PlantLife Volume 51.6, July 2021. The woody vegetation of Botsalano Game Reserve

The woody vegetation of Botsalano Game Reserve, North West Province

Text and photographs by Jeff W Morris

View across the Reserve from Sentinel Hill looking north towards Botswana.

 

Introduction

On our camping holidays to Botswana and Namibia, my wife and I often stopped over at Botsalano, a word meaning 'friendship' in Setswana. It was a convenient hopping off place and one of our favourite reserves. With lock-down and international borders being closed, I decided to write a sorely-needed guide for the reserve to keep me busy. What started off intending to be a map, some photos and a couple of check lists grew as I delved deeper into the history, geology and plant life. The tourist guide, a result of four visits over the past six months, is finished and this article shares some of the interesting features of the woody vegetation.

 

Location of Botsalano Game Reserve.

The Botsalano Game Reserve is located 40 km north of Mafikeng in the North West Province of South Africa, south of Lobatse, Botswana, and west of Zeerust. It is 5 742 ha in extent and is administered by the North West Province Department of Parks.

In terms of both geology and vegetation, the reserve borders on major formations. The rocks are of great interest to geologists as some are of the oldest found in southern Africa right next to some of the most recently formed. The vegetation also has components of two major biomes: the Grassland Biome and the Savanna Biome (Mucina & Rutherford 2006).

Climate 

The climate is warm-temperate with high temperatures in summer (exceeding 30 degrees C). Winter nights can be cold with occasional frost, as is usual in the dry hinterland of the continent. Average annual precipitation is about 530 mm, occurring in summer and autumn.

Topography and hydrology of the area

Looking across a shallow valley towards Sentry Hill that lies on the Southern border of the Reserve.

The reserve has a flat to undulating topography with a fall from the highest elevation at 1 440 m to the lowest of only about 120 m. The landscape tilts gently from the higher lying northeastern end towards the southwestern lower lying areas. The central part is characterised by gentle slopes and shallow valleys. Sentry Hill, the location of one of the bush camps, is a high point in the reserve  (1 431 m). The many dams in Botsalano effectively block the natural flow of streams, resulting in mostly dry streambeds except after heavy rains.

Geology

The geological evolution of the area is fascinating and has been researched at length. Deep below the surface, magma cooled slowly 3 000 million years ago to form granite, while at about the same time (geologically speaking) on the surface massive volcanoes ejected molten lava and ash to cover thousands of square kilometres in North West Province and Botswana (Grobler & Walraven 1993).

Close-up of rocks of the Kanye Formation (left) and typical exposure of rocks between grass and shrubs (right).

Underlying most of the reserve are volcanic sediments of Ash flow tuff and Block-and-ash flow tuff of the Kanye Formation. This Formation consists of black, fine- to very fine-grained felsitic rocks. The felsitic rocks have a red-brown to yellow-brown weathered surface while fresh samples have a black vitreous appearance (Michaluk & Moen 1991). This rock has been highly weathered. On the surface all one finds is crumbly small stones. Watch out for these on many road surfaces.

East-west aligned dolerite dykes of very hard rock occur in the northern part of the Ash flow tuff area. They are particularly abundant in the area south of Thutlwa Road near the junction with Kukama Drive. These are not easy to spot when the grass is long and are best observed after a fire. Dykes are near-vertical, sheet-like features formed when molten magma is forced to the surface through longitudinal cracks in the bedrock.

Typical dyke exposures.

The southwest corner of Botsalano is considered to be on the arc of an extinct volcano which had a diameter of about 30 km. The volcanic cone has been eroded away completely and the best evidence that it existed comes from specialised aerial surveys.

Over time, the granite below the Kanye lava and ash was pushed upwards so that it is now exposed as the Gaborone Ring Complex (Grobler 1996) on the southern and western edges of the Reserve. The granite is composed of quartz, alkali feldspar and plagioclase. 

 

Gaborone granite boulders

Alluvial deposits comprising mostly clayey soils cover geological formations in the beds of drainage lines. These deposits are easily eroded and often result in unvegetated areas with dongas.

Tongues of Kalahari sand extend into the reserve from the southwest corner. These sedimentary deposits, which cover vast areas of the Kalahari to the west and north of southern Africa, represent some of the most recent geological formations in southern Africa. They date from the Quaternary, a period about 2.6 million years ago, and overlay the granite rocks. Only very small remnants of this formation occur in the reserve and most have been disturbed by human activities in the past, including cultivation. It is identifiable by deep, orange-brown loose sand on the surface.

Visitors should look out for rhino rubbing rocks. These interesting objects are rocks rubbed as smooth as silk by rhino rubbing off parasites after a mud bath.

Example of Rhino rubbing rock with A4 binder for scale.

Plant communities

Five plant communities have been identified on the basis of a quantitative analysis of the woody species. The revised map is based on an earlier study by Bosch (2011). You can read about the methods used in Morris (2021). There is a link at the end of this paper.

Vegetation map of Botsalano Game Reserve.

 

Bushwillow woodland

The plant community immediately behind Sentry Hill Bush Camp is named after the dominant species, Combretum molle (common names follow Van Wyk & Van Wyk 2013). The area is a steep-sided kopje with large boulders of Gaborone granite exposed on the surface. Species diversity is low. The most common species are Combretum molle, Gymnosporia tenuispina, Ozoroa paniculosa, Senegalia hereroensis, Croton gratissimus, Dombeya rotundifolia, Ximenia caffra, Vachellia robusta subsp. robusta and Vitex zeyheri.

 

Bushwillow woodland with Vitex zeyheri and Combretum molle on left.

Bushwillow woodland with Gaborone granite rocks in foreground.

Vachellia robusta subsp. robusta; Ankle thorn; Enkeldoorn on left. Tapinanthus oleifolius mistletoe on V. robusta on right.

Croton gratissimus on left; Fruits of Combretum molle on right.

Mature specimen of Ozoroa paniculosa on left; O. paniculosa leaves on right (the red patches are the exotic Peruvian zinnia growing under O. paniculosa.)

Camel thorn sandveld

This community occupies a small area of the reserve, although it is widespread in the arid northern and western parts of South Africa. In the reserve it occurs in patches in the southwestern corner extending northwards towards the main camp. Two small patches also occur along the fence line of the western and northern boundaries, often in combination with the Black thorn scrub community.

 

A magnificent specimen of Vachellia erioloba next to the reserve boundary.

The sandveld is usually on deep Kalahari sand or on shallower sand forming a veneer over sheets of Gaborone granite.

The community typically consists of tall, free-standing trees or bush clumps of combinations of the common species with open grassland between the trees. The indicator species is Vachellia erioloba (Camel thorn), a magnificent gnarled tree when mature. Good examples of the tree occur near the tourist entrance gate (Main Camp). Other commonly occurring trees include Searsia lancea, Ehretia alba, Searsea pyroides var. pyroides (uncommon in other parts of the reserve), Ziziphus mucronata, Grewia flava and Vachellia hebeclada subsp. hebeclada. Occasionally occurring trees are Vachellia karroo, Tarchonanthus camphoratus, Senegalia hereroensis, Ozoroa paniculosa and Vachellia tortilis subsp. heteracantha. Notable for being absent or rare are Euclea undulata and Searsia leptodictya.

Because of the deeper soils, large areas were cleared and ploughed in the past. These areas are often invaded by Vachellia karroo and Senegalia detinens subsp. detinens when left fallow.

 

Searsia lancea; Karree; Karee. There are two main species of Karee in the Reserve. Botanists have recently decided to re-name Rhus species as species of the genus Searsia. S lancea is easily recognised by its long, narrow drooping leaflets arranged in threes. There are some good specimens around the Mogobe Dam at the Tented Camp.

Vachellia karroo; Sweet thorn; Soetdoring. Recognised by its dark foliage, yellow globular inflorescences and black trunk. Usually with large white, straight spines. Well represented near the entrance gate and at Safari Bush Camp.

Vachellia erioloba; Camel thorn; Kameeldoring. This is a large tree and is restricted to areas of deep Kalahari sand. It is common in the Main Camp. It has characteristic woody pods that are brownish when young, as in the photo. They are grey and velvety when mature.

Black thorn scrub

This community is found along all the dry drainage lines in the reserve and can be seen where tracks cross drainage lines. Good examples are on Tilodi and Kolobe Links. Soils are heavy and clay-rich. This soil type is highly erodible and dongas occur in places. Bare soil is often visible with low grass cover.

Black thorn scrub.

The community is named after one of the common short trees, Senegalia mellifera subsp. detinens which can occur as a free-standing tree but is often an impenetrable thorny thicket. Other common woody species are Vachellia tortilis subsp. heteracantha, Ziziphus mucronata, Searsia lancea, Grewia flava, Vachellia hebeclada subsp. hebeclada and Vachellia karroo. The following also occur: Euclea undulata, Senegalia hereroensis, Carissa bispinosa and Vachellia robusta subsp. robusta.

Clumps of mixed species of short trees often occur. C. bispinosa and E. undulata, for example, are seldom standing alone but occur in dense clumps with Ziziphus mucronata, Senegalia mellifera subsp. detinens and Searsia lancea.

Heavy clay soils of the drainage lines support Senegalia mellifera subsp. detinens.

 

Senegalia mellifera subsp. detinens; Black thorn; Swarthaak; This short tree is common in the drainage lines.

Vachellia hebeclada subsp. hebeclada; Candle thorn; Trassiedoring. You will certainly know about it if you walk into one of these low shrubs with their very sharp thorns. The seed pods stand upright giving the impression of candles. Candle thorn is most common in previously overgrazed, low-lying areas.

 The earth walls of most dams are covered by large Searsia lancea trees.

Umbrella thorn savanna-woodland

The bulk of the reserve is covered by two intermingled plant communities. They cover the higher-lying areas where soils are generally shallow or very shallow. Rocks of the Kanye Formation form the substrate of most areas. The two communities are a fairly closed, dense woodland and a more open, grassy savanna with intermediates occurring. The main features distinguishing this community from Black thorn scrub are shallower soils and situation on higher-lying ground.

The following woody species are common to both variants: Vachellia tortilis subsp. heteracantha, Searsia leptodictya, Ziziphus mucronata, Grewia flava, Ehretia alba, Vachellia hebeclada subsp. hebeclada, Mundulea sericea, Vachellia robusta var. robusta, Vitex zeyheri and Euclea undulata. Notable absences are Searsia lancea, Vachellia karroo and Senegalia mellifera subsp. detinens.

The Umbrella thorn, Vachellia tortilis subsp. heteracantha, after which this community is named, has many growth forms and is not only a tall tree with a flat crown. Shapes also vary from a straggly shrub or a short tree with a dense round crown to the characteristic umbrella-shaped tall tree. In this reserve the growth form is usually a straggly shrub or short tree. The Umbrella thorn is found in all communities within the reserve and is not restricted to Umbrella thorn savanna-woodland.

Umbrella thorn woodland variant

Areas with apparently more dense woody growth are mapped as the woodland variant. The soil is very shallow in these areas. Outcrops of Kanye Formation rocks and dykes are visible in many places. A good example is along Kukama Drive near the junction with Thutlwa Road where there is a dense concentration of dykes.

Species that are specific to this variant and not the savanna variant, include Ozoroa paniculosa, Senegalia hereroensis and Tarchonanthus camphoratus.

Senegalia hereroensis can be confused with Senegalia caffra that has similar drooping, large leaves. It has been established from distribution patterns and photographs that the individuals in the reserve are most likely S. hereroensis (pers. comm. Braam van Wyk 2021).

     

Typical scene in the woodland variant.

Woodland variant on dolerite dyke.

   

Ehretia alba; White puzzle-bush; Bergwitboom. This tall shrub is recognisable by its dark green foliage and dense branches.

Senegalia hereroensis; Mountain thorn; Bergdoring. This tree can be confused with Senegalia caffra, which has not been found in the Reserve but has smaller leaves which do not droop as much. It has the same hook spines.

Searsia leptodictya; Mountain karree; Bergkaree. Leaflets are wider than those of S. lancea, with coarsely serrated edges.

Tarchonanthus camphoratus; Camphor-bush; Kamferbos. This is another easily-recognised small tree. It has grey, velvety leaves which give off a strong aroma of camphor when crushed.

Vitex zeyheri; Silver fingerleaf; Vaalpypsteelboom. This is an easily-identified shrub with its grey 5-foliolate leaves. It often occurs with Tarchonanthus camphoratus, another grey-leaved shrub.

Umbrella thorn savanna variant

Areas with open grassland and scattered trees are mapped as the savanna variant. The soil is slightly deeper than in the woodland areas but remains shallow and rocky. The only woody species specific to this variant and not the woodland variant, is Carissa bispinosa. This species is also common in Black thorn scrub.

There are many good examples throughout Botsalano but look out for it along the Thutlwa Road near the turnoff to None Link.

 

Savanna variant. Most woody plants are Vachellia tortilis subsp. heteracantha.

Savanna variant with typical Vachellia tortilis subsp. heteracantha.

Savanna variant.

View across Savanna variant towards Black thorn scrub in valley.

Vachellia tortilis subsp. heteracantha; umbrella thorn; haak-en-steek. This is probably the commonest tree/shrub in the reserve. It is readily recognized by the presence of both straight and hook thorns, white globose inflorescences and the spirally twisted pods. Next four photographs are of the same species.

Globose inflorescences on left; lone tree in Savanna variant on right.

Mature specimens in a wide drainage line on left; typical curled pods on right.

Learn more about this Reserve

There is a comprehensive Tourist Guide in pdf format available here:

https://www.scribd.com/document/513783370/Botsalano-Visitor-Guide-v9

and a short video here:

https://youtu.be/B810bEr6WpM 

and a short description of the quantitative technique used:

https://www.scribd.com/document/513785209/Use-of-PCA-for-Vegetation-Mapping-Revisited-v3

The official website for the Reserve is: http://northwestparks.org.za/botsalano game reserve.html 

 An iNaturalist Group has been started for all records of plants and animals in the Reserve. Please add your sightings to this valuable source of information for tourists and scientists.

Acknowledgements

This compilation was not possible without reference to published and unpublished studies of the area. Information from Bosch (2011), Mucina & Rutherford (2006), Grobler (1996) and Grobler & Walraven (1993) formed the basis of the work. Help with species identification was provided by Braam van Wyk. Mike Panagos provided a copy of an unpublished thesis for reference. Dawid Mouton made valuable suggestions regarding the geology section.

The Reserve manager and staff are thanked for allowing me access to areas not open to the public and for supporting the work in various ways.

References

Bosch, AD (2011) The Vegetation Management of the Botsalano Game Reserve in the North West Province, South Africa. Unpublished Magister Technologiae: Nature Conservation thesis, Faculty of Science, Tshwane University of Technology, Pretoria.

Grobler, DF (1996) The geology, geochemistry and geochronology of the Gaborone Granite Suite and Kanye Formation north of Mafikeng, South Africa. Unpublished PhD thesis, Univ. Witwatersrand, Johannesburg, 459 pp.

Grobler, DF & Walraven, F (1993) Geochronology of Gaborone Granite Complex extensions in the area north of Mafikeng, South Africa. Chemical Geology (Isotope Geoscience Section) 105: 319-337, Elsevier Science Publishers BV, Amsterdam.

Michaluk, E & Moen, HFG (1991) The Geology of the Mafikeng Area. Explanation of Sheet 2524 Scale 1:250 000. Government Printer, Pretoria.

Morris, JW (2021) Use of PCA for vegetation surveys - revisited. Unpublished report.

Mucina, L & Rutherford MC (eds) 2006 The vegetation of South Africa, Lesotho and Swaziland. Strelitzia 19. South African National Biodiversity Institute, Pretoria.

Van Wyk, B & Van Wyk, P (2013) Field Guide to the Trees of Southern Africa. Struik Nature, Cape Town.

About the author:   Jeff Morris started his career in the 1960's as plant ecologist with the Botanical Research Institute (now National Biodiversity Institute). He spearheaded the introduction of quantitative techniques and computerisation in those early days. After some years he left the scientific field, moving to the corporate world and eventually started his own computer software house. He is now retired and lives in Pretoria. He enjoyed this citizen science exercise of getting back to sweaty and dusty field work, collecting plants for identification, interpreting satellite photographs and researching literature on the Reserve and surrounds.