. Landforms were was measured for fourteen circles at sample locations 2 recognised as stripes because they had a length width and 3 (fig. 2.2). Soil samples were taken at ∼ 10 cm ratio of approximately more than two. There is no uni- depth or if present, below the purely organic layer. In the versal definition for activity and of non-sorted circles and laboratory, the samples were sieved to remove branches, stripes and there is also no universal method to deter- roots and stones. Approximately 10 grams of soil were o mine activity (Klaus, 2012). Klaus methods included the weighed and dried in an oven at 105 C for 24 hours. The measuring of differential frost heave or vertical and radial dried samples were weighed again and the soil moisture soil movement. These methods require a longer period of content was calculated as measurements or repeated measurements which was not mwet soil − mdry soil possible within this study. Therefore, the activity of pat- % soil water = ∗ 100 mwet soil terned ground was based on whether recently brought up material was visible at the surface. Soil without a moss For two circle centre samples, the samples were not cover and stones without lichens were an indication of weighed correctly and therefore the results of 14 inter- recently brought up material. circle and 12 circle centre positions are presented. 6 | Chapter 2. Methods

Figure 2.2: The ﬁeld sample locations are indicated at the digital elevation model.

During the sampling and sieving, water in the samples order polynomial equation was used for the transforma- had condensated and therefore, the results underestimate tion, with a total root mean square error of 21 m. The the true soil moisture content. DEM was checked for voids and outliers and sinks were filled using the function ’sink fill’ in ArcMap. Lakes that could be identified from a topographic map were excluded 2.2.2 DEM analysis from the sink fill process. The sink fill tool changed 0.26% A Digital Elevation Model (DEM) was used to analyse on of the DEM. which landscape positions non-sorted circles are present and absent. A LiDAR DEM with a 2 m resolution was The topographical parameters used in the analysis available from Landmäteriet, the Swedish National Land were the altitude, slope, curvature, Topographic Position Survey. Since the DEM is used to study the landscape Index, Topographic Wetness Index, northness and east- rather than the non-sorted circles themselves, a coarser ness. The altitude was derived from the DEM and the resolution was preferred. To remove noise introduced by slope was calculated in ArcGIS. The Topographic Po- the patterned ground, the DEM was resampled to 20 m sition Index was calculated with the eponymous tool in using the cubic convolution method. Also a geomorpho- SAGA GIS (Guisan et al., 1999). The TPI is calculated logical map of the area was available (Melander, 1977), as the altitude minus the mean altitude of the surround- where the presence of ’patterned ground at flat terrain’ ing area (fig. 2.4). The value for TPI depends on the was marked, which were assumed to be non-sorted circles. scale factor, which is the diameter chosen to calculate This assumption is supported by the high agreement of the mean altitude (fig. 2.4). The TPI was calculated non-sorted circles observed in the field and marks on the with a scale factor of 60 and 400 m (fig. 2.4). For the map. Furthermore no other types of ’patterned ground north border of the study area, the TPI at 400 m scale is at flat terrain’ were found during the field work. The overestimated, since the lake level was used to calculate scanned geomorphological map was georeferenced with the average altitude instead of the lake bottom. The To- help of 16 control points on a topographic map. A second pographic Wetness Index was calculated in SAGA GIS 2.2. Materials and methods | 7

(Boehner and Selige, 2006). The TWI is calculated as: SCA ln tan(β) where the SCA is the specific catchment area, which is the inverse of the value calculated with the tool ’catchment areas (parallel)’ and tan(β) is the local slope. In ArcGIS the curvature is calculated using the function of Zevenbergen and Thorne (1987). The values for curvature are positive for a convex surface and negative for a concave surface. In ArcGIS the aspect was calculated and afterwards, the cells with an aspect of −1, flat areas, were removed. The northness and eastness were calculated as resp. the sine and cosine of the aspect. The north- and eastness range from −1 to 1, where −1 indicates resp. south and west and 1 indicates north and east (fig. 2.4). The areas with non-sorted patterned ground according Figure 2.3: The in the field measured altitude and slope com- to the geomorhological map were marked in ArcMap and pared with the altitude and slope derived from the 20 m res- a grid was created with the same cell size as the DEM olution DEM. For slope and altitude, the correlation between the measured and DEM derived values is resp. 0.71 and 1.00. with 0’s (non-sorted circle absent) and 1’s (non-sorted circles present). For logistic regression it is recommended to use equal portions of 0 (absence of non-sorted circles) and 1 (presence of non-sorted circles) (Ayalew and Yam- agishi, 2005) and therefore, 2500 points with and 2500 points without non-sorted circles were randomly selected using the ArcGIS tool ’Create Random Points’ (Appendix A). These 5000 points represented 2.4 and 0.58% of the total number of points resp. with and without non-sorted circles. The points were coupled to the in that grid cell present values for altitude, slope, curvature, aspect, TPI (a) and TWI. Also the coordinates for the in the field analysed non- sorted landforms were coupled to corresponding values for altitude, slope, curvature, aspect, TPI and TWI. A comparison between the in the field measured slope and altitude and the DEM calculated slope and altitude show that there is a high agreement between the two mea- (b) sures for altitude, but for slope the DEM derived slope is 0.15o ± 3.0 higher. The large difference for slope can be explained by a difference in scaling: in the field, the slope was measured very local, while the DEM had a resolution of 20 m. In the analysis, the field measured slope and DEM derived altitude were used.

2.2.3 Data analysis For the measurements of the fieldwork, the relationships between the measured landscape properties altitude and (c) slope and the non-sorted circle parameters were analysed using linear regression. A relationship was assumed to be Figure 2.4: (a) The TPI is calculated as the altitude minus significant for p-values ≤ 0.05. the average altitude of the surrounding cells. The scale factor determines the width of the area wherefore the average altitude Landscape properties of the active and inactive circles is calculated. (b) shows the effect of different scale factors, were compared visually and with t-tests. The landscape 400 m compared to 60 m. (c) The northness and eastness are calculated as resp. the sine and cosine of the aspect. 8 | Chapter 2. Methods properties of active landforms were tested against those For the DEM analysis, locations with and without non- of all inactive landforms and against the in the same area sorted circles were compared using binary logistic regres- found inactive landforms. sion. Binary logistic regression estimates the probability For the soil moisture samples, a comparison was made that non-sorted circles are present given the landscape between the circle centre and inter-circle area. Also pos- properties. Logistic regression requires the explanatory sible relationships between soil moisture content and non- variables to be uncorrelated. Curvature & TPI on sixty sorted circle- and landscape properties were tested using m scale were correlated (r=77%) and are therefore not linear regression. combined in the regression models. Different regression models were compared based on the ROC-curve and percentage false positive and false negative estimations. 3| Results

3.1 Fieldwork For the sample fields 2, 3, 5 and 8 (fig. 2.2), non-sorted circle fields were characterised as locally elevated, convex In total, 85 non-sorted landforms were sampled in the ridges in the landscape. The lower surroundings were Abisko region, spread over eight locations (fig. 2.2). Of covered with trees, high shrubs or a peaty vegetation. the 85 landforms, 72 were circles and 13 were stripes. The stripes were found at higher slopes than circles (p < All stripes were inactive. Of the 72 circles, seven were 0.05) (a mean slope of 11.9o for the stripes versus a mean classified as active. The inactive circles were covered by slope of 4.2o for circles). Three stripes did not follow the vegetation and lichens were found at the surface stones. steepest slope, e.g. one stripe flowed down a slope of 4o The active circles were to a less extent covered by mosses, while the slope in the perpendicular direction was 10o. forbs and shrubs. Furthermore, only a few small lichens were found on the surface stones. The absence of lichens Soil and presence of soil without a moss crust may indicate The non-sorted circles were characterized by a grey that this material was brought to the surface recently. silty soil, with reddish and bluish colors, while the soils An other evidence for activity on these circles was a 40 of the inter-circle areas were dark and contained more cm high willow tree that was tumbled over by a stone. humus. In the inter-circle areas, dark organic layers were Apart from the difference in vegetation- and lichen cover found on top of the silt, with a depth of several cm up to between the active and inactive circles, more differences 20 cm. In these organic layers, little to no silts and stones were observed. The active circles were lower than the sur- were present. rounding inter-circle areas (±20 cm lower), while the in- Soil moisture samples were taken in the circle centre active circles were elevated above the surroundings. Also and the inter-circle area. The soil moisture content in the the active circles had a little pool in the back. All stripes vegetated inter-circle areas was higher (average = 26.7%) in the area were classified as inactive, since they were all than in the circle centres (average = 12.4%) (p < 0.05). overgrown with mosses and lichens were present at the The soil moisture content in the circle centres was nega- surface stones. tively correlated with the coverage by stones < 7.5 cm (p < 0.05). No other relationships were found between soil Size and shape moisture content and non-sorted circle characteristics or The measured non-sorted circles had an average landscape properties. length of 237 cm in the downslope direction and an width of 275 cm. For the stripes, the length and width were Stoniness resp. 803 and 188 cm. Active circles were larger than The total surface stoniness (percentage of the surface inactive circles (length and width resp. 215 and 271 cm covered with stones) increased with altitude and slope (p for inactive and 390 and 296 cm for active circles) (P < 0.05) (table 3.1). On average, 45% of the non-sorted < 0.05 ). Longer landforms (both circles and stripes) circle and -stripe surfaces was covered with stones, of were found on steeper slopes and on higher altitudes (p which 31% of the stones was smaller than 7.5 cm. Below < 0.05) (table 3.1). Analysing 13 stripes only, longer 400 m a.s.l., the inactive circles had a surface coverage of stripes are found on steeper slopes, but this relationship 14% and the active circles had a stoniness of 46 %. Stones is not significant. The length in downslope direction is > 25 cm were sparsely present (0% on the inactive and 3% shorter than the width in the perpendicular direction for on the active circles). Above 400 m the surface coverage 58% of the non-sorted circles. The roundness, defined of stones > 25 cm was on average 5 % and independent as width divided by length is not dependent on slope or of altitude. Analysing circles only, there was no relation altitude. between surface stoniness and slope, but for stripes, the total stoniness increased with altitude and decreased with 2 Landscape properties slope (multiple linear regression, Radj = 74%). In the field, non-sorted circles and stripes were found at the north, east and west facing slopes of the mountains Vegetation at various altitudes. South facing slopes were also visited Considering vegetation, three different areas can be during the field work period, but no measurements were described, the non-sorted circles/stripes, the inter-circle done on these slopes. Non-sorted circle fields were most areas and the areas surrounding the non-sorted circle present above the treeline and below the treeline non- fields. All of the studied non-sorted circles and stripes sorted circles were present on non-forested locations only. were (partly) covered with a lichen- and moss vegetation 10 | Chapter 3. Results

Length Width Stones Stones Stones Total Vegetation Green 0-7.5cm 7.5-25cm >25cm stoniness coverage vegetation [cm] [cm] [%] [%] [%] [%] [%] [%] All landforms (n=85) Mean 324 262 32 9 5 45 62 31 Standard deviation 266 151 20 8 5 24 24 21 Correlation with altitude + + + + + − − Correlation with slope + + + + Circles (n=72) Mean 236 275 30 8 4 42 66 34 Standard deviation 116 160 22 7 5 24 24 21 Correlation with altitude + + − − Correlation with slope + + Stripes (n=13) Mean 766 190 39 17 8 64 41 14 Standard deviation 362 56 11 7 6 10 14 10 Correlation with altitude + − Correlation with slope − + − +

Table 3.1: The linear regression results for the non-sorted landforms and for a subset of circles and stripes. The mean and standard deviation are presented and the results of the linear regression. For coefficients significantly different from zero (p ≤ 0.05) a + indicates a positive relationship, a - indicates a negative relationship. and also grasses and various types of dwarf shrubs (dwarf Explanatory Variable Coef. S.E. Sig. ROC birch, blackberry, salix, and heather were most common) Altitude [km] .503 .110 .000 .547 were found on most of the landforms. The coverage by Absolute deviation from -11.7 .342 .000 .830 grasses and dwarf shrubs was resp. 67% and 86 %. The mean altitude [km] vegetation cover of inactive circles and stripes decreased Slope -.077 .005 .000 .600 with altitude (p < 0.05) (table 3.1). Above 800 m a.s.l., Northness -.087 .041 .033 .544 the total and green vegetation cover was resp. 35% and Eastness -.065 .042 .123 .509 12%. The vegetation cover was highest in the areas be- Curvature .012 .032 .716 .502 low 400 m altitude, on average 92% of the inactive circles TWI .059 .017 .001 .532 was covered with vegetation, of which 45% with green TPI 60 m .003 .018 .853 .500 vegetation. For the active circles on the same altitude, TPI 400 m -.009 .002 .000 .507 Absolute deviation from the vegetation cover was 57% of which 46% was green -11.7 .346 .000 mean altitude [km] vegetation. .859 The inter-circle areas were characterized by a higher Slope -.074 .007 .000 Constant 2.73 .089 .000 and denser vegetation of mosses, grasses, forbs and dwarf shrubs. Table 3.2: Result of the binary logistic regression analysis Non-sorted landform fields were bordered by forest or showing the coefficient, standard error, p-value and area under by a higher and more dense vegetation of shrubs or peat the ROC curve. than the non-sorted landform fields itself. In some areas, the transition between a non-sorted circle field and the surrounding vegetation was sharp, but in other other non-sorted circles are relatively more abundant than areas areas, the transition was more gradual with overgrown without non-sorted circles compared to the north facing non-sorted circles at the side of a non-sorted circle field. slopes. The presence and absence of non-sorted circles does not change with eastness. The 5000 analysed random points are found between 3.2 DEM analysis altitudes of 342 and 1682 m. Ninety percent of the locations with non-sorted circles is found between 555 and Comparing locations with and without non-sorted 988 m, with a peak in abundance between 850 and 900 m. circles No non-sorted circles are present above 1080 m. The al- From fig. 3.2 it is clear that more than half of the titude of the area is different for the north- and the south study area is a north facing slopes. At south facing slopes, facing slopes and also the occurrence of non-sorted cir- 3.2. DEM analysis | 11

Figure 3.1: The altitude at which non-sorted circles (orange) are found compared to the altitude of all 5000 random selected points (green). The x-axis represents northness; at the left all positions with a northness between -1 and -0.75 and at the right all positions between a northness of 0.75 and 1. Other landscape properties did not change with aspect. cles is different for the north- and the south facing slopes (fig. 3.1). On south facing slopes, non-sorted circles are found at lower altitude than average, while on the north facing slopes, non-sorted circles are found at higher altitude than average. Regarding slope, non-sorted circles are most present on low slopes; 78% of the locations with non-sorted circles is found at slopes between 0 − 10%, compared to 60% of the locations without non-sorted circles. Non-sorted circles are more abundant on positions with a high topographic wetness index (TWI). For curvature, no relation with non-sorted circle occurrence is found. The average topographic position index (TPI) for areas with non-sorted circles differs for a 60 m and 400 m scale. On a 60 m scale, the average TPI of locations with non-sorted circles is 0.0033, which is −.64 for the 400 m scale. This means that at smaller scale, non-sorted circles are found on at average, slightly more convex locations, while they are on average found at more concave positions when looking at a larger scale (p < 0.05). For the 60 m scale, there is no significant difference in TPI for locations with and without circles, but for the 400 m scale the TPI for locations without non-sorted circles is higher.

Binary logistic regression Non-sorted circle occurrence does not increases linear with altitude and therefore ’the absolute deviation from mean altitude’ was included in the analysis as an extra explanatory variable, which was derived from the altitude. The variable was calculated as: Absolute deviation from mean altitude = |Altitude Figure 3.2: The absence and presence of non-sorted circles −786.3| at diﬀerent landscape positions. Note that for both groups, 2500 points were random selected and that this selection is not representative for the ratio between locations with and without non-sorted circles and that the sum of 2500 with 2500 points without non-sorted circles is also not representative for the whole area. 12 | Chapter 3. Results

Where 786.3 m is the mean altitude for the 5000 random selected points. This variable was chosen as explanatory variable for the prediction of non-sorted circle occurrence, since this gave a better prediction for non- sorted circle occurrence than altitude (table 3.2). Alti- tude, slope, northness, TWI and TPI (at 400 m scale) are significant predictors for non-sorted circle occurrence. Non-sorted circle occurrence decreases with increasing distance to the mean altitude. Furthermore, the probability of non-sorted circle occurrence decreases with increasing slope and increases with increasing TWI. The coefficient of northness is negative; there are more non- sorted circles at south facing slopes. TPI at the 400 m scale decreases with non-sorted circle occurrence, which indicates that non-sorted circles are more often found at areas which are lower than the surrounding 400 m. Using slope and absolute deviation from mean altitude, the probability of non-sorted circle occurrence is best estimated. The probability for non-sorted circle occurrence is then calculated as: ey P = 1+ey , where y = −0.012∗|(Altitude−786.3)|−0.074∗Slope+2.729 For the 5000 points, the percentage correctly estimated is 78, 6%. The probability map for non-sorted circle occurrence created from this formula is presented in fig. 3.4. For median altitudes non-sorted circle occurrence is correctly estimated. In the low altitude, north facing area, a few non-sorted circle fields are present, while the probability of occurrence is below 20%.

3.3 Activity analysis

Comparing seven 7 and 78 inactive landforms (circles and stripes), the active landforms are found at lower altitude, close to lake Torneträsk (fig. 3.3). Furthermore, the active circles are found at lower slopes and at locations with a higher TWI and lower TPI (p < 0.05). The latter is most pronounced for the 400 m scale. The curvature does not differ between position with active and inactive circles. When zooming in on the shore of lake Torneträsk, active and inactive circles were found within a distance of 100 m. The active circles were found at a lower altitude, lower slope, higher TWI and lower TPI at both the 40 and 60 m scale (fig. 3.3). Furthermore, the active circles are found on average 45 m closer to the lake than the inactive circles. In the field, no differences in surrounding vegetation, solar radiation and soil texture was visible between active and inactive circles. Figure 3.3: Comparison of active and inactive circles in the study area. In total, there were seven active circles measured, but because they were found in the same DEM raster cell, they appear as four. 3.3. Activity analysis | 13

Figure 3.4: Probability map of non-sorted circle occurrence. The presence of non-sorted circles is based on a geomorphic map of the area. The probability is calculated as a function of altitude and slope. 4| Discussion

4.1 What are characteristics of 4.2 On what landscape position are non-sorted landforms in the Abisko non-sorted circles present and area? absent?

Altitude, slope, northness, TWI and TPI are significant predictors for non-sorted circle occurrence in the Abisko region. Below the different hypothesis are discussed regarding the occurrence of non-sorted circles and land- Non-sorted stripes were found on steeper slopes than cir- scape properties. cles. This was expected because solifluction rates increase with inclination (Matsuoka, 2001). Non-sorted stripes are reported to be oriented down the steepest slope (Åker- Hypothesis 1: "Since temperature decreases with man, 1980), but in the Abisko region three stripes were altitude, non-sorted circle occurrence increases with oriented down a less steep slope. An explanation for this altitude." can be that flow down the steepest slope was hindered by the occurrence of larger stones, since they can stop Hypothesis 2: "The limited availability of soil moisture soil movement (Matsuoka, 2004). The coverage of land- and fine materials at high altitude limits non-sorted forms by stones was highest on the stripes. The amount circle formation." of surface stones is found to increase with elevation and is correlated to a decrease in vegetation. A decrease in veg- From the binary logistic regression analysis, non-sorted etation can be expected because of higher wind, less soil circle occurrence was found to increase with altitude. Dis- and lower temperatures. Since vegetation covers the soil tance to the mean altitude was however a better predic- (and stones) the observed correlation between vegetation tor for non-sorted circle occurrence. Ninety percent of and surface stoniness was not unexpected. the points with non-sorted circles was found between 555 and 988 m. The tree line, at 550 m a.s.l. (Milbau et al., 2013) and a relation with non-sorted circle occur- During the field trip, one location was found to have rence above the the tree line was also shown by (Hjort, active circles and no other active circles were observed in 2006). Below the tree line, higher and denser vegeta- the area. Klaus (2012) studied activity of non-sorted cir- tion limits the formation of non-sorted circles, through cles in approximately the same area and he did measure the insulating effect and stabilization by the roots. As differential frost heave and lateral soil movement in areas hypothesized, non-sorted circle occurrence was limited at that were classified ’inactive’ in this study. Ridefelt and higher altitude. A lower Topographic Wetness Index was Boelhouwers (2006) concluded that their landforms were found at higher altitude. But there are more possible ex- active because ’outflows of silt and clay slurry’ were visi- planations than only the absence of finer materials and ble. This was also observed in the field, but these circles soil moisture. An explanation for the lack of circles at were classified as inactive if they were covered with mosses higher altitudes can be found in the presence of higher and the stones were covered with lichens. There are dif- slopes at higher altitude. Also a lack of vegetation and ferent proxies for activity and no definition of activity is a prolonged period of snow cover can explain non-sorted given in literature. This leads to inconsistent conclusions circle absence at high altitudes. regarding activity. In this study, by activity was meant that radial movements in the soil were occurring, mean- ing that fresh stones are brought up and organic matter is Hypothesis 3: "Non-sorted circles are found in areas transported down. Differential frost heave itself will not with a low slope, because at steeper slope angles, result in a downward transport of organic material and solifluction will lead to the formation of stripes." is therefore of less interest when for example the effects of activity on soil carbon storage are studied. But when Both the results of the field work and the DEM analy- vegetation and hydrology pattern are of interest, differen- sis support this hypothesis. In the field, non-sorted stripes tial frost heave is a useful measure for activity since ice were found at on average higher slopes than non-sorted lenses cause damage to plant roots and affect soil water circles and also the geomorphic map shows stripes on movement. higher slopes. 4.2. On what landscape position are non-sorted circles present and absent? | 15

Hypothesis 4: "More non-sorted circles are found in In this study in south facing areas, non-sorted circles depressions, because these are the locations where fine were found at higher altitudes than on north facing areas. materials and soil moisture collect." A similar relation was found by DeGraff (1976) for relict According to the DEM analysis, non-sorted circles patterned ground. In this study, no explanation for this were found to have a higher Topographic Wetness Index can be found in a comparison between the other studied (TWI) than locations without non-sorted circles. This landscape properties and aspect and altitude. The found was expected because ice lens formation requires a high relationship can be an artefact of the study area, because water availability. The Topographic Position Index (TPI) > 60% of the 5000 points lie on north facing slopes and indicated that flat and slightly concave or convex ar- the average altitude of north facing slopes (741 m) is eas are most favourable for non-sorted circle formation, lower than the average altitude of south facing slopes which can be explained by the corresponding low slopes. (873 m) (p < 0.05). It is also possible that other factors The higher occurrence of non-sorted circles at areas with than studied here which are influenced by altitude and as- a negative TPI at 400 m means that areas below the pect affect occurrence of patterned ground. For example surroundings (associated with concave areas) are more the higher solar radiation, more freeze-thaw cycles and favourable for non-sorted circle formation. This result a higher soil moisture content may make soils up to a was also found by Hjort (2006) who studied non-sorted higher altitude suitable for non-sorted circle formation on circle occurrence in Finnish Lapland, 300 km northeast of the south facing side. Abisko. In my study, for a 60 m scale factor, the mean TPI for non-sorted circle presence was positive. This is Predicting non-sorted circle occurrence in agreement with the field observations that, on a local Of the studied parameters; altitude and slope are the scale non-sorted circles were found on low convex ridges most important predictors for non-sorted circle occur- in the landscape, instead of in valleys. The concave posi- rence. The binary logistic regression model including al- tions are the areas where fine material and soil moisture titude and slope predicted 78, 6% of the 5000 random accumulates, which favours ice lens growth. The con- chosen points correctly. A part of the variation is not ex- cave areas are however also the areas where wind driven plained by this regression model. Variables that were not snow accumulates which limits ice lens formation. Within included in this model, but that might improve the predic- larger concave valleys, my results suggest that the formation of non-sorted circle occurrence are the canopy cover, tion of non-sorted circles in the small scale concave areas presence of fine materials/bedrock and relative radiation. is limited by snow, while availability of fine materials and Also Hjort (2006) found that slope and altitude are impor- moisture is not limited on the small scale convex loca- tant predictors for non-sorted circle occurrence. Hjort also tions. Summarizing, on a 400 m scale, hypothesis 4 can successfully included canopy cover and relative radiation be confirmed, but on a 60 m scale, non-sorted circles are as explanatory variables. On a larger scale, precipitation found on convex positions. is important to include in a model, since yearly precipitation varies strong in the Abisko area and soil moisture Hypothesis 5: "More non-sorted circles are found at is found to be important. Variance in soil temperature is the north facing slope. At south facing slopes, solar partly explained by differences in aspect and altitude, but radiation is higher which raises the air and soil small-scale variation as a result of cold-air drainage, snow temperature and thereby limits non-sorted circle formation." Non-sorted circles are found to be more abundant at the south facing side than on the north facing side of the mountains. In the study area precipitation and solar radiation are higher at the south facing side than at the north facing side. Higher precipitation and consequent soil moisture content favours ice lens growth. At first sight, a higher solar radiation and snowfall seem to inhibit non-sorted circle formation. But on the other hand, snow cover will thaw more at the south facing side or will melt completely away in the beginning of winter and at the end of spring which increases the number of freeze-thaw cycles. Furthermore, a higher daily temperature differences on south facing slopes can also increase freeze-thaw cycles Figure 4.1: All seven active circles were bordered by a pool (DeGraff, 1976). upslope (north) of the circle. 16 | Chapter 4. Discussion depth and vegetation are not captured by these parame- Daanen et al. (2008), who found higher activity in areas ters. Also vegetation coverage is expected to improve the with more precipitation or a higher activity during peri- model, since below the tree line, non-sorted circles were ods of permafrost melting. This does however not directly found on non-forested sites. explain why the majority of circles became less active in In this study, it was not tested if the regression model the area during the last decades, because precipitation can be extrapolated to other areas. Both this study and did not decrease since 1980. Long-term soil moisture the study of Hjort (2006) indicated that altitude and slope measurements for the active and inactive circles at the are the most important predictors for non-sorted circle oc- lake shore would be interesting to test the soil moisture- currence; also both studies showed that non-sorted circles activity hypothesis, but soil moisture measurements are are most abundant above the tree line and that their num- unfortunately not possible in this position, because dis- ber decreases at higher altitudes. This makes it likely that turbing the protected area is not allowed. a similar relation exists for other subarctic areas. 4.4 What are the effects of a changing 4.3 How does the activity of non-sorted climate? circles change with landscape properties? During the last fifty years, a net overgrowth of non-sorted circles by vegetation was found in the Abisko (Becher Comparing the active circles with the 78 inactive land- et al., 2013). Also the circles that were classified as forms, they differ in altitude, slope and TPI. Air tem- inactive during this study were active in the past. Ac- perature is expected to be lower at higher altitude and tivity is dependent on climate (Hopkins and Sigafoos, low temperatures favour ice lens formation and thereby 1950) and therefore the for the 21st century predicted activity of circles. Low in the valley, close to the lake changes in the climate are expected to influence activity temperatures in winter can however be lower because of of non-sorted circles. Since 1980, average temperature cold air drainage (Geiger et al., 2003). increased and snow depth decreased in the Abisko region In the same location as the active circles, also inac- (p < 0.05) (fig. 4.2). An increase in air temperature tive circles were found. The active and inactive circles leads to warmer soil temperatures (Helama et al., 2011) were found close together. When comparing these active and this may have reduced ice lens growth during the and inactive circles at the shore of lake Torneträsk it was last decades. Soil temperature however also depends on found or can be expected that air temperature, aspect, snow depth (e.g. Helama et al., 2011; Zhang, 2005) and soil texture, slope and vegetation do not explain the dif- the observed decrease in snow depth could have decreased ference in activity. However, two other differences might soil temperature during winter (Overduin and Kane, 2006; explain the difference in activity. First, the seven active Roennefarth, 2015) and thereby increased freeze-thaw cy- circles were found six meters lower in the landscape and cles and needle ice growth (Smith, 1987). Overduin and second, the active circles were found 45 m closer to the Kane (2006) researched differential frost heave in Alaska lake compared to the eight inactive circles. Probably, the and did not found a different differential frost heave for active circles are in contact with the groundwater which years with a thin and thick snow pack and suggested that makes that ice lens formation is not limited to the little soil moisture content is more important. This is in line precipitation in the area. This possibility is supported by with the results of this study. the presence of little pools bordering the north or east Sælthun and Barkved (2003) studied different climate of the active circles (fig. 4.1). If these active circles models and concluded that in the Abisko region, air tem- are indeed groundwater fed instead of rainwater fed, this peratures will rise (highest increase in winter) and pre- explains why they are not correctly predicted by the re- cipitation will increase (highest increase in winter and gression model. Unlike the regression model suggests, the autumn). How will these changes influence non-sorted active circles are found below the tree line. And the TWI, circles? which is a measure for surface water rather than ground- As described above, a direct influence of warmer air water accumulation might also not be applicable to model temperatures is a higher soil temperature. Furthermore groundwater fed non-sorted circles. warmer air temperatures are also related to shallower Together with the at a large scale observed differences snow depth (Helama et al., 2011). Over a longer pe- in TPI and slope between active and inactive circles, it is riod, warming will reduce the extent of permafrost occur- likely that in this area soil moisture content is the most rence (Åkerman and Johansson, 2008) and deepen the important parameter to determining activity under cur- active layers which will lead to deeper drainage and con- rent climate. This result is in line with the conclusions of sequent lower soil moisture content (Leffer et al., 2016). several other studies Becher et al. (2013); Klaus (2012); Rising temperatures are expected to increase the quan- 4.4. What are the effects of a changing climate? | 17 tity of vegetation (more shrubs) (e.g. Sturm et al., 2001; The above described effects of changes in temperature Walker et al., 2006) and the tree line is expected to ad- and precipitation and their combined effect on patterned vance (Grace et al., 2002). Vegetation insulates the soil ground occurrence are uncertain. Other uncertainties lie and the roots of vascular plants stabilize the soil, therefore in the effect of a changing temperature on the decomposi- vegetation will decrease the activity and occurrence of cir- tion of organic matter and the positive feedback between cles. The circles that were classified as inactive will be organic matter content and vegetation growth. Also the susceptible for getting overgrown with vegetation, since effect of an earlier snow melt on ice lens formation is vegetation is less hampered by disturbances. Another ef- unknown. Overall, the effects of a warmer climate and fect of vegetation on ice lens growth is trough their effect higher precipitation are expected to differ throughout the on soil moisture content. Vegetation is found to reduce area. Directly above the current tree line, non-sorted cir- water movement to the freezing front and thereby lim- cles are expected to get overgrown with vegetation, al- its frost heave (Daanen et al., 2007). And with a higher though reindeer grazing will slow down the overgrowth. If vegetation, transpiration will increase and soil evaporation temperatures stay sufficiently low to allow for freezing, ac- will decrease by shadowing. A higher autumn and win- tivity might increase at higher altitudes as a consequence ter precipitation increases water availability for ice lens of more vegetation growth and higher soil water availabil- growth and since water was found to be a limiting fac- ity. More insights in the magnitude of the different effects tor for non-sorted circle activity, higher precipitation will is important if we want to study future arctic ecosystem promote activity. A higher winter precipitation increases functioning in terms of water and nutrient fluxes, vegeta- snow depth and an increased snow depth is found to alter tion pattern and carbon storage. Interesting questions to vegetation species composition (Leffer et al., 2016). gain more insight in the drivers of activity are: When estimating non-sorted circle activity, differential • What requirements for non-sorted circle formation frost heave is important. Highest differential frost heave is are not met at altitudes above 1000 m? found in areas where the centres are unvegetated and the • Why did they majority of the non-sorted circles be- inter-circle areas are vegetated (Walker et al., 2008; Klaus came inactive during the past decades? et al., 2013). An increase in vegetation at higher bare areas might therefore promote non-sorted circle formation and activity with the proposed increase in temperature. 18 | Chapter 4. Discussion

Figure 4.2: The average yearly temperature, precipitation and snow depth measured at the Abisko Research Station (Abisko Scientiﬁc Research Station, unpublished data) 5| Conclusions

When studying the effects of climate change on subarctic ecosystems, it is important to study the responses of non- sorted circles and other forms of patterned ground. For non-sorted circle forming processes, a cold temperature, high soil moisture content and the presence of fine materials are the main prerequisites and therefore it was hypothesized that non-sorted circles would occur at higher altitudes, concave positions and positions with a high Topographic Wetness Index. In total 72 non-sorted circles and 13 non-sorted stripes were measured in the field. Of the circles, seven were classified as active, while no active stripes were found. The active circles were found at the shore of Lake Törnetrask. From the DEM it was concluded that non-sorted circle occurrence is highest at median altitudes: above the tree line, but not above 1000 m a.s.l. At higher altitudes, the slopes were steeper and non-sorted circle formation may be limited by a low soil moisture content and limited presence of fine materials and vegetation. Both the results of the field and the DEM analysis suggest that circles are found at lower slopes because at steeper slopes, solifluction leads to the formation of stripes. Non-sorted circles were found at concave positions at larger scale where fine materials and soil moisture are present. On smaller scale however, non-sorted circle fields were found at small ridges in the landscape. Suggested is that a higher snow depth and longer period of snow cover in the lowest positions limits differential frost heave. As hypothesized positions with non-sorted circles have a higher Topographic Wetness Index than positions without non-sorted circles. Altitude was the most important predictors for non-sorted circle occurrence, followed by slope. Differences in predicted and observed occurrence were found mainly below the tree line. Including vegetation cover is expected to improve the model. Active circles were found at the shore of the lake. A comparison with the inactive circles in the close surroundings indicated that, under current climate, water availability is most important driver for activity. Considering the main prerequisites for non-sorted circle formation: cold temperatures, high soil moisture content and fine materials, what will be the influence of a warmer and wetter climate? Presence of fine materials will not change on a short time scale. An increase in temperature will increase vegetation and decrease freeze-thaw cycles. Both will decrease non-sorted circle activity and occurrence. Soil moisture content will increase with increasing precipitation and favour non-sorted circle occurrence and activity. Deeper active layers will however decrease soil moisture content. The magnitude of the different processes should be studied to gain a better insight in future non-sorted circle occurrence and the effect on vegetation, soil moisture, nutrient and carbon storage dynamics of subarctic ecosystems. Bibliography

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Figure 5.1: The locations used in the logistic regression. In total 2500 points with and 2500 points without non-sorted circles were selected randomly.