University of Cape Town I

University of Cape Town I

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BOLUS LIBRARY

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Can soil factors of Namibian fairy circles explain the absence of vegetation from them?

By: Fabian van Hase¹ Supervisors: M.D. Cramer¹

M. Picker²

1Department of Botany, University of Cape Town, South Africa

2De.partment of Zoology, University of Cape Town, South Africa

A dissertation submitted to the University of Cape Town in partial fulfilment of the honours degree in Botany

November 2010

1

0\ ,\Ti~ED

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The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source.

The thesis is to be used for private study or non- commercial research purposes only.

Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author.

University of Cape Town

I I I I I I I

Can soil factors of Namibian fairy circles explain the absence of vegetation from them?

Abstract

Fairy circles are an enigmatic feature of the pro-Namib zone of Namibia which is expressed in countless barren circles of 2 -12 m in diameter. In April 2010, 54 fairy circles were studied in the Namibrand Nature Reserve to investigate the hypothesis that their soils show differences to the soils of the surroundings which would lead to suppressed plant growth and the absence of vegetation from the circles. Characterisation, temperature profiling and a transplantation experiments were carried out in the field and soil collected in the field (from on-, off-circle and ant nests) was analysed for nutrients, field capacity, drying rate and used in a growth experiment (using wheat) in the glasshouse of the University of Cape Town. No distinct differe₎ nces in soil nutrient levels, water holding capacity or temperature were found for circle soils and the transplantation experiment did not yield any significant fitness reductions of plants on circle when compared to the matrix.

However, the growth experiment showed increased growth on antrn,,--est nd suppressed growth on

circle soils and the drying rate of circle soils was significantly higher than that of matrix soils (F,3,33i =

9.91, p < 0.001 after 96 h of drying). I conclud~ that circle soils could dry out faster than roots

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manage to grow and that this may kill grass seedlings and keep circles bare. Secondly, the inhibited r,,.,,

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growth on soil from circles may be due to an inhibitory substance released by an organism other than ants (due to increased growth on ant nest soili nto these soils; however no culprit for this

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process was found and further study into this is wa ranted. """'

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Keywords: Fairy circles, pro-Namib, drying rate, grassland, inhibitory substance, soil moisture

Introduction

Fairy circles are ellipsoid, often slightly concave depressions which are devoid of vegetation and are bounded by a rim of tall grasses. They were first mentioned in a scientific paper in 1971 and

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subsequently reported on in at least 10 papers (van Rooyeiy2004}. They occur in an irregular north- south belt of the pro-Namib zone of Namibia and are regufahy spaced or 'overdispersed' in the landscape (R-value of l.68}(Tinley 1971, Albrecht

et al.

^2001}. This zone is the transition between the

2

I I I I I I I I I I I I I I I I I I

great escarpment and the Namib Desert on the west coast of southern Africa and is defined by low scrubby bush and grass plains (Joubert 2008, van Rooyen 2004). The distribution of these fairy circles extends from southern Angola through Namibia and just beyond the Orange River (Moll 1994, Jurgens

et al.

1997, Becker and Getzin, 2000) . .. _.,

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The vegetation in which the circles occur is generally sparse grassland and is based on sandy plains and only after unusually good (above 120 mm) ·summer rains are these

Stipagrostis

grasslands lush (Viljoen 1980, Albrecht 2001). Despite this, circles remain obvious in the landscape. In general fairy circles show a trend of decreasing size and less developed edge vegetation from north to south, ranging from 10 m in southern Angola to 2 m in southern Namibia and northern South Africa (van Rooyen 2004).

Since 1971, numerous hypotheses have been put forward to explain the origin of fairy circles. The

more notable hypotheses in terms of studies p�rformed suggested the influence of areas of localized

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fe".,vµJ radioactivity, termite a�!Yil.Y, fungi and allelopathic compounds from

Euphorbia

plants, (Joubert to �

2008, van Rooyen 20()fl,, Moll 1994, Becker and Getzin 2000, Jurgens

et al.

1997). The radioactivity,

J.-,,•,/�

fungi and

Euphorbia

hypotheses have since been discredited (van Rooyen 2004, Joubert 2008).

However, the termite hypothesis has had support in the literature and recently an alternative ant hypothesis (concerning an

Anoplolepis

species) has been proposed (Albrecht

et al.

2001, Picker 2010, pers. Comm.). Albrecht

et al.

(2001) proposed that termites have their colonies directly underneath fairy circles and that they release a biologically active compound that diffuses to the soil surface where it inhibits root hair growth which decreases drought tolerance of these plants. Due to the temporally patchy distributior 'of rain, grasses in the area would have to be well adapted to drought

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stress due to short terrrl wetting and drying. Thus, not having extensive root hairs for optimized

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water uptake would make the plants above termite' nests vulnerable to desiccation which eventually would kill them, producing bare fairy circles. It was suggested that termites would be interested in clearing the area above their colo�y to enhance water uptake and retention through reduced transpiration by plants. However, Albrecht

et al.

(2001) were not able to isolate such a biologically active compound thus far. Similar biologically active compounds may also be produced by other plants, similar to the initial idea of allelopathy by

Eupliorbia

plants; however, so far no suitable culprit has been identified. Recently an alternative ant hypothesis is also being studied (Picker 2010,

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^{

pers. Comm.). It was sugg�sted that ants rather than termites may be the causal factor of circles as

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observations during fie!dtrips to the fairy circles of north and south of Namibia showed the absence of termites and the presence of ants on circles. The exact mechanisms of this hypothesis however, have not been proposed yet.

3

I I I I

Fundamental support of the basic principle of the above hypotheses was given by Seely and Hamilton (2003 ). They argue that the regularly spaced distribution of the circles indicates intra- specific competition of a settling organism (such as a social insect or plant), supporting the idea that

the circles must be of biological and not of physical origin.

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I studied 54 fairy circles within the Namibrand Nature Reserve located in the south-/est of Namibia

11

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(Fig. 1) with the aim of isolating possible causal factors of fairy circles. It was hypothesised that circle soils differ from soils of the surroundings leading to decreased plant growth on them. Possible

~ s of differing soils were hypothesised to be attributable to hi~er soil surface temperatures on

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circles than the surroundings, soil nutrient levels being lower on circles, circle soils having a lower

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field capacity (water hoJding capacity) and a faster drying rate than the soils outside the circles. All of these could potentially lead to inhibited growth or higher mortality rate of grasses on circles.

I tested these hypotheses by collecting soil samples for nutrient analyses, growth experiments, and measurements of field capacity and soil drying rates. Soil temperatures were also measured and in- situ transplantation experiments performed. The study was motivated by the evident lack of a central theory that can adequately explain the phenomenon of fairy circles.

Methods

Site description

Fieldwork was carried out at Namibrand Nature Reserve (15.01203E, 16.00249W) between the 10th and 24th of April and the growth experiment was performed at the University of Cape Town between the 20th of August and 11th of October 2010. The study area was characterised by sandy flats with underlying calcrete at about 2 m depth and the vegetation was composed of homogenous Stipagrostis grassland (matrix dominated by 5. obtusa). Fairy circles were found on all sandy areas

and were a prominent feature of the landscape. The grasslands receive an annual average

precipitation of 90 - 120 mm (between January- April), are subject to herbivory by antelope and are prone to fires after goGd rain years (Viljoen 1980). Being the oldest desert in the world, the Namib has seen little climatic change since its formation, with no major vegetation shifts since at least the Oligocene (van Zinderen Bakker 1975). An easily accessible area was chosen north of the road leading to the Wolwedans Lodge and all 54 circles contained within this area were studied.

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Fig. 1 Map of the study area {15.01203E, 16.00249W) in the Namibrand Nature Reserve located in the south-west of Namibia (image adapted from www.namibrand.com/images/nrnr-map.jpg)

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Fairy circle characteristics

To establish the shape and orientation of the circles, the long and short axes of 54 circles were measured and the orientation of the long axis was determined with a field compass. A paired sample t-test was then performed on the data to test for significant distortion of circles (i.e. non-roundness).

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Community compostion

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Vegetation transects were done on 26 circles to establish community composition as well as prevalence of vegetation on and off the circles. Transects were taken on a northerly bearing from the centre of each cirr!e, the length of transects being equivalent to twice the radius of the individual circle. At 0.5 metre intervals a 2 m long stick was placed perpendicularly with its centre mark on the transect. This created a fishbone transect on which the number of plants of each species were recorded. An Analysis of variance (ANOVA) and Post hoc Fisher LSD were performed on these data.

Temperature

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On 10 circles temperature readings were taken three times over the course of one day (7h30 - 8h30, - ~ 10h40 - 11h40 and 16h30. - {7h30). An industrial electronic thermo me er was used (Fisher Scientific§ l ~

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Digital Thermometer).':~eadings were obtained by pushing the 6cm loni, probe vertically into the v.,<'h,

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gcound. An ANOVA and Post hoc f;she, LSD wece perl'o,med on these data.

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Plant transplantation experiment

Nine circles, each with an associated matrix area 10 m from the cir e, ge, were marked. Five green

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seedlings of StipiJf;rostis obtusa were transplanted from the ma rix (out ide of the marked matrix areas) onto each circle and five into their associated matrix area. In bot! the matrix and circle areas,

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five 5. obtusa seedlings that 'Nere already growing on those sites re s.•lected and used as a control group for the transplan,ts-(termed "natives"). All plants were watered de ly and a "greenness" index was recorded in the afternoon. Four categories of "greenness" were ass, ;ned based on an equal division of the natural colour range observed in the plants; in decreasing order of plant fitness these

were: dark green, green, pale green and yellow (assigned colour codes 4, 3, 2 and 1). _I\

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Soil samples

Soil samples were taken with a standard auger at a depth of 30 cm. Four •.oil samples were

combined from on-circle, four from the periphery and four from the matrix to ensure an average soil from each locabon. T~'.; ci'cl~s wece sam\ d ;nth;, way and anothec 7 s

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mples (of four subsamples

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each) were taken from the so~irectly above ant nests located in the matrix. The soil was stored in plastic bags and dried in the sun for 24 h before the bags were sealed and transported to the laboratory at the University of Cape Town.

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Nutrient analysis

Soil samples were sieved through a 2 mm mesh for nutrient analyses and mass spectrometer analyses. Soil electrical conductivity (EC) was determined as the resistance of saturated soil in a standard USDA soil cup. Soil pH was measured by shaking 2 g of soil in 20 ml 1 M KCI (at 180 r.p.m.) for 1 h, centrifuging at 10 000 g for 10 minutes and then determining the supernatant pH. Soil N was established via digestion with a FP-528 Nitrogen Analyser (leco Corporation, USA). 6.6 g of soil were extracted in Bray II solution (Bray and Kurtz 1945), filtered and then analysed using ICP-AES (Varian

l

Vista MPX, Australia). From 10 g of soil the exchangeable cations were displaced with 25 ml of 0.2 M ammonium acetate sc!ution; after filtration through a Whatman no. 2 they were made up to 200 ml and K, Na, Ca and Mg determined using the ICP-AES analysis. Zn, Mn, Cu and Fe were extracted with 0.02 M Na-EDTA and concentrations determined by ICP-OES against appropriate standards.

To determine B, 20 ml of deionised water were added to 10 g of soil; this was heated to 8D°C and three drops of calcium chloride were added. The solution was filtered through a filter paper and the resultant solution was submitted for analysis using ICP-OES. 40 mg of soil were weighed into a tin capsule and combusted in a Thermo Flash EA 1112 series elemental analyser. The gasses were conveyed into a Delta Plus XP isotope ratio mass spectrometer (Thermo Electron Corp., Italy). An IAEA and two in-house st2r.dards were used to calibrate the results and the C and N isotope ratios

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were expressed as c5¹³

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and c5¹⁵N respectively (Ehleringer and Rundel 1989, Evans 2001).

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A growth experiment 1as performed in the glasshouse of the Univ rsity of Cape Town using wheat

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Growth experiment

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(Triticum aestivu& aviaans). After germination in vermicur , seedlings of 60 mm(± 2 mm) from shoot base to tip were transplanted on the 20th of August

sieved (2mm mesh size) soil collected from the field. To prevent co etition for resources only one seedling was planted in the c;?.ntre of each pot at a depth of 15 mm (total of 37 plants). The pots were watered and mov,2d within the greenhouse three times a week to ensure equal exposure to

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environmental gradients. --(' 4" I- ~ '" ' ~ " " "

Height measurements were taken r~ to allow estimation of growth rates. Nineteen plants were harvested on the 11th of October 2010. All roots were carefully washed out of the soil and the plants were divided into flowers, stems, roots, live leaves and dead leaves which were weighed

7

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separately. The tissues were then dried in the oven at 80 °C for 48 hand reweighed. AN OVA and Post hoc Fisher LSD tests were performed on these results.

To investigate resistance to water stress of the remaining plants, watering was discontinued from the 9¹h of October 2010. A fluorometer (PAM-2100 Fluorometer, Heinz Walz GMBH) was used to measure Fm'/Fo', qN and ETR. Fm' is the maximum fluorescence yield, Fo' is the minimum

fluorescence yield (Fm'/Fo' indicates stress levels), qN quantifies non-photochemical quenching as a measure of stress and ETR is a measure of photosynthetic rate (Maxwell and Johnson 2000). A repeated measur~ iNOV.il. and Post hoc Fisher LSD test was performed on these data.

The remaining plants were harvested on the 22nd of October 2010. In order to be able to reuse the soil, these were harvested by sieving the roots from the soil with a sieve of 1.5 mm mesh diameter.

A similar procedure as during the first harvest was performed to measure wet and dry biomass.

ANOVA and Post hoc Fisher LSD tests were performed on the height data and a factorial ANOVA and Post hoc Fisher LSD test was done on the biomass data.

Soil field capacity and drying rate

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To investigate water holding ~apacity and the rate at which the different soils dried out a plug of ,:... /i

glass wool was placed in 10 ml syringes; these were filled with dry soil (ca. 10 g) and weighed after saturation of the soil with water. Soil field capacity (in gH20/gsoill was calculated as:

FC

=

^{(W - D) -;-(D)}, where FC is field capacity, Wis the weight of the soil at field capacity and Dis the weight of the air dried soil.

The soils in the syringes were then dried in an oven at 55°( for 96 hand reweighed at 24 h intervals to determine drying rates.

Statistical analysis

All statistical analyses

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performed using STATISTICA software (StatSoft 2009).

Results

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Fairy circle characteristics /

Fairy circles ranged in size from 1.9 m to 11.7 m (long axis), with a ; r .a\ of 6.7 ±0.26 m. The paired samples t-test yielded a statistically significant long to short axes i~ mean = 1.32 ± 0.028, t₁₀.05121,531 = -10.50, p < 0.001), showing an oval shape of the circles. The orientation of circles showed

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no preference for any particular bearing as they were randomly scattered across all bearings (Fig. 2).

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20.0

15.0

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5.0

0.0

N-S NNE-SSW NE-SW ENE-WSW E-W NNW-SSE NW-SE WNW-ESE Orientation

Fig. 2 Orientation of fairy circles shown by the bearings of their long axes, the y-axis shows the percentage of fairy circles per bearing (n = 54)

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Circle Matrix

Treatment

Periphery

• Stipagrostis ciliato

• Stipagrostis obtusa

• Schmidtia ka/ahariensis

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Fig. 3 Grass abundance shifts between fairy circle, matrix and periphery for the three most common species, individual grasses were counted on transects taken from circle centre into the matrix (length = 2*radius, n = 26). A one-way ANOVA on these data was followed with a Post hoc Fisher LSD test for homogenous groups; the bars indicate the mean ± SE and dissimilar letters above the bars indicate statistically significant differences among the treatments as per Post hoc test (p < 0.05)

9

The distribution of 5. ci/iata was ~tatistically significantly limjted to the periphery and the highest

'

abundance of grasses in terms of numbers was encountered on the periphery and the lowest on the circles (Fig. 3). Statistically significant differences supporting this were found for 5. obtusa and 5chmidtia kalahariensis between the circle and matrix (Fig. 3).

Temperature

Table 1 Surface soil temperatures through the day (mean± SE) with ANOVA and Post hoc Fisher LSD results for 10 fairy circles, meas~e~ ~ere taken with a temperature probe pushed vertically into the ground to a depth of 6 cm. Dissimilar letters indicate significant differences between treatments

Time Circle Matrix Periphery F p

Morning 19.7 ± 2.86 a 20.1 ± 2.86 a 20.0 ± 2.86 a 0.153 0.859 Noon 22.1 ± 1.03 b 22.1 ± 1.03 b 20.6 ± 1.03 a 5.7 0.0105

Afternoon 30.3 ± 0.80 a 31.3 ± 0.80 b 2.35 0.119

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No clear trends were evident in the soil surface temperatures throughout the day between the three treatments (Table 1).

Plant transplantation experiment

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Time in days 8

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~ Transplant-matrix

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Native-circle ...-Transplant-Circle

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Fig. 4 "G eenness" index changes of transplanted and native Stipagrostis obtusa seedlings throughout the transpla tation experiment as a proxy for plant fitness; 2.0 indicates green and 0.0 yellow grass which was repr sentative of the most vigorous and dead plants, respectively. The experiment was carried out in

lants were watered daily until day eight when watering was ceased

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Nutrient pH

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Mn B Fe C Molar N:P Molar C:P Molar C:N

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The only distinct difference in plant fitness was found in the consistently lower "greenness" values of native grasses in the matr.ix plots. These however, also had the lowest initial fitness and in fact increased in overall greenness in contrary to all other categories of grasses which decreased in greenness over the duration of the transplantation experiment (Fig. 4).

Nutrient analysis

Table 2 Soil nutrient contents of the four treatments; other details as for Table 1

Unit Circle Matrix Periphery Ants nest F

(KCI) 6.5 ± 0.5 a 6.6 ± 0.5 ab 6.7 ± 0.5 ab 6.8 ± 0.5 b 1.86 mS/m 5.7 ± 0.002 b 5.0 ± 0.002 a 5.7 ± 0.002 b 6.0 ± 0.002 b 4.929 mg/kg 3,0e ± 0.004 b 333 ± 0.004 b 313 ± 0.004 b 263 ± 0.004 a 4.32 mg/kg 49.7 ± 0.136 a 51.1 ± 0.136 a 44.6 ± 0.136 a 39.5 ± 0.136 a 1.599

:~:~ -. Exchangeable cations

0.44 ± 0.005 a 0.46 ± 0.005 a 0.42 ± 0.005 a 0.47 ± 0.005 a 0.62 (cmol(+)/kg)

Exchangeable cations

33.1 ± 0.002 a 32.6 ± 0.002 a 33.7 ± 0.002 ab 35.8 ± 0.002 b 2.356 (cmol(+)/kg)

Exchangeable cations

13.3 ± 0.005 b 13.4 ± 0.005 b 13.2 ± 0.005 b 10.9 ± 0.005 a 3.022 (cmol(+)/kg)

mg/kg 0.31 ± 0.003 b 0.29 ± 0.003 b 0.24 ± 0.003 a 0.32 ± 0.003 b 9.767 mg/kg 0.3 ± 0.005 a 0.37 ± 0.005 b 0.33 ± 0.005 ab 0.37 ± 0.005 b 3.828 mg/kg 44 ± 0.001 C 39 ± 0.001 b 34 ± 0.001 a 36 ± 0.001 a 21.01

·'•'.

mg/kg c:05 ± 0.006 b 0.05 ± 0.006 b 0.05 ± 0.006 b 0.07 ± 0.006 a 4.155 mg/kg 12.8 ± 0.003 a 12.7 ± 0.003 a 12.4 ± 0.003 a 13.9 ± 0.003 a 1.14

% 0.13 ± 0.015 a 0.25 ± 0.015 b 0.25 ± 0.015 b 0.13 ± 0.015 a 15.506 mol/mol 2.7 ± 0.023 ab 2.0 ± 0.023 a 2.2 ± 0.023 ab 3.1 ± 0.023 b 2.204 mol/mol 11.1 ± 0.018 a 19.6 ± 0.018 b 21.5 ± 0.018 b 12.8 ± 0.018 a 9.077 mol/mol 4.69 ± 0.333 a 10.6 ± 0.333 b 10.1 ± 0.333 b 4.29 ± 0.333 a 10.994

Nutrient levels in circle soils were not significantly different from soils of the other three treatments for all nutrients except for8 which was higher in soils from in-circle than those taken outside of circles (Table 2). Notable howe!v'er, were the very low N levels found in all soil types.

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p 0.155 0.006 0.011 0.208

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0.09

0.043

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0.019

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0.013 0.346

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0.106

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Growth experiment

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Circle

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Matrix

a

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Periphery Treatment

b

• Flowers

• Stems

• Roots

• Dead leaves

a

• live leaves

Ant nest

Fig. 5 Plant biomass from the first harvest of the growth experiment carried out in the glasshouse at the University of Cape Town (19 wheat plants harvested on the 11 October 2010). These plants were growj

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for 53 days in 1.5 kg pots filled with soil collected from the field. An ANOVA was performed o~ ata, followed by a Post hoc Fisher LSD test for homogenous groups. The bars are divided into the biomass o plant components (adding up to the total biomass) and dissimilar symbols indicate significant!, differences among treatments.

The total biomass of plants grown on ant nest soils was significantly higher than that of the other treatments. Additionally, plants from circle soils had significantly less biomass in terms of plant components (except for flowers} than the remaining treatments (Fig. 5}.

12

2.5

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1.5 • Flowers

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• live leaves 0.5

a

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Circle Matrix Periphery Ant nest

Treatment

Fig. 6 Plant biomass from the second harvest of the growth experiment carried out at the University of Cape Town (18 plants harvested on the 22nd of October 2010). Wheat plants were grown for 64 days in

soil collected from the field and received no water during the last 13 days of the experiment; other details as for Fig. 5.

The overall biomass was significantly higher for plants grown on ant nest soils than the other three treatments and plants derived from circle soils had significantly lower biomass in terms of separate

components than the other treatments (except for live leaves which showed no difference between treatments}{Fig. 6).

0.8 0.7

0.6

i

^0.5

0.4 _{- 9 -0rcle}

- . -Matrix

0.3 - Periphery

- Antnest 0.2 -i--~~~~---.~~~~~-.-~~~~~.--~~~~-,

13-0ct-10 15-0ct-10 17-0ct-10 Date

19-0ct-10 21-0ct-10

13

Fig. 7 Non-photochemical quenching (qN - indicating stress) of plants grown during the growth

experiment in the glasshouse at the University of Cape Town (n = 18); watering was ceased on the 13th of October 2010. Data points represent daily mean values per treatment and the bars indicate the standard error

Plants grown on ant nest soils had the lowest, and plants from circle soils had the highest qN towards the end of the experiment {Fig. 7). Increasing qN indicates increasing plant stress due to desiccation as watering was ceased at the onset of the experiment, therefore ant nest plants were least stressed and circle plants were most stressed for water.

2 1.8 1.6

1.4

1 0.8

0.6 -+--~~~~~~

13-0ct-10 15-0ct-10 17-0ct-10

Date

19-0ct-10

- Circle Matrix

- -Periphery -:,!--Ant nest

21-0ct-10

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Fig. 8 Maximum-(Fm') to minimum (Fa') fluorescence yield ratio of plants from the four soil treatments;

other details as for Fig. 7

The maximum- to minimum fluorescence yield indicates plant stress; higher values indicating decreasing stress levels. According to this, ant nest and periphery plants were least stressed and circle plants were most stressed (Fig. 8). An overall increase in stress showed that all plants were undergoing some constraints since watering was ceased (Fig. 8).

14

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60

so

a:::40 t;j

30

20

- -arde _ .Matrix - - . -Periphery --,..-Antn~ t

10 -+-~~~~~~~~~~~~~~~~~~~~~~~~~

13-0ct-10 15-0ct-10 17-0ct-10 Date

19-0ct-10 21-0ct-10

Fig. 9 ETR readings (indicating photosynthetic rate) of plants grown on the different soils; other details as for Fig. 7

Although the trend in ETR readings was generally decreasing through the course of the experiment, circle plants showed lower photosynthetic rates than the remaining plants on six out of nine days (Fig. 9}.

Soil field capacity and drying rate

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0.16 0.16

Circle Matrix Periphery Ant nest

Treatment

Fig. 10 Field capacity (gH20/gsoill of the 4 soil types. n ANOVA was performed on the data, fo=l-1 ~·- by a

~

Post hoc Fisher LSD test. The bars represent the mean with standard errors indicated; dissimilar letters in ~ r e n c e s between treatments as per Post hoc test

15

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Table 3 Water loss (gH20/g,0;1) on drying of the four soil types; repeated measures ANOVA and Post hoc Fisher LSD test were performed (results shown) and the table shows mean water loss± SE. Dissimilar letters indicate significantly different water losses between treatments

Drying time {h) Circle Matrix Periphery Ant nest F p

24 48 72 96

0.08 ± 0.002 C

1

^{0.06 ± 0.003 a 0.07 ± 0.004 be r ,0.06 ± 0.005 ab 4.52 0.009}

0.13 ± ^{0.003 b} ~ 0.006a 0.12 ± 0.006 b ,; ~0.007 a 4.62 0.008 0.18 ± 0.003 cd _,, 0.007 ab 0.17 ± ^{0.005 be .16} ± ^{0.002 a} 4.3 0.011 0.19 ± 0.002 d 0.18 ± ^{0.003 be 0.19} ± ^{0.002 cd 0.17} ± ^{0.003 abc} 9.91 <0.001

Significantly higher water loss was found for circle soil when compared to matrix and ant nest soil

· and only periphery showed similar water losses to the circle (Table 3) .

. ;,

Discussion

/ L-i.,n J

Circle size in the study area conformed to the north-south decrease in size as outlined by van Rooyen (2004). The random orientation of fairy circles suggested that causal factors of circles do not conform to any environmental factors such as the main wind direction or influences from solar radiation. Also supportive of previous findings was the almost complete absence of plants from fairy circles and increased vegetation on the periphery. Stipagrostis ciliata was also almost entirely confined to the periphery_ which might be a result of the high field capacity of th~ periphery soil and the access to the abundant on-circle water resources which are available due to the lack of

4,..

competition from plants there (Albrecht et al. 2001). This idea is supported by anecdotal

observations of 5. ciliata in drainage lines and on road verges where increased water availability can

. ~ r ,

be expected. Grasses on fairy circles also appeared to be no older than one growing season, which/ " ' ~ - ties in with observations made by Albrecht et al. (2001) and will be discussed later in more detail. ~

re,~ r.J

No increased soil surface temperatures were found on circles when compared to the matrix. This ( / ,

contcad;cts my hypothes;s of elevated soU tempe,atu,es of ,;,de soUs wh;ch m;ght have kmed young

~

^,

plants by damaging their root,$ (Hendrick and Pregitzer 1993). ..._;

J

Furthermore, the tranci!Jiant experiment carried out in the field only showed a lower fitness of native plants on the matri~ plo{ (Fig. 4). This experiment as such therefore failed to provide substantial evidence to suppoi the idea that in-situ plants would fare better off-circle than on-circle, at least while constant wat\ng would inhibit effects of desiccation.

~" ~J r~I.,._ - (A-I-er < A v ~ ~""~ 1,)

G,orl-,_• ~ " ' tJ:+

¹⁶

:.r-'

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t-'1/'

.tv " ' ~ ~~M ^{- ~." M J:vd} f ^e1

/ Jh6\I\I" r~,,. 6h.,r4

~ t e , , . . . ,

Q'L.-

The nutrient analysis ~ e d that circle soils were always similar to at least one other soil type, except for Mn. This means that, contrary to my hypothesis, they did not display distinctly lower nutrient levels which could have explained the absence of vegetation from circles. Furthermore, the circle soils were not similar to ant nest soils which undermines the idea that ant nests may be

permanently located on fal ry circles or that fairy cl rcles are tied to ant nests.

ilv._

"41.,~

I " u-,. , .; ~

When gro~n in t h ~ o ~ l a n t s on ant nest soils yielded a significantly higher total biomass

~

^•

than the Pe;¥,&fr,ing soils. This finding further undermines the notion of on-circle ant nests being directly responsible for increased mortality of grasses, at least in the absence of the ants themselves. Possible causal factors such as volatile organic compounds contained in the soils above ant nests can therefore not be supported by my findings. Furthermore, when plants were analysed in separate components those on circle soils generally yielded significantly less biomass than the rest (Fig. 5 and Fig. 6). As the nutrients contained in the soils do not explain this trend, it can only be attributed to other differences in soil relating either to soil properties or an unknown inhibitory substance from an organism other than ants.

Possible sources of inhibitory substances may be extinct or extant plants on or near fairy circles or some other organism that dispi~ys competition which may explain the overdispersion of fairy circles. During my fieldwork nc

0

'i~-~h agent could be identified in the study area; however, within a 100 km north and south of the study area Boscia bush clumps were encountered which showed similar dispersions in the landscape and had a similar size as fairy circles. It seems unlikely that these could have given rise to fairy circles in my study area as they were not observed there and a climate shift would presumably be necessary as their current distribution enjoys higher rainfall. No such climate shift has been reported for the pro-Namib as far back as the Oligocene and it seems implausible for potential inhibitory substances released by Boscia bushes to remain in the soil since the Oligocene (van Zinderen Bakker 1975). However, no other culprit could be identified and a study into possible

,j '

organisms producing inhibi.tory substances would be of great value here .

. ) ':;.

The indicators of stress.levels in the plants, qN and Fm'/Fo', both suggested higher stress levels in plants grown on circle soils than in the remaining plants (Fig. 7 and Fig. 8). Stress was due to desiccation after watering was ceased; this means that circle plants were not able to adapt to decreasing water availability in the soil as well as the remaining plants. Furthermore, the

otosynthetic rate of on-circle plants was generally lower than that of the other plants suggesting that on-circle plants showed inhibited photosynthesis due to water stress (Fig. 9). These findings are similar to observations made by Albrecht et al. (2001) who argued that the difference in drought tolerance is due to the lac!< _.,. cf root hairs in plants grown on circle soils. This result therefore supports my hypothesis that circ:e soils show an inhibitory influence on plant growth.

17

4 ^'241 tsu.,; ~ ~ ~

G rJ

^<..,v~

Lu'{/

I

J-rn { -.1

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~ 1

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I

My findings furthermore showed that the field capacities of matrix and ant nest soils were

significantly lower than those of circle and periphery soils (Fig. 10). This contradicts my hypothesis of circle soils having the lowest field capacity. However, despite the higher field capacity of circle soils,

' '·

they (together with periphery soils) exhibited the fastest drying rate (Table 3). This in turn supports the hypothesis of circle soils showing elevated moisture loss on drying and could be a possible explanation for the absence of vegetation on circles.

, Therefore, I propose the following mechanism for fairy circles remaining bare: Seedlings are able to

k <:J~e-,

germinate but cannot survive until the end of the growing season as the soil dries out quicker than, ; . ~

r "1 ~

roots can grow. Periphery plants that are faced with similar soil drying conditions may survive by ~

~ l f t

establishing rootlets towards the matrix where soil drying is slower. Coupled with the lack of root ~ {

'/\-b?'~v

hairs, on-circle plants would be at a significant disadvantage especially in light of the low background ^~ soil Nitrogen availability and 1'.he short and often temporally rainy season which could exacerbate soil

o,}-/)1

t,;, , moisture problems by ~;:~~~ing wetting and drying cycles (Albrecht et al. 2001). These findings tie in

.A

I J#>/J f

. uV·•~N~~

with previous observations which also found suppressed growth of plants on circle soils. The

r.+-~ ( ,z~

1

differences were ascribed to cyclical wetting and drying of these soils which becomes plausible in light of the elevated drying rates found in my study (Albrecht et al. 2001).

Conclusion

In conclusion, my results support the hypothesis of differences in fairy circle soils which cause suppressed plant growth and t e absence ~f vegetation when compared to soils from their surroundings. I was able

t o

ascribe some of these differences to the increased drying rate of fairy circle soils. The fast m~i~ture loss would bring about soil desiccation faster than roots can grow, thereby causing grass seedlings to die before the end of the growing season. Furthermore, it is plausible that an additional factor such as a biological inhibitory substance be present in these soils which would further depress plant growth. However, no culprit could be identified to date and further study is warranted to identify possible organisms giving rise to such a substance.

Acknowledgments

I would like to thank Michael Cramer and Mike Picker for guidance during field work and formulating

the concepts of this study as well as for the necessary funding. A special thanks goes to Michael Cramer for accurate directions in the writing process and the many lessons learnt during data processing. I would also like to thank Kelly Vlieghe, Vere Ross-Gillespie and Nils Odendaal for their

18

I

I I I I

help with fieldwork and the accommodation at Na mi brand Nature Reserve. Last but not least, I would like to thank the University of Stellenbosch for the provision of the wheat seed material.

References

i /Albrecht, C.F., J.J. Joubert and P.H.d. Rycke (2001). Origin of the enigmatic, circular, barren patches ('Fairy Rings') of the pro-Namib. South African Journal of Science 97, 23-27.

v Becker, T. and S. Getzin (2000). The fairy circles of Kaokoland (North-West Namibia) -origin, distribution, and characteristics. Basic and Applied Ecology 1, 149-159.

V B

^{ray, R.H}. and L.T. Kurtz (1945). Determination of total, organic, and available forms of phosphorus in soils. Soil Science 59, 39-45.

v l:hleringer, J.R. and P.V: \~undel (1989). History, units and instrumentation. Rundel, P. W., J. R.

Ehleringer and K. A. Nagy. Springer-Verlag, Berlin. 1-15.

0 vans, ^D.R. (2001). Physiological mechanisms influencing plant nitrogen isotope composition. Trends in Plant Science 6, 121-126.

V

Hendrick, R.L. and K.S. Pregitzer (1993). Patterns of fine root mortality in two sugar maple forests. Nature 361, 59-61.

V

Joubert, A. (2008). ^lnve_~_tigation on selected biotic and a biotic factors in the maintenance of the

"fairy circles" (barren p;tches) of Southern Africa. 141. University of Pretoria, Pretoria

V

Jurgens, N., A. Burke, M.K. Seely and K.M. Jacobsen (1997). Desert. Vegetation of Southern Africa.

Cowling, R. M., D. M. Richardson and S. M. Pierce(Eds) Cambridge University Press, Cambridge. 613.

Maxwell, K. and G.N. Johnson (2000). Chlorophyll Fluorescence -A Practical Guide. Journal of Experimental Botany 51, (345): 659-668.

/ Moll, E.J. (1994). The

c )i~i~

and distribution of fairy rings in Namibia. 1203-1209. Malawi 19

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I

I

V

Picker, M. (2010). A n t ~ i r y Circles Formation. Personal Communication. Cape Town

U~I' .

Rooyen, M.W.v., G.K. Theron, N.v. Rooyen, W.J. Jankowitz and W.S. Matthews (2004). Mysterious

_•;:·

circles in the Namib Desert: review of hypotheses on their origin. Journal of Arid Environments 57, 467-485.

v seely, M. and 8. Hamilton (2003). Fairy Circles in Western Namibia. The Barking Gecko 4, 2.

V StatSoft, I. (2009). Data Analysis Software System.

\/' Tinley, K.L. (1971). Ethosha and the Kaokoveld. Supplement to African Wildlife 25, 1-16.

t ~ . . '. ~--;

~ an Zinderen Bakker, ·E.M. (1975). The Origin and Palaeoenvironment of the Namib Desert Biome. Journal of Biogeography 2, (2): 65-73.

V

Viljoen, P.J. (1980). Veldtipes, verspreiding van die grater soogdiere en enkele aspekte van die ekologie van Kaokoland. university of Pretoria, Pretoria

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