T h o r n b u s h invas ion in a savanna ecosys tem in eastern Bot swana*

J. A. van Vegten K. Straatweg 135, 1851 BD Heiloo, The Netherlands

Keywords: Africa, Biomass assessment, Bush encroachment, Overgrazing, Regressive Succession, Savanna

Abstract

The paper describes the thornbush invasion (bush encroachment) found in the savanna at Olifants Drift in eastern Botswana. Overgrazing by cattle is responsible for the opening up of the grass sward and has enabled woody species to establish. A quantification of this process between 1950 and 1975 in an area of ca. 100 km 2 is given in terms of plant density as well as aboveground biomasL The loss of herbaceous biomass which generally accompanies bush encroachment and the possibilities for pasture restoration are mentioned.

Introduction

Destruction of the grass cover by overgrazing, mainly by domestic herbivores, is common in tropical and subtropical savannas (Walter, 1954, 1973; Walter & Volk, 1954). It is a clear example of man's detrimental exploitation of the environment and of the process that is popularly called bush encroachment: the transition from grassy to more shrubby ecosystems induced by overgrazing. Quan- titative data on the subject are scarce and refer mostly to its reversal, i.e. the effects of bush clearing (e.g., Lamotte, 1975; Philipps, 1974; Pratchett, 1978; Pratt , 1966; Trollope, 1980; Walker, 1979; Walker et al, 1981).

The present study attempts to provide some in- formation on the dynamics of bush encroachment, its speed and magnitude, quantified in space and time. In this case it concerns thornbush invasion between 1950 and 1975 around Olifants Drift, a settlement in eastern Botswana on the west bank of the Limpopo River.

* Species nomenclature follows the Flora of Southern Africa and the system in use at the herbarium of the Botanical Research Institute at Pretoria, South Africa.

Vegetatio 56, 3 7 (1983). © Dr W. Junk Publishers, The Hague. Printed in the Netherlands.

The study area

The area covers about 108 km 2 of undulating savanna land around Olifants Drift in Botswana. See Table 1 for climatic conditions, including the average annual water deficiency, i.e. the moisture required by vegetation which soil storage and pre- cipitation have failed to provide. From Table 1 it can be seen that the area appears to suffer from drought at least part of the year, usually during May - November.

Geology

The area is part of a slightly undulating plain west of the Marico River (which is called Limpopo River below its junction with the Crocodile River). Most of the relief represents a late Tertiary erosion surface which covers Precambrian rocks. A poly- cyclic Quaternary erosion surface runs immediately along the river and includes series of alluvial terra- ces (Mitchell, 1976). The Precambrian material consists of Basement Complex granites, granitoid gneisses and migmatites (Olifants Drift formation). In the southern part of the area faulting along a WNW-ESE axis has produced a graben, which is

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Table 1. Climatic conditions of the Olifants Drift area (after Schultze & McGee, 1978. and personal observations).

January mean daily temperature: July mean daily temperature: Nocturnal frost is common during

June- August Mean annual precipitation: Mean January precipitation: Mean July precipitation: Rainy season, i.e. months with

mean precipitation i> 50 mm:

Dry season, i.e. months with mean precipitation 0 25 mm and with total precipitation of the driest months 0.1-10.0 mm:

The rainfall is unevenly distributed during October - April

Relative air humidity, ! m above ground surface:

Average annual water surplus < 100 mm,, occurring during

December - April Average annual water deficiency:

22 24 ° C '12 14°C

400-600 mm 50-100 mm 0- 10 mm

December March

June - September

between 15% (winter minimum) and 85% (summer maximum)

400 600 mm.

filled in with Cave Sands tone , an aeo l ian sediment abou t 200.106 years old (Jones, 1959-1963).

Soil

Q u a t e r n a r y eros ion cycles, which spread the vir- tual ly un in te r rup ted mant le of desert sand over the Kga lagad i basin, fo rmed the present penepla in of which the s tudy a rea is a small par t . The surface consists of weakly deve loped soils derived f rom and over lying the geological fo rmat ions just ment ioned. These soils are fersiall i t ic yel low and gray sands and sandy loams. Al luvia l soils const i tu te a minor com- ponent of the area. These are deve loped on weakly conso l ida ted depos i t s and show a very low degree of genetic hor izon di f ferent ia t ion. Thei r d ra inage general ly is poor (Mitchel l , 1976).

Vegetation

The vegeta t ion is a shrub savanna wi thout large trees or open grassy stretches, typical for wide areas

under s imilar cl imatic arid soil condi t ions (Werger & Coetzee, 1978). The herb layer is d o m i n a t e d by grasses. Eragrostis rigidior is widespread and dom- inates the vegeta t ion as a whole, while Aristida congesta and Schmidtia pappophoroides occur f requent ly . The fol lowing grasses have a more pa tchy d is t r ibut ion: Urochloa trichopus, E. leh- manniana, A. stipitata, Chloris virgata, Cynodon dactylon and Sporobolus pyramidalis. The most c o m m o n forbs are Tribulus terrestris, Acanthos- permum hispidum, Xanthium spinosum, Pavetta harboriL Dicerocarium zanguebarium, Solanum incanum, Sesbania bispinosa, Asparagus spp., Hib- iscus meeusii and Alectra vogelii.

The shrub low tree layer is domina t ed by Legu- minosae, par t i cu la r ly Acacia tortilis, A. erubes- cens, A. mellifera, A. fleckii and Dichrostachys cinerea. Of less impor t ance are A. nilotica, Grewia flava and G. retinervis. Still less c o m m o n and with a more pa tchy d i s t r ibu t ion are, in o rder of impor- tance, Terminalia sericea, Combretum apiculatum, Peltophorum africanum, Maytenus heterophylla, Ziziphus mucronata, Croton gratissimus, Boscia albitrunca, B. foetida and Euclea undulata.

Fauna

The indigenous wildlife popu l a t i on has been re- duced to v i r tua l ly zero. Only in the thickets a long the bo rde r river kudu (Tragelaphus strepsiceros) and impa la (Aepyceros melampus) are seen occa- sionally. These ante lopes general ly live on the east bank where they find relat ive peace on the ranches of T ransvaa l fa rmers who offer p ro tec t ion to the animals . I t is the dense popu la t ion of l ivestock animals , pa r t i cu la r ly cattle, which has a s t rong im- pac t on the ecosystem. The a rea has been es t imated to be grazed by twice as much catt le as would be a l lowed in o rder to ma in ta in its present vege ta t ion (Sandford , 1977).

Fire and grazing

Est imates are that a r e s to ra t ion of the fo rmer grass savanna would require debushing and some decades of very light s tocking at abou t 15% of the present s tock ing rate, combined with a d rop in savanna burn ing f requency f rom the present once every 1-3 years to once every 5-8 years.

The area conta ined abou t 750 inhabi tants in 1975

and 900 in 1980 (Dept. of Town and Regional Planning, 1980). The main way of life of these peo- ple is livestock rearing on communal pastures. The animals are kept in enclosures overnight and are free to roam at will between their enclosure and waterpoint whilst grazing during the day. Herding is no general practice and animals are not guided into particular areas, but are just accompanied by one or more men. Crop cultivation is uncommon, so that the principle influence on the vegetation is grazing by domestic stock and with less impact the frequent, man-induced bush (grass) fires. These fires are never very intense or widespread, because continuous overgrazing precludes the accumula- tion of sufficient fuel for massive fires. The effect of wood harvesting and bush clearing for crop fields and homesteads is insignificant due to the low pop- ulation density. It is emphasized here that in this paper only net biomass increment and production will be discussed.

Materials and methods

In order to assess the thornbush encroachment through time the canopy cover of the woody vegeta- tion was mapped from aerial photographies taken in June 1950, August 1963 and June 1975. With the aid of transparent grid sheets it was possible to distinguish 8 canopy cover classes, namely: canopy cover0%(ofgroundsur face) ,0 i, ! -5 ,5 10, 10-30, 30-50, 50-75 and 75-100%.

In order to relate canopy cover to aboveground biomass a 2-step procedure was adopted. Firstly, after initial vegetation surveys had revealed that over 95% of the shrub-tree stratum was constituted by the 5 species listed in Table 2, the relation height biomass was established by selecting at random 448 shrub/ t ree specimens (in Table 2 the number of

Table 2. The numbers of thornbush specimens measured, cut and weighed.

Acacia tortilis 92 A. erubescens 79 A. Jleckii 84 A. mell([~'ra 79 Dichrostachys cinerea 114

Total 448

specimens of each species is given), measuring their height and, after cutting them at ground level, weighing them (fresh biomass). Scandent and cop- piced individuals were taken to be one specimen whenever all aerial parts originated from one stem, stump or (underground) growth center. This as- sessment was conducted during dry season months. Secondly, 30 squares of 50 X 50 m 2 were selected at random in each of the canopy cover classes, disre- garding the 0% class. Within each square the total aboveground woody biomass was estimated, using the height biomass relation, again during dry sea- son months. The average biomass of each group of 30 squares was considered stochastically represen- tative for its particular canopy cover class (after Golley, 1978).

Results

The height biomass correlation obtained from the 448 cut specimens is shown in Figure 1. The relation canopy cover class average aboveground woody biomass, as deduced according to Golley (1978) is shown in Table 3, as well as the surface areas covered by each canopy cover class in 1950, 1963 and 1975. From these data the increase in total aboveground woody biomass of the Olifants Drift area between 1950 and 1975 can be derived (Table 4).

In height (cm)

i t 0 0

o i

o A A A A

In mass (kg)

Fig. 1. The correlation between height and biomass (natural logarithms) of the 448 cut specimens. The correlation coeffi- cients are 0.92 (Dichrostachys) and 0.94 (Acacia, all species). Frequencies have been grouped and their means are given as circular (Dichrostachy.£) and triangular (Acacia) symbols.

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Table 3. The relation canopy cover class-woody biomass and the surface areas covered by each canopy cover class in proport ion to the total study area in 1950, 1963 and 1975.

Canopy cover Aboveground woody biomass Surface area covered by each canopy class (fresh mass) in kg. ha I (cover class given as % of total area) (%)

Average Range 1950 1963 1975

0 0 7.6 6.1 5.7 0-1 59 32 98 41.4 23.8 3.0 1-5 2 050 521- 3 038 38.1 40.2 29.6 5 10 2860 440- 4 170 7.1 10.1 28.2

10 30 3 740 1 280- 6 820 3.6 12.0 13.6 30 50 5820 2610 9160 1.3 6.0 7.2 50-75 9 860 3 290-22 400 0.5 0.6 11.6 75-100 21 210 6420 37840 0.4 1.2 1.1

Table 4. The estimated total aboveground woody biomass in the Olifants Drift area in 1950, 1963 and 1975, and the derived net increment and production.

Estimated total aboveground woody biomass 1950:14 705 649 kg 1963:24 883 803 kg 1975:39 030 581 kg

This represents an average net biomass increase o~ Average biomass Increase 1950:1362 kg .ha l

1963:2304 kg .ha -I 942 kg .ha I 2252 kg .ha j 1975:3614 kg .ha ] 1310 kg .ha

Or expressed as average net production: 1950 72.5 kg .ha J .y r ~ 1963 1975 109 kg .ha I.yr I

Discussion

It appears from Table 3 that between 1950 and 1975 the surface area with relatively little woody biomass has decreased strongly while that with a denser woody covering has increased. The surface area with a canopy cover of 1-5% expanded initial- ly by an increase of woody growth in open, grassy terrain. After 1963 progressive woody growth turn- ed a considerable proportion of this savanna type into that of a higher canopy cover class, thereby decreasing its surface area. At the same time insuf- ficient area of lower canopy cover developed into 1-5% canopy cover terrain to maintain or increase the latter's surface area, because not sufficient en-

tirely open savanna was left by that time. The con- tinuous increase of 5-10% canopy cover savanna is a reflection of this development, i.e. a large propor- tion of 1-5% terrain turning into more wooded savanna. The aerial photographs showed this clear- ly.

The development of the 10-30% and 30-50% savanna categories is less pronounced. Their in- crement in woody vegetation is asymptotic after 1963. This might be partly related to the rapid area increase of 50-75% cover class terrain between 1963 and 1975. This increase demonstrates the inade- quacy of having only 3 snapshots from which to consider a period of 25 years and thus being unable to deduct at which speed a certain canopy cover savanna develops into one of a higher order. Aerial photographs showed certain parts of the study area to develop from 5-10% savanna into 30-50% and even more within 12-13 years.

The changes within the 75-100% class are rela- tively minor, and so are those within the 0% class and the 50-75% class between 1950 and 1963. This suggests that the maximum potential shrub bio- mass is not uniformly distributed throughout the study area, but rather varies with the edaphic and hydrologic characteristics of the different spots of the area. This could not be studied in detail, howev- er.

It appears that within a relatively brief time in- terval bush encroachment can considerably change a landscape such as the Olifants Drift area; it almost tripled its total woody biomass within 25 years. Since the related loss of herbage biomass could not be included in this study, this is hardly the place to

discuss this point. With reference to Donaldson & Kelk (1970) and APRU (1979), who showed a nega- tive linear correlation between biomass values of woody plants and herbage in the same area, it should suffice to conclude here that this loss is substantial.

References

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Accepted 2 I. 10.1983.