ARBETSRAPPORTER
Kulturgeografiska institutionen
Nr. 690
___________________________________________________________________________
Small scale spatial and temporal variability of
micro-climate in a fellfield landscape, Marion Island
Oskar Berg
Uppsala, Maj 2009 ISSN 0283-622X
3 PREFACE
The theoretical preparation for this journey and the project itself was arranged and implemented during spring 2008. The thesis was later compiled during fall 2008. The project included two weeks residence in South Africa and four weeks on Marion Island. The project and the journey has for me been a unique experience and I got the chance to perceive a different environment and also meet a new culture.
I would like to thank my supervisor Jan Boelhouwers who gave me the opportunity to perform the project and the journey. I would also like to mention Werner Nell, Mphumzi Zilindile and Ian Meiklejohn who made the practical work on the island possible to perform.
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ABSTRACT
Marion Island is situated in the South Indian Ocean and belongs to the sub-Antarctic island group, Prince Edward Islands. The islands in the sub-Antarctic have over the past few decades been exposed to a warmer and drier climate trend. The aim of this thesis is to achieve better understanding of the small-scale spatial and temporal variability between Azorella selago and the surrounding microclimate. Due to the consequences of climate change, the interactions between Azorella selago, landforms and soil processes are important for the future of the terrestrial ecosystems in the sub-Antarctic. The theory part in this thesis describes different processes and features that are essential to understand the context of this thesis. The energy balance and the insolation is shown to be an important aspect when looking at the spatial variability of the microclimate. The summary of the results in the thesis is based on temperature and moisture measurements within two grids. One on the east and one on the west side of the island
The most important result from the measurements is that different weather conditions create different situations for the microclimate. The weather condition ‘sunny no wind’ created a high spatial variability in temperature on the ground, which was completely absent during overcast days. Temperature variability is highly dependent on cloud cover according to these results. Moisture changes also seem to be less weather dependent than temperature changes.
The data provide a first confirmation that an increase in sunshine hours gives increased spatial variability in temperature (not moisture) and soil frost. An increase in spatial variability of the microclimate within small areas could give rise to an expansion in the patchiness of soil frost processes in the landscape. The representivity of single point measurements of ground surface temperature should be questioned.
Patterns of areas with low moisture content within the grid correlate with points where measurements were taken on Azorella selago. The Azorella cushion could, according to the results of this thesis, be associated with dry areas within the grid. Azorella selago is thereby suggested to increase the spatial variability of moisture and also contribute to a locally drier microclimate. Moisture variability varies more between the east and west side of the island, than that it is weather dependent.
Shaded areas show a pattern of lower temperature than for the other variables under sunny conditions. If more shaded areas are created by for example landforms like Azorella
selago or solifluction deposits, the temperatures would probably be lower and also create a
wider spatial variability.
This study provides first data on the important interactions between Azorella selago and how it affects through spatial variability in micro-climate, ground frost potential and resulting soil disturbance by frost creep and solifluction.
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1 INTRODUCTION... 7
1.1 Aims and objectives ... 8
1.2 Study area... 9
2 THEORY... 11
2.1 Microclimate ... 11
2.2 Energy balance ... 12
2.2.1 Thermal properties and sub-surface climates... 13
2.3 Azorella selago... 13
2.4 Soil frost processes and landforms... 15
2.4.1 Frost creep ... 16
2.4.2 Gelifluction... 16
2.4.3 Solifluction deposits and landforms... 17
2.4.4 Patterned ground ... 19
2.4.5 Sorted circles ... 19
2.4.6 Sorted stripes ... 20
3 Method ... 20
3.1 Study site locations... 20
3.2 Grid... 21
3.3 Measurements... 22
3.4 Mapping ... 23
4 RESULTS... 24
4.1 Temperature variations at Tafelberg ... 24
4.2 Temperature variations at Mixed Pickle ... 26
4.3 Temperature values at Tafelberg vs Mixed Pickle... 27
4.4 Moisture variations at Tafelberg ... 28
4.5 Moisture variations at Mixed Pickle ... 29
4.6 Moisture values at Tafelberg vs Mixed Pickle... 30
4.7 Moisture and temperature influence from Azorella selago... 30
4.8 Statistical correlations between different variables at Tafelberg and Mixed Pickle 31 5 DISCUSSION ... 34
5.1 Temperature ... 34
5.2 Soil moisture ... 35
5.3 Comparison between sites... 35
6 CONCLUSIONS... 36 SOURCE REFERENCE ... 37 APPENDIX ... 40 A1 ... 40 A2 ... 41 B1 ... 42 B2 ... 43 C1 ... 44 C2 ... 45 D ... 46
1 INTRODUCTION
The climate changes of the sub-Antarctic are among the fastest globally. South Africa’s Prince Edward Islands have been exposed to a warming and drying trend over the past few decades. The mean annual temperature has risen by more than 1.2 ºC and the annual precipitation has declined by 25 mm each year1 over the last 50 years. The changes have considerable implications for biotic and abiotic features for the Sub-Antarctic.
Therefore these islands are well-suited to test the consequences of climate change on biology and landforms.2
Climate change experiments show definite negative effects for keystone plant species on the Prince Edward Islands. As a result a reduction in biomass of the microarthropods has occurred. The climate change on the species will effect the whole terrestrial ecosystem of the Prince Edward Islands such as nutrient cycling, primary production, succession and geomorphological dynamics such as soil stabilisation and soil frost patterns. Thus, the interactions between climate, geomorphologic processes, nutrient cycling and species of plants and animals are crucial for the future biodiversity of the Prince Edward Islands. 3
To understand the poorly examined interactions between biological and geological systems the scientific discipline of geobiology has recently developed. Geobiology examines the interface between the biosphere and the geosphere.4 The relationship between ecology and geomorphology is close. Therefore a subdisipline of geobiology has risen in the shape of biogeomorphology. Geomorphological forms affect distribution and development of flora and fauna while ecology can control the development of landscape morphology by changing the surface processes. Because biogeomorphology is the primary discipline for this thesis it demands a further explanation. Biogeomorphology essentially involves cooperation between ecology and geomorphology and focuses on the two-way linkages between ecological and geomorphological processes. The discipline biogeomorphology evolved in the late 1980s and is explained as an area of “no man’s land” and also to be uncoordinated, adjustable and interdisciplinary at this stage.5 The processes that constitute the linkages between the different disciplines within biogeomorphology are bioweathering, bioerosion, bioconstruction and bioprotection.
Bioerosion is defined as an activity that involves weathering and/or removal of material directly by organic activity. Bioerosion can also be passive and through indirect biological processes reduce protection and encourage erosion. An example which includes both active and passive bioerosion could be the trampling within a penguin colony. The scratching and trampling by the penguins correspond to active removal of substrate and the erosion caused by removing of protective vegetation explains the passive bioerosion.
Bioconstruction is the “production of sedimentary deposits, accretions or
accumulations by organic means and may involve both active and passive organic influences”6 The last of the three dominant processes is bioprotection. This term can be
explained as the roles of organisms that in a passive or active way prevent or delay the action of other earth surface processes. A coral reef protecting the coast within is a good example. To understand how biogeomorphology and the connection and interaction between the bio- and geosphere can be used practically three examples will be presented. The first field is
1 Smith 2002 p. 345-357
2 Frenot et al. 1997 p. 358-366; Bergstrom & Chown 1999 p. 472-477; Convey 2001 p. 17-42.
3 Hugo et al. 2004 p. 466-473; LeRoux et al. 2005 p. 1628-1639; Boelhouwers 2003 p. 67-71; Boelhouwers et al.
2003 p. 39-55
4 Noffke 2005 p. 1-3; Naylor 2005 p. 35-51 5 Naylor 2005 p. 35-51; Naylor et al. 2002 p.8-9 6 Naylor et al. 2002 p.8
when to simulate models for carbon sequestration for the mitigation of climate change. This is an emerging topic within the disciplines of biogeomorphology and palaeoecology. The study of morphometry of other planets in the solar system constitutes a second field. By examining other planets, without the influence of living organisms, it becomes more easy to understand the effect of morphometry and also the interaction between geomorphology and ecology on the earth. Bioengineering for restoration of ecosystems is the third field. The understanding of interactions between microbial life and different minerals such as clays together with the understanding of the carbon cycle (and other nutrient cycles) has led to the observation that bacteria can be used for remediation of water and soil that has been polluted by oil.7 Biogeomorphology can in a capacity of a linkage between two other disciplines elucidate several environmental issues that are difficult to understand if you only use the single disciplines of geomorphology or ecology. Problems which can be identified by biogeomorphology are biochemical cycles, biological diversity and ecosystem functioning, climate variability, hydrologic forecasting, infectious diseases in the environment, institutions and resource use, land-use dynamics and reinventing the use of materials.8
The structure and the terrestrial biodiversity of the sub-Antarctic islands are influenced by the three-way interaction between climate, vegetation and landform processes. The plant
Azorella selago represents the vegetation in this interaction because of its widespread
distribution and important functional roles in the sub-Antarctic.9 Azorella is a long lived keystone species and colonizes loose scoriaceous slopes and newly exposed ground. Azorella is sensitive to the local abiotic environment and is also known to vary morphologically in response to environmental conditions.10 The features that decide the Azorella size and establishment patterns are related to soil structure, frost dynamics, microclimate and nutrient avalibility. The microenvironmental conditions (together with geomorphological soil quality and frost processes) may play a considerable role in the growth, mortality and population structure of Azorella selago.11 Azorella is also a part of the processes that alter the
environment in the sub-Antarctic. It is associated with processes such as terrace formation and soil accumulation in the landscapes. The root system of the plant stabilises the slopes and influences the formation of lobes, sheets and terraces. Thereby it affects the spatial distribution of material deposition, soil frost processes and landscape structure. The erosional and sorting effect through frost heave is different on leeward and windward sides.12
Because of the consequences of climate change on the different factors described above the interactions between Azorella selago, landforms and soil processes are important for the future of the terrestrial ecosystems of the sub-Antarctic.
1.1 Aims and objectives
This thesis aims to achieve a better understanding of small-scale spatial and temporal variability between Azorella selago and the surrounding ground surface, in selected microclimatic parameters. This will be achieved by detailed thematic mapping on a scale of 25x25m and 15x15m. The mapping, together with measurements (temperature, soil moisture) will be performed on two places representative for the island, one on the west (Mixed Pickle)
7 Naylor 2005 p. 35-51 8 Renschler et al. 2007
9 LeRoux & McGeoch 2004 p. 608-616
10 Huntley 1972 p. 103-113; Frenot et al. 1993 p. 140-144; LeRoux 2004 p. 608-616 11 Boelhouwers et al. 2000 p. 341-352
12 Hall 2002 p 71-81; Boelhouwers et al. 2003 p. 39-55; Boelhouwers & Hollness 1998 p. 399-403; Holness
and one on the east (Tafelberg) at c.300-400m above sea level. Data will then be analyzed for statistical correlations and interpretations.
Specific research questions:
- What are the small-scale spatial patterns between surface materials, Azorella selago spatial distribution and morphology and micro-climate (surface temperature, soil moisture)?
- What are the spatial and statistical correlations between the different variables mapped?
- How can the spatial correlations be interpreted in a dynamic model of geobiological system interactions and feedbacks?
1.2 Study area
Marion Island (46 º 54´ S, 37 º 45´ E) constitutes one of the sub-Antarctic islands in the South Indian Ocean. The position of the island and the two study sites are shown in Figure 1. The Island belongs to South Africa and together with nearby Prince Edward Island, constitutes the Prince Edward Island group. Marion consists of the peak of a shield volcano13, has an area of
approximately 290 km² and, rises 1230 m above sea level. The age of the island is estimated to be less than one million years.14 The climate in the maritime sub-Antarctic is characterised by low annual temperature ranges, high precipitation and frequency of cloudiness.15 Marion Island is situated in the latitudes called “the roaring forties” named after the strong winds. Gusts of over 200 km/h (ca 55 m/s) have been recorded and the wind blows most frequently from northwest with an average velocity of 32 km/h. (ca 9 m/s). Calm conditions are rare. West winds are normally associated with wet, cloudy conditions and south winds with dry and clear conditions.16
The synoptic weather of the island is characterised by mid-latitudinal depressions, passages of frontal systems and the influences of anticyclones.17 The South Indian Ocean
anticyclone also influences Marion Island with warm air from the north and moderate north-westerly to north-easterly winds.18 A change in cloudiness and sunshine hours is considered to take place on Marion Island. Between the years of 1951 and 1999 measurements show a large degree of interannual variability in total annual sunshine hours. But 30 % of that variability points at an average increase of 3,3 sunshine hours each year during the measurement period. The strongest increase can be observed during April, May and August but sunshine hours increased for all months during this period. An increase in air and sea temperatures has occurred over the same time period. Together these increases can be associated with changing atmospheric circulation patterns. The radiation and air temperature increases can also be explained by the positions of cyclone tracks relative to the island.19
The mean summer maximum and minimum temperatures at sea level on Marion Island are 10.5 and 5 ºC and the corresponding mean temperatures for the winter are 6 and 1 ºC 13 Verwoerd, 1971 p. 40-61 14 McDougall et al. 2001 p. 1-17 15 Boelhouwers et al. 2003 p. 39-55 16 http://marion.sanap.org.za/index2.html 2008-03-04 17 Vowinckel, 1954; le Roux, 2008 18 Rouault et al, 2005 19 Smith 2002 p. 345, 349
(1961-1990). The mean annual precipitation total is 2576 mm and precipitation occurs for 25 days per month, on average (1961-1990). 20
The grey lava topography of the island is dominated by two dominant features (Long ridge and Feldmark Plateau) as a result of faulting and are proposed to be horst structures. The surface topography is mostly dominated by the youngest black lavas and is said to be
“sectioned by radial faults along which recent eruptive centres are found in the form of scoria cones”.21 Figure 2, a picture taken from Long Ridge, gives a view of the inland environment of the island. Grey lava is dominating the right corner of the picture and black lava extends in the middle between the two scoria cones.
Marion Island has been almost entirely covered by ice during the last Glacial Maximum and the landforms created by the ice are moraines, striations and sculptured bedrock (eruptions after the ice cover has formed most of the island). Periglacial solifluction landforms on Marion Island are represented by stone- and vegetation banked lobes, terraces and sheets.22 Blockstreams and blockfields also constitute periglacial landforms.23 Patterned ground also occurs on the island, specifically sorted stripes and sorted circles.24 The
geomorphology of the island reflects its volcanic origin. The landscape of the island is highly dynamic and affected by many factors such as coastal cliff erosion, weathering of lava and basalt outcrops and slope processes on grey lava derived till, debris slopes and scoria cones. Interactions between animal activity and erosion processes also play a role, especially in coastal bird and seal colonies.25 Aeolian processes are also likely to play an increasing
20 http://marion.sanap.org.za/index2.html 2008-03-04 21 Boelhouwers et al. in press
22 Hall 2002 p. 71-81; Boelhouwers et al. 2003 p. 39-55 23 Sumner & Meiklejohn 2004 p. 395-398
24 Holness 2001 p 30 25 Boelhouwers et al. in press
geomorphic role in the future as wind speeds appear to be increasing while the island is becoming drier, and will impact on the colonisation of vegetation.
The areas chosen as investigation areas on the island were selected on the eastern side, Tafelberg with the slope facing an easterly direction (S46° 53, 186´ E 37° 48, 265´ alt 338 m)and on the western side Mixed Pickle with the slope facing a westerly direction (S46° 52, 391´ E 37° 39, 458´ alt: 348 m) Both investigation areas were situated on grey lava and with similar slope angle between 1º and 8º.
2 THEORY
In this part different processes and features are presented that are essential to an understanding of the integrity in this thesis. The theory part starts with a description of the microclimate which constitutes the area of all the processes and also where all the landforms can be found.
2.1 Microclimate
The climate close to the ground to which species are directly exposed is defined as the microclimate. It is affected by landforms, vegetation, fine scale topography, substrate and
Figure 2. Black lava dominates the landscape and extends between the three scoria cones. In the right corner grey lava from the top of Long Ridge 2008-04-15. Photo: Oskar Berg
aspect.26 Microclimate immediately surrounds the individual organisms and constitutes a relevant factor in understanding of the relationship between species characteristics and climate. To maintain their own survival, species can create and manipulate the microclimate to which they are exposed.27 To develop and reproduce under microclimate conditions species also rely on phenotypic plasticity.28 Microclimate is therefore substantially for the functioning, behaviour and survival of the species.29 Compared to other climate readings as
meteorological temperatures, microclimate may differ significantly, especially in temperature extremes and in the rate that changes occur in time and space. 30
The microclimate (studies of temperature and the impact on microhabitat) therefore constitutes a fundamental part in the understanding and predicting effects of the climate change on species.31
2.2 Energy balance
An aspect that effects the microclimate and thereby all its features is the different processes in the energy balance together with thermal properties and sub-surface climates.
The component that constitutes the base of radiation flux of energy to the earth is called insolation (the amount of incoming radiation from the sun) There are four components in this radiation balance and they consist of incoming short wave radiation from the sun (K↓) and shortwave outgoing radiation (K↑) which is short wave radiation reflected from the surface of the earth. The other two components are the long wave radiation that is emitted from the earth itself (L↑) and the long wave radiation that is reflected from the atmosphere back to the earth surface (L↓) The net all-wave radiation (Q) on an ordinary day is described in an equation as: Q = K↓ – K↑ + L↓– L↑.
Q is an important energy exchange for most systems because “it represents the limit to the
available source or sink”. 32 Q is effected by latitude, clouds, surface type (albedo) and inland or coast location. E.g. cloudy weather is often associated with uniform temperatures because of the reduction of the daytime solar heating and night time long-wave cooling. A surplus of energy on the day and a deficit of energy on the night often occurs during ordinary conditions. Q is also influenced by other factors. Conduction describes direct heat flow between different substance, e.g. heat flow from the heated air down in the soil during a day. During the night a new heat flow is created between the heated soil and the colder air.
Latent heat flow also constitutes an important part. When water evaporates from the earth surface, the heat is transferred from soil moisture into vapour, resulting in a cooling of the surface. If the water vapour is condensed at the surface, latent heat is released and creates a warming at the surface.
Sensible heat flow is a flow of heat from the surface to the atmosphere. The flow arises through convection and could create down- or upward currents of air which can conduce heating (day) or cooling (night) of the surface. The surface is a thin layer located between the atmosphere and the soil. The surface can transform one energy flow to another flow but it is
26 Bale et al. 1998 p. 363-377 27 Unwin 1980; Körner 2003 p. 344
28 Schoettle and Rochettle 2000 p.1797-1806; Hummel et al. 2004 p. 705-715; Terblanche et al 2005 p.
1013-1023
29 Griffiths 1976; Körner 2003 p. 344 30 Rosenberg et al. 1983.
31 Griffiths 1976; Wookey et al. 1993 p. 490-502 32 T. Oke 1978 p. 23
too thin to maintain heat itself. The temperature and the amount of moisture at the surface are important aspects when it comes to ground frost. These factors can be explained by the energy balance equation:
Energy balance = K↓ + L↓+ latent heat flow + sensible heat flow + Conduction.33 2.2.1 Thermal properties and sub-surface climates
Heat transfer and thermal climate in the soil are controlled by four linked thermal properties. These are thermal conductivity, heat capacity, thermal diffusivity and thermal admittance. Thermal conductivity is a measure of the ability to conduct heat and is defined as the quantity of heat flowing through unit cross-sectional area (m2) of the substance during a certain time (s). Thermal conductivity varies with depth and time but is most dependent on the porosity of the soil and of the quantity of moisture in the soil. Adding moisture to the soil increases its thermal conductivity. This is explained partly through the coat that is created around the particles and increases the thermal contact between the grains. But also through the fact that soil pore space is finite and the addition of pore water thereby expels the similar amount of soil air. This increases the thermal conductivity because water conducts better than air.
Heat capacity is described as the quantity of heat necessary to raise the temperature of a substance volume with 1˚C. Heat capacity explains the substance ability to store heat and it expresses the transformation in temperature related to the loss and gain of heat. The amount of heat needed to warm the soil is strongly dependent on the moisture content. Adding water to soil exceeds the soil’s heat capacity as water has a high heat capacity.
Thermal diffusivity is the soil’s ability to diffuse thermal influences. It is a measurement of the time it takes for temperature changes to travel in the soil. The daily heat input creates a temperature wave that travels fast and to a considerably depth in the soil where conductivity is high. If it takes large amounts of energy to warm up intermediate layers on the way down (because of high heat capacity) the heat wave slows down and does not penetrate as far. The thermal diffusivity are also affected by soil moisture which can exceed the diffusivity. But if more than 20 % moisture is added the thermal diffusivity will be reduced. Soil with high thermal diffusivity allows rapid penetration and the temperature changes affect a thick layer. If the soil has low thermal diffusivity the temperature change becomes less extreme.
Thermal admittance is rather a surface then a soil property. It is a measurement of the ability of a surface to embrace or release heat when the surface senses the temperature change of a given change in heat flow. This explains why some material, e.g. metal, with high thermal admittance feels colder to the touch than those with low thermal admittance, e.g. wood, even though the materials are in the same room with the same temperature.34
2.3 Azorella selago
Azorella selago is a vascular species and belongs to the family of Umbelliferae. It grows
commonly in feldmark regions and locally on slopes. The status of the plant is described as native and the distribution area is Marion Island, Prince Edward Island, Crozet, Kerguelen, Heard, Macdonald, Falkland Islands and South America. This species forms dense cushions which can grow 10 - 30 cm high, see Figure 3.
33 T. Oke 1978 p. 20-27 34 T. Oke 1978 p. 37-49
However, cushions up to 70 cm high are sometimes seen. The flowers of the species are small and greenish yellow and the leaves have 3-5 acute lobes. The Azorella often grows isolated but sometimes they merge together and form extensive mats.35 Azorella, known as the cushion plant, is considered to be one of the keystone species of the fell field vegetation of Marion Island and has the greatest altitudinal range of all the vascular plants. Azorella selago is a pioneer species and colonizes loose scoriaceous slopes, recent lava flows and glacial forelands. In Figure 4 Azorella selago is growing on an area of grey lava at Tafelberg.
With the characteristic of being a long lived species Azorella is known to vary morphologically in response to environmental conditions.
Azorella growing in sheltered locations has faster stem growth, larger leaves and a
longer growing season than those in more exposed environments.36 Azorella is also considered to be one of the most vulnerable species to the changing climate on Marion
35 http://marion.sanap.org.za/index2.html 2008-05-25
36 Huntley 1971,1972 p. 103-113; Frenot et al. 1993 p. 140-144; le Roux 2004 p. 608-616; le Roux et al 2005 p.
1628-1639
Figure 3. Isolated Azorella cushion. Photo: Oskar Berg
2008-04-05
Figure 4. Azorella cushions, altitude 338 m at Tafelberg grid. Photo: Oskar Berg 2008-04-05
Island.37 Azorella recycles its own nutrients which lead to an accumulation of nutrients in the local area with normally poor soils.38 The accumulation occurs because of the plants ability to retain senescent leaves that form a rich, moist, humus-like collection of organic material inside the plant thorough decomposition.39 The Azorella cushion, with its nutrient support and interactions with landforms and soil frost processes, is predicted to cause significant consequences for the Sub-Antarctic terrestrial biodiversity.40
Azorella is sensitive to the local environment and the microclimate of an area can
explain differences in growth form and compactness. Microclimate and micro topography are also thought to modify cushion morphology and survival.41 The establishment patterns of the plant are also related to soil structure, frost dynamics and nutrient dynamics to some extent. The leaf morphology varies with changes in the climate and responds strongly to differences in altitude, such as radiation, precipitation and temperature fluctuations. The east-west aspect on Marion Island with its climate differences also emerge as an important aspect and driver of pattern in this system.42 Biotic factors such as cushion age, ephiphyte load or nearest neighbour characteristics can not explain the variability in Azorella selago size and growth rate.43 Instead it is suggested that microenvironmental features with their properties and
geomorphology (particularly soil quality and soil frost processes) may play an important role concerning Azorella selago and its growth, mortality and population structure.44
Azorella selago is associated with geomorphological processes such as terrace formation
and soil accumulation and is therefore responsible for altering its environment. The root system of the plant may affect formation of lobes, sheets and terraces and also constitutes a stabilising factor for the slopes. Affecting these aspects Azorella selago influences spatial distribution of material deposition, landscape structure and soil frost processes.45 Azorella
selago is also sensitive to its environment and interactions between the plant and landform-
and soil processes together with alteration by climate change has substantial consequences for the future and the terrestrial ecosystem of Marion Island46
2.4 Soil frost processes and landforms
Some of the processes and landforms within the microclimate on Marion Island are described below.
One of the most widespread processes of soil movement in periglacial environments is solifluction. The process is described as “Slow flowing from higher to lower ground of waste
saturated with water”.47 The term gelifluction describes solifluction associated with frozen ground48 In discussions of solifluction you also have to mention frost creep (cryogenic deserption), which is a down slope movement of particles. Gelifluction and frost creep
37 Barendse and Chown 2001 p. 73-82; le Roux et al. 2005 p. 1628-1639 38 Nunez et al. 1999 p. 357-364; Körner 2003; le Roux 2004 p. 608-616 39 Huntley 1971 p. 103-113
40 Hugo et al. 2004 p. 466-473
41 Jumppponen et al 1999 p. 98-105; Sage & Sage 2002 p. 501-508; Kleier & Rundel 2004 p. 461-470 42 McGeoch et al. subm
43 le Roux & McGeoch 2004 p. 608-616 44 Boelhouwers et al. 2000 p. 341-352
45 Hall 2002 p. 71-81; Boelhouwers etal. 2003 p. 67-71; Boelhouvers & Holness 1998 p. 399-401; Holness 2003
p. 69-74
46 McGeoch et al. Submitted chapter. unpublished 47 J.G Andersson 1906 p. 36
constitute the movement that is generally termed solifluction.49 In solifluction there are three other components;
1. Potential frost creep (how much a grain will move in respect to the ground heave horizontally)
2. Horizontal component of gelifluction movement. (down slope movement of a grain in excess of maximum potential creep)
3. Retrograde movement ( how much the grain will move back in respect to resettlement of the ground)50
When one-side freezing occurs solifluction consists of two components. These are frost creep, which is associated with needle ice, diurnal frost and gelifluction which occurs in spring when seasonally frozen ground thaws from the surface downwards. During two sided freezing there is a third component in the solifluction. This is called “plug like” flow and starts when the thawed active layer is capable of sliding across the lubricated slip plane provided by the ice rich zone at the top of the permafrost. The importance of each component varies with different location attributes such as present moisture or ground temperature. In alpine regions frost creep and needle ice may be the most common mass wasting processes because of strong diurnal and seasonal temperature rhythms.51
2.4.1 Frost creep
The term is described as ratchet-like down slope movement of particles caused by frost heaving off the ground. The down slope displacement results from settlement upon thawing. The heaving is predominantly normal to the slope and the settling more vertical. Frost creep relates to one-sided and two-sided freezing. One-side frost creep concerns movement decreasing from the surface downwards and depends on available moisture for heave, angle of slope, frequency of freeze and thaw cycles and frost susceptibility of the soil. The equation for frost creep is:
∆l = h tan σ
∆l = potential frost creep, h = normal heave to ground surface, σ = slope angle.
Two-sided frost creep arises in environments of permafrost. Frost creep may here exceed the amount compared with one-sided frost creep. The presence of ice lenses at the top of the permafrost layer makes a larger contribution to total movement of frost creep.52
2.4.2 Gelifluction
Gelifluction describes solifluction associated with frozen ground and occurs in areas where downward percolation of water is limited by frozen ground and where segregated ice lenses melt and thereby provide excess water. This reduces internal friction and cohesion in soil. Locations rich in moisture, such as a late lying snow bank, are particularly good places for gelifluction. Gelifluction occurs mostly during thaw periods. The volumetric transport of material by gelifluction accounts for 6-60 cm³ a year. The movement of the gelifluction decreases with depth and is usually restricted to the uppermost 50 cm of the active layer.
49 French 1996 341 pp 50 Matsuoka 2001 p. 107-133 51 French 1996 341 pp 52 French 1996 341 pp
In plug-like flow the profile of the flow is convex downwards. Movement often occurs in summer when the active layer is at its thickest. The volumetric transport of “plug-like” flows are 7-52 cm³ a year. Small mudflows in mid- and late summer on lower slopes are related to “plug-like” flows.53
2.4.3 Solifluction deposits and landforms
Solifluction deposits are different kinds of sediment, largely unstratified, that have been transported by frost creep and gelifluction processes and thereafter deposited. The deposits are commonly matrix-supported and consist of fine-grained sediment that has low liquid limits. Platy microfabrics are typical of solifluction deposits and show the previous existence of segregated ice lenses that rises localized slip during thaw.54
The landforms created by solifluction are lobes terraces and sheets and they will here be explained in association with Marion Island. On Marion you can find stone-banked lobes, stone-banked terraces, vegetation-banked terraces and stone banked sheets. Marion is a periglacial area and therefore a variety of process contributes to the formation of landforms.55 Lobes
A lobe is a landform created by solifluction processes and with the characteristics of length>width. The landform consists of a tread (the length or body of the lobe from where the lobe starts till it ends) and a riser (the up rise of the lobe compared to the ground below) Lobes can be either stone-banked or vegetation-banked. Stone-banked lobes are described as gelifluction and other deposits confirmed by crescent (half moon shaped) stony embankments.56
Stone-banked lobes exist in a limited distribution on Marion Island but are often present in scoria cones. The stone-banked lobes are composed of a surface layer of gravel and small cobbles. The local slope orientation is the primary control of lobe orientation and the lobe size increases with altitude and local slope angle.57 The best developed stone-banked lobes are found on slopes of 7-35º. Treads of stone-banked lobes are often free of vegetation which is the result of frost processes. Bases of lobes frequently show colonization of Azorella selago.
58
Vegetation-banked lobes have similar characteristics to stone-banked lobes with the big difference that the embankment consists of vegetation. Because no descriptions of vegetation-banked lobes on Marion Island exist there will be no further explanation.
Terraces
Terraces are also a landform created by solifluction processes and similar to lobes but are described as more extensive. The characteristics of a terrace is that width<length. A terrace also consists of a tread and a riser and could be stone-banked or vegetation-banked as seen in Figure 5 and 6.
Stone-banked terraces are described as “terrace or garland-like accumulations of stones
and boulders overlying a relatively stone-free moving subsoil”.59
Stone-banked terraces are often found in areas with lower slope angles than stone-banked lobes. On Marion Island stone-banked terraces are best developed on slopes with 7- 11 º.
53 French 1996 341 pp 54 French 1996 341 pp 55 Holness 2001 p. 175-183 56 Embleton and King 1975 p.112 57 Holness 2001 p. 189-194 58 Holness, 2001 p. 176-178 59 Benedict 1970 p. 174
Terrace length could vary between 1 -30 m and riser height between 0,2-3,4 m. The tread is dominated by fine and matrix-rich diamict, which is covered by surface pavement or gravel. Larger clasts tend to increase down slope within the tread which gives a sorting. The treads are often free of vegetation due to intensity of diurnal-frost processes. There could also be
Azorella selago in connection with risers and the clasts in the riser are often bigger than those
in tread. Stone banked terraces are affected by soil frost activity on Marion Island which could have impact on the maintaining the terrace form. But stone-banked terraces are largely inactive under present conditions.60
The vegetation-banked terraces are described as “bench-like accumulations of moving
soil that lack conspicuous sorting”.61 The spatial distribution of vegetation banked terraces is limited on Marion Island but they are common on scoria cones below 600 m a.s.l where plant growth occurs. Vegetation banked terraces are best developed on leeward slopes of 5-30º. Vegetation-banked terraces consist of a matrix-rich tread banked up behind Azorella selago, as in the right corner of Figure 6. Treads are generally free of vegetation, consist of fine pavement and the length varies from 2-14 m. Terrace treads are also covered with a surface gravel layer, with gravel size increasing in a down slope movement. The risers are vegetated with a general height of 0,25 – 1,6 m. Just like stone-banked lobes and stone-banked terraces, large vegetation banked-terraces are found at high altitudes with high slope angles.62
Sheets
Sheets are landforms created by solifluction processes. A sheet is a large solifluction surface and could be described as a big terrace. Sheets also consist of a tread and a riser and could be banked or vegetation-banked. The only feature described on Marion Island is stone-banked sheet.
Stone-banked sheets consist of an accumulation of moving soil confined by lobate or straight stony embankments. They are widespread in scoria areas of Marion Island. A big difference between stone-banked lobes, stone-banked terraces and stone-banked sheets is that the tread angle of the sheet is close to the angle of the local slope. The angles of stone-banked lobes and stone-banked terraces are lower than the angle of the local slope. The riser is generally 0,25-1m high and consists of coarse openwork material. It is hard to define a sheet
60 Holness 2001 p. 183-189 61 Benedict 1970 p. 170
Figure 5. Stone banked terrace at Tafelberg. Figure 6. Azorella banked terrace at Tafelberg Photo: Jan Boelhouwers 2007-04-17 Photo: Oskar Berg 2008-04-05
from the slope itself. But where the tread could be defined it could range from 5-60 m and it is generally fine-grained and shows little down slope sorting.63
2.4.4 Patterned ground
Patterned ground is different kinds of landforms and refers to polygons, nets, stripes and circles. The Mechanisms behind the landforms are not well understood but most forms are frost related. The patterned ground could be both sorted and non-sorted. The sorted patterned ground is described as polygons, nets, stripes or circles with defined rocky borders as a result of sorting in soil subject to frost action. Because of the frost action there are mostly no vegetation in association with sorted-patterned ground. Figure 7 presents some examples of the shapes of different sorted- patterned ground.
Non-sorted patterned ground could have the same forms as sorted-patterned ground but have no defined rocky borders and are often associated with vegetation.
The forms of patterned ground that have been reported from Marion Island to a wider extent are sorted circles and sorted stripes. These two landforms will therefore be described in more detail.64
2.4.5 Sorted circles
The term is described as “patterned ground whose mesh is dominantly circular and has
sorted appearance commonly due to a border of stones surrounding finer material”. 65
Sorted circles are widespread within previously glaciated areas of Marion Island from sea level to 1100 above sea level. Sorted circles consist of a roughly circular fine centre surrounded by a boarder of coarser material. The depth of sorting increases with altitude and on lower altitudes the circles are broader and shallower. The circles are generally restricted to areas with s low slope angle or near horizontal ground and are often found in areas of till. Needle ice occurs in sorted circles in grey lava and needle ice together with ice lenses create the frost heave responsible for much of the morphology of sorted circles.66
63 Holness 2001 p. 189-194 64 Holness 2001 p. 194-196 65 Washburn 1956 p. 836 66 Holness 2001 p. 41-57 Circles polygons Nets Stripes
Figure 7. Shapes of sorted-patterned ground. O.B 2008.
Figure 4. Sorted Patterned ground. O.B 2008. Figure 4. Sorted Patterned ground. O.B 2008.
2.4.6 Sorted stripes
The term is described as “Patterned ground with a striped pattern and sorted appearance due
to parallel lines of stones and intervening stripes of dominating finer material orientated down the steepest available slope”.67 Sorted stripes are the most commonly reported periglacial landforms in the maritime sub Antarctic and the landform often occurs in grey lava areas where till is present as in Figure 8. The landform also occurs on other types of landform as the treads of stone-banked lobes and terraces. The sorted stripes on Marion Island can get as long as 30 m and are divided in coarse and fine stripes. The depth of vertical sorting of the stripes increases with altitude. Needle ice activity is likely the dominant factor in stripe formation. Another factor of the sorted stripes on Marion Island is that they mostly occur on windward slopes. They also occur in leeward slopes but not to the same extent.68
3 Method
The method was first planned theoretically in Sweden and practically implemented on two sites, Tafelberg and Mixed Pickle, on Marion Island. The theoretical method had to be changed a few times due to lack of time, time-consuming work and unstable weather conditions. The same method, except for some small changes, was used on both sides of the island.
3.1 Study site locations
Approximately 6 km in a south west direction from the Marion base a site selection was made on Tafelberg, with the elevation of 338 m. The chosen area was representative to the environment in terms of geomorphological-, Azorella patterns and also representative to local topography such as slope angles and orientation, see Figure 9. The material of the ground was
67 Holness 2001 p. 63 68 Holness 2001 p. 63-89
Figure 8. Sorted stripes at Mixed Pickle, altitude 300m. Photo: Jan Boelhouwers 2007-04-17
grey lava. The sizes of the material varied from small grain to big boulders. Patterned ground (sorted circles, and sorted stripes) were observed in the area. The dominating vegetation in the area is Azorella selago. On the Azorella the grass Agrostis magellanica often occured. In some areas features of the moss Ditrichum strictum occurred.
On the western side of the island a similar site, in an almost straight westerly direction from Marion base, was chosen at Mixed Pickle with an elevation of 348 m. The chosen area was also representative of the environment in terms of geomorphological and Azorella patterns and also representative of local topography such as slope angles and orientation, see Figure 10. The area was also chosen taking into consideration similarities between Tafelberg and Mixed Pickle, in the different patterns as mentioned above. The site was chosen on grey lava, though the area was more dominated by black lava than Tafelberg. The dominating vegetation in the area is Azorella selago, with features of Agrostis magellanica and Ditrichum
strictum here as well.
3.2 Grid
A grid was created at an altitude of 338 m with wooden poles and plastic string. The size of the grid was 25×25 with a cell size of 5×5m, see Figure 11a. Erosion pins and coloured stones were put on the ground every 3 m in all directions starting from 0,0. The stones and pins were put in the grid for later examination. The idea from the beginning was to put stones and pins every 1 m but that would have taken too much time. The points of the stones and pins also became measurement points. One problem here was that the last stone and pin in one row ended up at 24 m. To secure problem-free map creations, the distance between the two last stones/pins in each row is only 1 m.
Figure 9. The grey lava slope at the study site of Tafelberg was photo documented for future comparisons. 2008-04-24. Photo: Ethel Phiri
Almost the same method was used at Mixed Pickle, on the west side of the island. Because of the time limitation on our stay there a smaller grid of 15×15 m was created, see Figure 11b. All distances within the grid were the same as Tafelberg, except that no 1 m distance was needed in the end rows to complete the map surveys.
3.3 Measurements
Measurements on the ground surface at each point were taken every 2 hours starting at 10 am and ending at 16 pm. The temperature measurements were made by an IRT (Infra Red Thermometer, accuracy 2 oC, precision 0.5 oC). The IRT was held 1 dm from the ground surface when the measurement was taken. One measurement at each point was taken, which makes the final numbers of points and measurements 100. The moisture measurements were also taken at each point with the same time interval using a TDR (Time Domain Reflectometry) probe (Theta probe®). One measurement at each point was taken here as well. At each point it was noted if the landform was an Azorella selago or if the measurement was taken in the shade. The temperature and moisture measurements at Tafelberg were taken on 3 different days with different weather conditions. Day 1 (20080402) was a sunny day with no wind. No measurement was made at 10 am this day because of setting up the grid in the morning. Day 2 (20080403) was sunny and windy and partly cloudy starting at 14 pm. Day 3 (20080404) had the conditions overcast windy, misty. Every measurement was made within 25-30 minutes and started at 0,0 and then 3,0, 6,0 and so on. With the different x and y coordinates and their own unique value the data was processed in Surfer 8 and different temperature and moisture maps were created.
Measurements were taken in the same way with the only difference that the measurement points were reduced to 36 at Mixed Pickle. The time to complete one set of
Figure 10. Study site with grid situated on grey lava at Mixed Pickle. In the close background black lava and beyond snow-cowered mountain peaks.
measurements reduced from 25-30 minutes at Tafelberg to 10-15minutes at Mixed Pickle. The different days of moisture and temperature measurements at Mixed Pickle were limited to 2 and the different conditions were: Day 1 (20080420) partly cloudy/partly sunny, windy, misty, Day 2 (20080421) overcast, misty light wind. The weather conditions on the west side of the island were characterized by a large variability and fast changes. Thereby it is hard to decide the exact conditions for a whole day.
3.4 Mapping
The mapping of the grid included elevation curves and the different landforms. The material used was paper and black lead and during rough weather conditions white painted Perspex plates and black lead. The elevation curves were created by slope angle measurements using an abney level. The elevation of x0, y0 was 338 m (through gps) and then slope angle measurements were taken each 5 m within the grid. Trigonometry was used to calculate the different altitude of each point. That gave 36 different altitudes within the grid and enough data to create a model of height curves (digital terrain model) using Surfer 8 (See Appendix Figure 27 and 28). The purpose of these slope angle measurements was to create a detailed model. The different landforms in the grid to be mapped constituted of Azorella selago and the ground material is described as grain and stones for the whole grid. The Azorella were first mapped using the same method used on the morphology, 25 × 25 cm grid on paper/ white perspex, black lead and also coloured soft pencils. The cushions were drawn on the paper/Perspex grid using the plastic strings from the cells as direction points.
Starting to draw the material and different stone sizes the same way appeared to be a massive job and there was no time to finish such a task. Another strategy was tested using a digital system camera, camera tilt and 1,5 m wooden pole. With this new technique pictures could be taken from above and thereby cover the whole grid. When testing which area one
Figure 11a. Grid with measurement points at Tafelberg.
O.B 2008
Figure 11b. Grid with measurement Points at Mixed Pickle. O.B 2008
picture could cover 2 × 1 m showed to be the best alternative. That is a small area of a 25×25 m grid and over 350 pictures had to be taken. The pictures were taken systematically and also using a pen pointing out the north direction to ensure easier mapping afterwards. In the pictures both vegetation and material are represented.
An abney level was also used to take slope angles on interesting small slopes within the grid. One difference in the mapping at Mixed Pickle was the method of creating the morphology. To create a more detailed mapping every cell was drawn on a paper with 10 ×10 cm on the sides. Instead of drawing the whole grid on one paper/perspex this scale created a more detailed mapping. The amount of photos was also smaller at Mixed Pickle because of the size of the grid. The elevation curves were measured and mapped in the same way as at Tafelberg. The photographs are going to be presented in Geografiska Annaler as a photomosaik, but will not be presented in this thesis.
4 RESULTS
Here the results from moisture and temperature measurements taken on the two sites at Tafelberg and Mixed Pickle are presented. The temperature measurements for both sites are presented first, followed by the moisture measurements. The influence of Azorella selago on moisture and temperature values and statistical correlations ends the results part.
4.1 Temperature variations at Tafelberg
The first day of fieldwork at Tafeberg (2008-04-02) was a sunny day with no wind. Temperature measurements were taken at 12:00, 14:00 and 16:00. The daily trend in temperature variations can be seen in Figure 12 and in Figure 21 (Appendix A1). The highest maximum and average temperature occur at 14:00 with a maximum temperature of 26, 2 °C and an average of 17,5 °C. The minimum temperature is 2, 2 °C which gives a temperature range of 24 °C. The lowest maximum temperature, (23, 2 °C), temperature range (20 °C) and average temperature (14, 3 °C) occur at 16:00. The lowest min temperature is the same at 12:00 and 14:00 at 2,2 °C. A trend in cold and warm temperature areas is consistent within the grid during the whole day, see Figure 21(Appendix A1). The highest temperatures of all the measurement days survey during 2008-04-02 and conditions Sunny no wind. The temperature range is also highest within the grid at any specific point in time during the conditions sunny no wind, see Figure 12 and compare the values in the different diagrams.
On the second day of fieldwork (2008-04-03) measurements were taken at 10:00, 12:00, 14:00 and 16:00 and the conditions were sunny and windy with a change to partly cloudy at 14:00 and 16:00. The maximum and minimum temperature during the day were 16,4 °C and 0,8 °C and occur at 12:00. When conditions change to partly cloudy at 14:00 and 16:00 a reduction in max temperature gives a reduced temperature range and at 16:00 the range is only 9,8 °C compared to 15,6 °C at 12:00. A spatial pattern within the grid of warm and cold areas is also consistent during the sunny windy day, but not as apparent as on the day before, see Figure 21 (Appendix A1). The maximum temperature and the temperature range are in general lower at sunny windy conditions than at the conditions sunny no wind. The minimum temperature does not change as much as the max and temperature range, see Figure 12. Because of the change in weather conditions to partly cloudy at 14:00 and 16:00 a whole day comparison can not be made according to conditions sunny no wind and sunny windy. But at 12:00 the conditions are sunny no wind at 2008-04-02 and sunny windy at 2008-04-03, therefore a comparison between the different conditions at 12:00 could explain variations
during different conditions. The max temperature during sunny no wind conditions was at 12:00 24,6 °C and at sunny windy conditions 16,4 °C. In the same comparison order the minimum temperature were 2,2 °C and 0,8 °C.The average temperature distinguished from 16,7 °C to 9,4 °C. The chill effect from the wind is therefore obvious.
On the third day of field work (2008-04-04) measurements were taken at 10:00, 12:00, 14:00 and 16:00 and the conditions were overcast, windy, misty during the whole day. The temperature range during the day is, as showed in Figure 12, much reduced compared to the other days. The daily temperature trend in mean temperature is slightly affected by wind and cloud conditions. Temperature variations (max-min) are 2 °C or < 2 °C except for a single value of 5,6 °C at 12:00. No spatial variations can be seen within the grid during these conditions according to Figure 21 (Appendix A1).
080402 Sunny no wind. 080403 Sunny windy. (Partly cloudy 14:00 and 16:00) 080404 Overcast, windy, misty.
080420 Overcast, partly misty, partly windy, partly sunny. 080404 Overcast, Light wind, Partly misty.
4.2 Temperature variations at Mixed Pickle
At Mixed Pickle the field work started a few weeks later (2008-04-20) and measurements were taken over two days. Temperature measurements were implemented at 10:00, 12:00, 14:00 and 16:00, see Figure 13. At 10:00 conditions were overcast, misty, light wind, at 12:00 overcast, windy, partly sunny. At 14:00 misty sunny light wind and 16:00 had partly cloudy, windy misty. The weather conditions changed a lot during the day (according to weather circumstances on the west side of the island). The first day of measurements at Mixed Pickle had the coldest surface temperature for both Mixed Pickle and Tafelberg and were noted as – 3,6 °C at 16:00. It is also notable that during the first day all minimum temperatures were below 0 °C. Figure 22 (Appendix A2) show that spatial variability is low during the whole day, but a pattern of higher variability during 12:00 and 14:00 compared to 10:00 and 16:00 can be observed.
On the second day (2008-04-21) measurements were taken at 12:00, 14:00 and 16:00 and conditions were more stable. All of the times had overcast with small changes in wind and mist. The temperature range is lower during these conditions and all minimum temperatures were above 0 °C compared to the day before. The spatial variability is also lower on this day, see Figure 22 (Appendix A2). The highest temperature range occurs at 14:00 both days. A daily temperature trend can be followed in Figure 13 with maximum temperature at 14:00 both days.
Figure 13. Maximum, average and minimum temperature values at Mixed Pickle during two
4.3 Temperature values at Tafelberg vs Mixed Pickle
The fact that weather conditions vary a lot on the west side of the island and that only two measurement days were implemented makes the comparison grade between Mixed Pickle and Tafelberg small (Figure 12 vs 14 and Figure 21 vs 15). One thing worth mention is that the minimum temperature seems to be generally lower at Mixed Pickle compared to Tafelberg. A day with variability in weather conditions at Mixed Pickle presents minimum temperatures below 0 °C at all times. At Tafelberg all minimum temperatures were above 0 °C. (The points for measurements were also much reduced at Mixed Pickle). One apparent trend on both days for Mixed Pickle is a relatively big drop in temperature between 14:00 and 16:00. At 2008-04-21 (overcast) the difference in mean temperature is 3,6 °C between the different times. The minimum temperature at 14:00 (4,2 °C) is also higher than the maximum temperature at 16:00 (3,8 °C). Compared to Tafelberg 2008-04-04 with similar conditions the mean temperature between 14:00 and 16:00 only differs by 0,9 °C.
On the last day (2008-04-04 and 2008-04-21) of measurement, on respective sides, both implicate overcast conditions and are therefore comparable. At Tafelberg temperature variations (max-min) are 2 °C or < 2 °C or less except for a single value of 5,6 °C at 12:00. At Mixed Pickle temperature variations are >2 °C of all times with a maximum temperature range of 3,8 °C at 12:00. Maximum temperatures are higher at Tafelberg at all points with a maximum difference of 4,8 °C (Tafelberg: 12 °C, Mixed Pickle: 7,2 °C) at 12:00. Minimum temperatures are lower at Mixed Pickle at all points with a maximum difference of 5,4 °C (Tafelberg: 6,6 °C, Mixed Pickle: 1,2 °C) at 16:00.
4.4 Moisture variations at Tafelberg
Moisture values were measured over two days at Mixed Pickle. First day (2008-04-02) measurements were implemented at 12:00, 14:00 and 16:00. Weather conditions were observed to be sunny no wind. No daily trend in moisture can be seen the first day. According to Figure 14 the variability in moisture content is large. Apparent patterns of areas with low moisture content can be observed in Figure 23 (Appendix B1) and the areas correlate with the points were measurements have been taken on Azorella selago, Se chapter 4.7 and Figure 25 (Appendix C1) for more info.
2008-04-03 moisture measurements were implemented at 10:00, 12:00, 14:00 and 16:00. Conditions were sunny windy at 10:00 and 12:00 but became partly cloudy at 14:00 and 16:00. The average and minimum moisture contents are slightly lower than the day before, see Figure 14. No apparent daily trend in moisture change and large variability in moisture content during this day. Low moisture areas still follow the pattern from the day before, see Figure 23 (Appendix B1).
On the second day (2008-04-04) conditions were overcast, windy, misty at all times 10:00, 12:00, 14:00 and 16:00. No daily trend can be seen and moisture variability is large. A trend of lower maximum moisture content can be seen during the last day with overcast conditions, see Figure 14. Low moisture areas still follow the pattern from the day before, see Figure 23 (Appendix B1). 080402 Sunny no wind. 080403 Sunny windy. (Partly cloudy 14:00 and 16:00) 080404 Overcast, windy, misty. Figure 14. Maximum, average and minimum moisture values at Tafelberg
4.5 Moisture variations at Mixed Pickle
Moisture measurements were also taken on two different days at Mixed Pickle starting on 2008-04-20. The moisture measurements were implemented at 10:00, 12:00, 14:00 and 16:00. Weather conditions were observed to be overcast, partly misty, partly windy, partly sunny. No daily trend in moisture change can be observed. The variability in moisture content is large, see Figure 15. Low moisture areas have a consistent pattern during the day, see Figure 24 (Appendix B2).
On the second day (2008-04-21) moisture measurements were implemented at 12:00, 14:00 and 16:00. Conditions were observed to be overcast, with a light wind and partly misty. No daily trend can be observed during the new conditions and no trends or specific differences can be observed between the two different days according to Figure 15. The variability in moisture content is large. Areas with low moisture content are easy to find within the grid at Mixed Pickle as well as at Tafelberg see Figure 24 and 23 (Appendix B2, B1). The areas with low moisture content correlate to the points marked with Azorella selago, see chapter 4.7 and Figure 25 (Appendix C1).
080421 Overcast, light wind, parlty misty. 08020 Overcast, partly misty, partly windy, partly sunny
Figure 15. Maximum, average and minimum moisture values at Mixed Pickle on two different days O.B 2008
4.6 Moisture values at Tafelberg vs Mixed Pickle
The maximum values in moisture content are higher at Mixed Pickle than at Tafelberg, comparing Figures 16 and 18. At all times maximum values are > 80% at Mixed Pickle. At Tafelberg maximum values are < 80 % at all times. The maximum value at Mixed Pickle is 86,9 % compared to Tafelberg with 76 % which gives 10 % higher maximum moisture value at Mixed Pickle.
Minimum moisture value is > 20 % at all times at Mixed Pickle with a lowest value of 22, 2 %. Minimum moisture value is <10 % at all times at Tafelberg with a minimum value of 0, 2 %. The average values also differ between the two sites. At Mixed Pickle the average values are > 60 % on all occasions compared to Tafelberg with average values between 38,2-42,5 %. Moisture content at ground surface is higher at Mixed Pickle than Tafelberg at the time of measurement. The spatial variability is large for both sites, see Figures 23 and 24 (Appendix B1 , B2). Dry areas are associated with Azorella selago, se Figure 25 (Appendix C1) for more information.
4.7 Moisture and temperature influence from Azorella selago
In Figure 25 (Appendix C1) two different rows are presented. The first row represents moisture measurements 2008-04-20 at Mixed Pickle with measurements taken on the plant
Azorella selago. The other row represents moisture measurements from 2008-04-20 without
measurements taken on Azorella sealgo. There are clear differences in moisture curves between the different rows. There are several dry areas on the first raw that can be observed consistent during all the different times on the first day. These areas represent areas with measurements taken on Azorella selago. In the second row, with the measurement points for
Azorella selago taken away, there are no distinct dry areas. One dry area can still be seen and
is also consistent during the whole day.
In Figure 26 (Appendix C2) the same procedure is used with temperature measurements at Tafelberg 2008-04-02. The first row represents temperature measurements including measurements taken on Azorella selago. In the second row these last named measurements have been taken away. Compared to Figure 25 (Appendix C1) almost no differences can be observed in Figure 26 (Appendix C1). A small tendency to a wider, warmer area in the left corner at 1600 can be seen in row two. But at 1200 and 1400 small tendencies of the same warmer areas can be seen in row one. No distinct patterns can thereby be observed. According to this data Azorella selago affects the moisture values more than the temperature values when it comes to spatial variability.
4.8 Statistical correlations between different variables at Tafelberg and
Mixed Pickle
There are four different variables in the following diagrams. Variable Rock/grain represents points in the grid where temperature and moisture measurements were taken on a rock or grain. Therefore every point has its own moisture and temperature value. The other variables are shade, Azorella selago and Azorella/Shade. Variables are marked with different symbols, e.g. Azorella selago with green circles. All Azorella points together create a group. This makes it possible to distinguish different groups of variables from each other within the grid and thereby sort out if there are any patterns. The different groups also have a trend line to show whether there are any correlations between moisture and temperature within the groups. All the Figures show measurements from 1400.
The first day of measurements at Tafelberg are presented in Figure 16. The temperature variation during these conditions is high (see Figure 12). There are no clear patterns for
Azorella selago or rock/grain. Shaded areas are colder than the other areas during the
conditions sunny no wind. The correlation is low between moisture and temperature for rock/grain and shade. Correlation for temperature and moisture for Azorella selago is 58,5 %.
Figure 16. Moisture and temperature values of different variables at Tafelberg 2008-04-02, 1400. Conditions were sunny and no wind. O.B 2008
The second day of measurements at Tafelberg is presented in Figure 17. Temperature variation is lower during this day (see Figure 12). The shaded group is colder and also wetter. No clear patterns can be seen within the other groups. The correlation between the different groups is very low on this day.
Figure 18 and the last day of measurements at Tafelberg show no temperature variation but a wide variation of moisture. Correlation between moisture and temperature are low for both rock/grain and Azorella selago. The variables are less during this day because there is no shade during overcast conditions. Azorella selago are here associated with higher moisture values compared to the other two measurement days.
Figure 17. Moisture and temperature values of different variables at
Tafelberg 2008-04-03, 1400. Conditions were Sunny, windy and partly cloudy. O.B 2008
Figure 18. Moisture and temperature values of different variables at Tafelberg 2008-04-04, 1400. Conditions were overcast, windy and misty. O.B 2008
At Mixed Pickle conditions were more homogenous during the different measurements. Because of the small amount of measurements the reliability is lower at Mixed Pickle.
Azorella selago are drier and warmer and more clustered at Mixed Pickle according to Figure
19. The shaded areas are colder and also wetter than for the other groups.
In Figure 20 similar patterns can be observed. No areas with shade exist on this day because of no observed sunshine. Azorella selago are clustered here as well and associated with lower moisture values. Azorella cannot, as on the previous day, be associated with warmer temperatures. The temperature and moisture range are all smaller this day compared to the day before.
Figure 19. Moisture and temperature values of different variables at Mixed Pickle 2008-04-20, 1400. Conditions were Overcast, partly misty, partly
windy and partly sunny. O.B 2008
Figure 20. Moisture and temperature values of different variables at Mixed Pickle 2008-04-21, 1400. Conditions were Overcast, misty, slight windy. O.B 2008
