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127 B.J. STROHBACH * Vegetation degradation in Namibia Abstract Much concern is raised on issues like desertification (see the UN Convention on Desertification) and the conservation of biological diversity (see the Convention on Biological Diversity). In the Namibian constitution a paragraph is included relating to the conservation and sustainable use of natural resources (Article 95(l)). "Deforestation" and "bush encroachment" are the two most mentioned environmental problems facing the communal and commercial farming sectors respectively. Little is known about the processes of degradation. In this paper some examples of vegetation degradation are given. In the northern Oshikoto region, degradation is the result of a general overexploitation of the natural resources: overgrazing as well as deforestation. In the central parts of the country, with commercial cattle farming as main agricultural practice, overgrazing leads to a reduction of the grass cover and with it an increase in woody plants (“bush encroachment”) but also increased erosion. With these examples in mind, the need for long-term monitoring projects is discussed. Zusammenfassung Weltweit bestehen Bedenken über Desertifikation (siehe die UN Convention on Desertification) und den Behalt biologischer Diversität (siehe die Convention on Biological Diversity). In der Namibischen Verfassung besteht ein Artikel, nach dem der Schutz der Natur und die nachhaltige Nutzung der natürlichen Resourcen garantiert werden soll (Artikel 95(l)). Rodung („deforestation“) und Verbuschung werden derzeit als die größten Probleme angesehen, mit denen kommunale und kommerzielle Farmer des Landes zu schaffen haben. Wenig ist allerdings bekannt über die Prozesse der Degradation der Vegetation. In diesem Artikel werden zwei Beispiele der Vegetations-degradation erörtert. In der nördlichen Oshikotoregion findet Degradation statt wegen einer allgemeinen Übernutzung der Vegetation – sowohl eine Übernutzung der Forstprodukte als auch generelle Überweidung. In den zentralen Teilen des Landes, wo kommerzielle Viehhaltung der wichtigste Wirtschafts- zweig der Farmerei ist, führt Überbeweidung zu einer reduzierten Grasbedeckung und damit zu einer Zunahme holziger Pflanzen („Verbuschung“), als auch zu erhöhten Erosion. Anhand dieser Beispiele wird der Bedarf für Dauerbeobachtungen diskutiert. * National Botanical Research Institute, P/Bag 13184, Windhoek JOURNAL 48:127-156 Namibia Wissensschaftliche Gesellschaft / Namibia Scientific Society Windhoek, Namibia 2001 ISSN: 1018-7677

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127

B.J. STROHBACH*

Vegetation degradation in Namibia

Abstract

Much concern is raised on issues like desertification (see the UN Convention on Desertification) and the conservation of biological diversity (see the Convention on Biological Diversity). In the Namibian constitution a paragraph is included relating to the conservation and sustainable use of natural resources (Article 95(l)). "Deforestation" and "bush encroachment" are the two most mentioned environmental problems facing the communal and commercial farming sectors respectively. Little is known about the processes of degradation.

In this paper some examples of vegetation degradation are given. In the northern Oshikoto region, degradation is the result of a general overexploitation of the natural resources: overgrazing as well as deforestation. In the central parts of the country, with commercial cattle farming as main agricultural practice, overgrazing leads to a reduction of the grass cover and with it an increase in woody plants (“bush encroachment”) but also increased erosion.

With these examples in mind, the need for long-term monitoring projects is discussed.

Zusammenfassung

Weltweit bestehen Bedenken über Desertifikation (siehe die UN Convention on Desertification) und den Behalt biologischer Diversität (siehe die Convention on Biological Diversity). In der Namibischen Verfassung besteht ein Artikel, nach dem der Schutz der Natur und die nachhaltige Nutzung der natürlichen Resourcen garantiert werden soll (Artikel 95(l)). Rodung („deforestation“) und Verbuschung werden derzeit als die größten Probleme angesehen, mit denen kommunale und kommerzielle Farmer des Landes zu schaffen haben. Wenig ist allerdings bekannt über die Prozesse der Degradation der Vegetation.

In diesem Artikel werden zwei Beispiele der Vegetations-degradation erörtert. In der nördlichen Oshikotoregion findet Degradation statt wegen einer allgemeinen Übernutzung der Vegetation – sowohl eine Übernutzung der Forstprodukte als auch generelle Überweidung. In den zentralen Teilen des Landes, wo kommerzielle Viehhaltung der wichtigste Wirtschafts-zweig der Farmerei ist, führt Überbeweidung zu einer reduzierten Grasbedeckung und damit zu einer Zunahme holziger Pflanzen („Verbuschung“), als auch zu erhöhten Erosion.

Anhand dieser Beispiele wird der Bedarf für Dauerbeobachtungen diskutiert.

* National Botanical Research Institute, P/Bag 13184, Windhoek

JOURNAL 48:127-156 Namibia Wissensschaftliche Gesellschaft / Namibia Scientific Society

Windhoek, Namibia 2001 ISSN: 1018-7677

Vegetation degradation in Namibia 128

Introduction

Desertification is a mayor threat to sustainable land use in many parts of the world. Because of this, an international convention has been set up by the United Nations, trying to get to grips with the problem. Global warming is seen internationally as a possible cause for anomalies in the weather, with associated storms, flooding but also more severe droughts globally. This in itself is also a possible cause of desertification, however in most cases desertification is seen as caused by the land user himself (Booysen & Rowswell 1983, UNCCD 2000). Also of concern is the conservation of biological diversity. This, as well as the use of the diversity of species, was discussed and agreed upon at the Rio “Earth Summit” (the so-called CBD or Convention on Biological Diversity) (CBD 1998).

The sustainable use of our natural resources is a prime objective in agricultural development in Namibia. This objective is entrenched in the Constitution of the Republic of Namibia in paragraph 95(l), stating: “The State shall actively promote and maintain the welfare of the people by adopting, inter alia, policies aimed at ... the maintenance of ecosystems, essential biological processes and diversity of Namibia and utilization of living natural resources on a sustainable basis for the benefit for all Namibians, both present and future, ...”.

But how do we fare? How sustainable are our land-use practises? Or conversely, how bad is land degradation in Namibia?

Area-wise, we have three mayor land-use systems within Namibia: Commercial farming land covers 45 % of the land surface, communal farming is practised on 40 % of the land surface, whilst 15 % are managed by the government as nature reserves (van der Merwe 1983). The debate continues over the sustainability of these land uses; it is often assumed that commercial farming is more conservation-oriented whilst communal farming is assumed “destructive” (the “tragedy of the commons”).

However, degradation is prevailing in both commercial and communal farming areas. It is not the purpose of this paper to give a full report on the degree of degradation in the country (that is the purpose of the ‘State of the Environment’ reports of the Minister of Environment and Tourism (NRC-SPC-NEPRU Consortium 1999)). It is also not the purpose of this paper to define the underlying reasons for land-use practises resulting in land degradation, or to provide solution for these problems. It has long been realised that the primary causes of land degradation are of socio-economic origin, and any solutions will also have to be of socio-economic nature (Thomas 1997). This paper rather aims at giving some examples of degradation related to extensive stock farming, as practised in both the communal and commercial farming areas.

Example 1: Degradation in the communal grazing areas in the northern Oshikoto area1

The northern Oshikoto region (the south-western part of former Ovamboland) between Oshivelo and Ondangwa is dominated by 3 mayor landforms: To the south are the salt plains of the Etosha pan (the Andoni Plains as described by Cunningham et al. (1992) and le Roux (1980)), to the west the Oshana-system of the Cuvelai Delta with its Hyphaene petersiana plains and to the north and east the Broad-Leafed savannas of the northern Kalahari (including the Mangetti Block) (Cunningham et al. 1992). Between these mayor land types two broad bands of transitional vegetation (i.e. ecotons) occur: The Terminalia pruinoides woodlands occurs to the south between the Broad-leaved savanna and the Andoni Plains, whilst to the west

1 These examples are taken from Strohbach (1999).

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the Broad-leaved savanna is replaced by the Colophospermum mopane shrublands before the Hyphaene petersiana plains of the Cuvelai delta starts (Cunningham et al. 1992; Strohbach 1999).

The population in the ‘central North’ is concentrated in the Oshana system due to primarily the availability of water in the oshana’s and secondary the more nutrient-rich soils. With the development of the main road between Oshivelo and Ondangwa, settlement took a linear dimension along this road (Lund Consulting Engineers 1998; Lund Consulting Engineers et al. 1999). The ‘hinterland’ (the Andoni plains to the south and the Broad-leafed savanna to the north) was used as grazing land especially in the rainy season then water is readily available.

However, population growth (due to various socio-economic and political factors) meant that land became a scarce resource in the central oshana's. People started to settle further to the inland in the grazing areas. Water was always a problem, with only poor quality water being found in wells. When a rural water supply scheme was eventually planned, fears were raised that this will result in increased settlement in these previously unused areas (Mr F. Kuchling, personal communication2).

The communal farmers are typical ‘agro-sylvo-pastoralists’, meaning that their land use practices involve cropping, animal husbandry as well as the use of forestry products. This means that lands need to be cleared, whilst wood is collected for building material and firewood. Furthermore, various other forest products are collected and the area is also used for grazing. Degradation is thus not only related to overgrazing, but rather to a general over-utilisation of the natural resources.

In terms of the mechanistics, each vegetation type displays its own unique degradation trends.

The Hyphaene petersiana plains

The Hyphaene petersiana plains are characteristic of the eastern Oshana plains between Ondangwa and Okatope. These plains are fairly densly populated and degraded to a stable secondary succession state (Arcadis Euroconsult 1998). Hyphaene petersiana, the makalani palm, together with the shrubs Acacia arenaria, Acacia hebeclada subsp. hebeclada and A.

hebeclada subsp. tristris, and the grasses Wilkommia sarmentosa, Sporobolus ioclados and Eragrostis trichophora are characteristic. Typically the vegetation is a shrubland, but can vary between an open woodland (seldom), a bushland, or in degraded situations, a grassland (structure definitions after Edwards 1983). Figure 1 depicts a typical, degraded example of the Hyphaene petersiana plains.

In few isolated, protected areas (e.g. graveyards - see Cunningham et al. 1992 and Figure 2) some other trees and shrubs can be found, especially Terminalia sericea, Diospyros lycioides,

Elephantorrhiza suffruticosa and the grass Odyssea paucinervis. Due to widespread overutilisation, these species have disappeared in most cases from the landscape. Also occasionally found are some large trees of Sclerocarya birrea subsp. caffra and of Schinziophyton rautanenii.

Soil compacting and sheet erosion is common, indicating the generally resource-poor and degraded condition of these plains.

2 Mr F. Kuchling, Lund Consulting Engineers, P.O. Box 3106, Windhoek, Namibia.

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Figure 2: The graveyard at Onyaanya could serve as an example of the Hyphaene petersiana plains in conserved state. This relevé (87059) is 200 m from the relevé above.

Figure 1: Typical example of degraded Hyphaene petersiana plains at Onyaanya (relevé 87061).

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The Colophospermum mopane shrublands

From Okatope east- and northwards the Hyphaene petersiana plains are replaced by Colophospermum mopane shrublands. These are characterised by a fairly dense stand of Colophospermum mopane, or the Mopane, as tree (seldom) or shrub. These shrublands are basically the outer fringe of the Oshana system, the KAL9-4 AEZ (de Pauw et al. 1998/99). They occur in the far western parts of the study area, with patches occurring along the main road in the Terminalia pruinoides woodlands. To the north and north-east (towards Okankolo) this shrubveld is gradually replaced by broad-leafed savannas, whilst to the south-west, the vegetation is replaced by the Terminalia pruinoides woodlands. This transition is very patchy in nature. Occurring typically in these shrublands are the shrubs Croton gratissimus , Mundulea

sericea, Grewia flavescens and G. bicolor, and the grasses Stipagrostis uniplumis, Aristida

stipioides and Eragrostis trichophora. The structure is typically a high closed shrubland due to extensive wood harvesting and the strong coppicing ability of Colophospermum mopane

(Figure 3). Although not confirmed, these shrublands could develop into a high closed woodland if protected. In extreme cases of wood harvesting these shrublands turn into short open shrublands, sometimes even low sparse shrublands.

Degradation dynamics of the Colophospermum mopane shrublands.

Degradation has been studied in this vegetation type utilising the degradation gradient analysis technique of Bosch & Janse van Rensburg (1987) and Bosch et al. (1987), also applied by Strohbach (1992) and Joubert (1997). The degradation gradient is a mathematical abstraction of the change in species composition, not representing the actual type or intensity of the degrading factor.

Figure 3: Colophospermum mopane shrubland in fairly good condition. Relevé 87159 ca. 3.4 km south of Onanakali towards Ambende.

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The cover of trees in this vegetation type is generally low, and seldom reaches 10 % (Figure 4). This can be attributed to the fact that most of the trees have been cut down for poles and timber, which means that the remaining vegetation consists of coppicing shrubs only. Similarly, the shrub cover drops from an average 60 % cover to less than 20 % from “good” to “poor”. The fact that the shrub cover picks up again towards the extreme end of the gradient can be attributed to the fact that Pechuel-Loeschea leubnitziae invades the cleared, trampled areas (Figures 5). Although this shrublet is perennial, it is to be seen as a weed, and produces no usable wood at all. It is also not palatable as browse and has been reported to taint the taste of milk and meat (Wells et al. 1986). The vegetation height indicates the change very clearly: from an average height of about 5 m (with the possibility of reaching 8 m), the vegetation height drops to about 2 m under poor conditions and to about 1 m under extreme conditions.

Figure 4: Tree and shrub cover at various stages of degradation in the Colophosperm mopane shrubland.

Figure 5: Extreme degradation in Colophospermum mopane shrubland. Relevé 87163 is ca 0.9 km south of Onankali and is a trampled patch between 3 fields. Note that the remaining shrub is Pechuel-Loeschea leubnitziae.

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A common practice in this vegetation type seems to be the cultivation and re-harvesting of coppice. A striking example of this practice has been seen on relevé 87114 (Figure 6). The owner of this patch has removed about three quarters of the coppice, with only one or two coppice stems remaining per plant. Yet the wood demand was not satisfied; wood harvesting went on outside the fenced plot at the time of surveying.

Figure 6: Silviculture: coppice of Colophospermum mopane has been harvested. Only few coppice stems remain per plant (relevé 87114).

Figure 7: Grass and forb cover at various stages of degradation in the Colophosperm mopane shrubland.

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The response of the annual and perennial grasses, as well as that of forbs, to degradation, is shown in Figure 7. Perennial grasses only occur in the Colophospermum mopane shrubland in good condition. The main species are Stipagrostis uniplumis and Eragrostis trichophora. More seldom found are Schmidtia pappophoroides and Sporobolus fimbriatus. The perennial grass cover never exceeds 5 %.

Annual grasses are Aristida stipioides, which seems to be fairly unpalatable and also produced very little leaf mass, Urochloa brachyura, a highly palatable and valuable fodder grass, and Schmidtia kalihariensis, which becomes palatable as it dries out. The highest annual grass cover was observed in the better veld. Annual grass cover does not exceed 12 %.

A common problem with all annual plants is the nature of their life-cycle: being an annual plant, little is invested in rooting and/or leaf material production other than necessary to produce flowers and subsequently seeds. After seed setting, the life cycle is completed and the plant dies. These dry remains are soon uprooted by grazing animals and, if not eaten, trampled or blown away. A plain covered by annual plants with little or no perennial (woody) plant cover turns into a dust-bowl by late winter after the onset of the dry season. The soil is unprotected against wind erosion until the onset of the next rainy season in December / January.

The Broad-leafed savannas

Typical are the trees Terminalia sericea, Combretum collinum, Lonchocarpus nelsii, Burkea

africana and Acacia fleckii and the shrubs Combretum engleri, Acacia ataxacantha, Bauhinia

petersiana, Ozoroa schinzii, Grewia flava, G. flavescens and G. bicolor and Commiphora

angolensis, C. africana and C. glandulosa. In the KAL 8 (Omuramba-Dune association) (de Pauw et al. 1998/99) north of King Kauluma school some Baikiaea plurijuga was encountered on a dune. Although this popular timber species had only a DBH of 20 cm (thus far from exploitable), some of these trees were found chopped down in this remote area. In the north-eastern cattle post areas some Baphia massaiensis was found.

Typical perennial grass species are Stipagrostis uniplumis, Anthephora pubescens, Brachiaria

nigropedata, Digitaria seriata, Schmidtia pappophoroides and Eragrostis trichophora. Rare perennial species occurring here are Panicum kalaharense and Aristida meridionalis. The annual Aristida stipitata which is often dominant in the commercial farming areas in similar sands, was found twice only - at very low cover. Megaloprotachne albescens (a palatable annual climax grass) was found in a few relevés in the cattle post areas. Common annual grasses are Urochloa brachyura and Schmidtia kalihariensis. The latter is typical for trampled areas.

Degradation dynamics of the Broad-leafed savannas

The height of the vegetation is highly variable, with many of the relevés in better condition consisting of a shrubland rather than a bush- or woodland. This is especially true in the Mangetti east of Oshivelo, where in many places the vegetation is only 3 m high. Yet some of the relevés in the cattle post area to the far north-east of the study area were actual bushlands, with the tree layer reaching between 12 and 15 m height (Figure 8). Another feature of this vegetation type is the fact that in most cases of severe degradation, a few trees remain standing. Why these trees are not harvested, is not clear.

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In the KAL4 (de Pauw et al. 1998/99) (i.e. the north-west around Onyuulaye - Okankolo), a number of pans occur. These pans are used for planting, and are generally fenced. Around such a fenced pan (about 2 km in diameter) a fringe of roughly 150 m is cleared. In these cases the coppicing is only by Combretum collinum, reaching a height of between 1.5 and 2 m. Combretum collinum however does not coppice as strongly as Colophospermum mopane - it does not form long straight coppice branches which can be reharvested, but seems rather to remain a shrub.

The reduction in shrub cover (Figure 9) is far more clear-cut. The average shrub cover of 55 % drops to less than 10 % in the extreme degraded situations. Again, with a few exceptions, tree cover never reaches more than 10 %.

The woody biomass of Combretum collinum can be estimated as being as much as 1.2 t/ha under mediocre conditions, based on figures from Rutherford (1975). This figure drops to less than 0.4 t/ha under poor conditions (under the assumption that Combretum collinum is still a small tree at that stage of degradation). However, C. collinum being only a coppice shrub at that stage of degradation, virtually no harvestable wood is available. Fencing material is propably the most important sink of wood products in this veld type (Figures 10, 11 & 12).

Predictably, the perennial grasses react as typical climax or “decreaser” grasses (see Bosch & Janse van Rensburg 1987) (Figure 13). They reach their maximum abundance of about 12 % perennial grass cover under good conditions, and decrease to virtually zero cover under poor conditions.

The most drastic trend is shown by the annual grass species. These species, especially Schmidtia kalihariensis, reach cover values of over 70 % under poor conditions. Again, the danger here is that such annual-covered areas (see Figure 14) with little or no woody cover can turn into dustbowls by the onset of the dry season.

Figure 8: Broad-leafed savanna in good condition. Relevé 87096 in the north-eastern cattle-post area.

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An interesting feature of the trendline of the annual species is the fact that it does form a second peak under good conditions. This can be attributed to Megaloprotachne albescens, an annual climax grass which occurred on a few relevés in the cattle post area.

Figure 9: Tree and shrub cover at various stages of degradation in the Broad-leafed savanna.

Figure 10: Typical brush-fence at relevé 87131.

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Figure 11: A 4-wire fence is being strengthened with droppers. Average dropper spacing: 20 cm. North of Onankali near relevé 87121.

Figure 12: A gate to a homestead north-east of Okankolo consists of numerous poles leaned against a cross-member.

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Figure 13: Grass and forb cover at various stages of degradation in the Broad-leafed savanna.

Figure 14: Broad-leafed savanna in poor condition north of Onyuulaye (relevé 87128). Note what a few trees do remain.

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Roughly 20 % less species occur in this veld type in poor condition than in good condition. However, this trend is misleading in the sense that it does not account for the replacing of (more numerous) climax species with (fewer) subclimax and pioneer species. The loss of diversity from any veld/vegetation reduces the ability of that veld to regenerate to its original state after the removal of the “degrading” factor. This is due to the fact that the variety (both in species as well as in live strategies/adaptations) has been lost from the soil seed bank. It has been shown by O’Connor & Pickett (1992) that overgrazing reduces the ability of perennial grasses to produce seeds, and in that way reduces the regeneration ability of such species. This leads to a vicious circle leading to the extinction of such grasses from the veld type. Short-lived grasses however produced seeds profusely, in that way ensuring continued survival and in the end dominance.

The Terminalia pruinoides woodlands

The Terminalia pruinoides woodlands occur on more loamy (brackish?) soils in the KAL3-3 (de Pauw et al. 1998/99). It is essentially a transition zone between the Broad-leafed savannas on the deep sands in the north and the Andoni plains with its saline soils. To note: whilst the transition to the Andoni plains is abrupt, the transition to the Broad-leafed savanna is often gradual. The Terminalia pruinoides woodlands do occur in patches also in the Mangetti area.

Terminalia pruinoides and Albizia anthelmintica are characteristic trees occurring in this veld type, together with Lonchocarpus nelsii. Shrubs include Acacia mellifera subsp. detinens, Acacia nilotica, Croton gratissimus, Dichrostachys cinerea, Commiphora glandulosa and C.

africana, as well as Grewia flava, G. bicolor and G. flavescens. Some common dwarfshrubs are Hibiscus elliotiae, Rhus tenuinervis and Gossypium herbacium. Perennial grasses are Stipagrostis uniplumis and Eragrostis trichophora. The annual grasses are far more prominent, including Urochloa brachyura and Anthephora schinzii.

In the natural state the Terminalia pruinoides woodlands form a tall closed woodland (Figure 15). However, as these woodlands occur in the more densely populated areas along the main road near Omuthyia, they are subject to some severe wood harvesting. In this way the vegetation is reduced to an open to sparse woodland, sometimes even a shrubland.

Degradation dynamics of the Terminalia pruinoides woodlands

As was the case with the previous veld types, the vegetation height, tree cover and shrub cover reduces drastically as degradation progresses. The average height of the vegetation, being about 12 m (up to 15 m) under good conditions, drops to about 2 m, whilst the tree cover drops to zero. The shrub cover drops from well over 40 % to less than 10 % (Figures 16 & 17).

Terminalia pruinoides is very poor in its coppicing ability: a stump which has been cut down at ca 1 m height will form some 10 cm long sprouts with a dense leaf-mass, but will never grow out long, strong branches (Figure 18). Such a stump will eventually die. Terminalia pruinoides

shrubs (young plants) were seldom seen, meaning that regeneration is low.

The change in grass and forb cover over the degradation gradient is shown in Figure 19. The cover of perennial grasses drops from 30 % under good conditions to near zero over the degradation gradient. The annual grass cover shows an expected peak of roughly 15 % under mediocre conditions, although under these conditions the cover of annual grasses could be as high as 70 %. Typically Anthephora schinzii dominates the annual grass cover. According to Müller (1983), this species is well utilised by animals as it is soft and palatable. The cover of these species drops to less than 5 % under poor conditions.

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Figure 15: Terminalia pruinoides woodlands in fairly good condition in the cattle post area north of the King Kauluma school. Note the fairly dense undergrowth (relevé 87098).

Figure 16: Tree and shrub cover at various stages of degradation in the Terminalia pruinoides woodlands.

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Figure 17: Vegetation height at various stages of degradation in the Terminalia pruinoides woodlands.

Figure 18: The stump in the foreground right has been a Terminalia pruinoides tree which was chopped down at ca 50 cm. Note the little regrowth on top. This stump will eventually die (relevé 87159).

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The biodiversity decreases from an average 26 species per relevé to about 20 species per relevé as degradation progresses. Interestingly enough, species numbers vary widely, with some of the relevés in better condition showing numbers of only 20 species per relevé. This can be due to the fact that the closed woodlands (in good condition) allow only a limited development of the herb layer due to shading. Conversely, species numbers can reach more than 30 per relevé under good conditions, whilst species numbers under poor conditions never reach more than 25 species per relevé.

Example 2: Degradation on the commercial cattle farms in central Namibia

The aim of commercial farming in Namibia is primarily producing agricultural produce on an economically sound basis, rather than subsistence farming. The resources are different - amongst others more land and better access to capital are available to these farmers. This means that the farmers are dependent on the natural resources for their production system only, not their complete livelihood (i.e. cooking fuel, building material, food, etc.). Farming practises are then strongly commodity-linked like cattle, sheep or goats, wildlife, or, in a few cases, even agronomy. Mostly the farmer concentrates on one (or only a few) commodities like cattle farming.

Bush encroachment

Cattle (and many game species in central Namibia) are grazers, meaning that they feed predominantly on grasses. On a cattle farm this leads to the possible over-exploitation of this layer of the vegetation, with a general under-utilisation of the woody component of the vegetation.

Figure 19: Grass and forb cover at various stages of degradation in the Terminalia pruinoides woodlands.

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It is generally accepted that although numerous factors contribute to bush encroachment, the primary cause is a disturbance of the competitive balance for especially soil moisture between grasses and woody plants (Smit et al. 1996). This was already suggested by Walter (1971) and is also supported by Knoop & Walker (1985).

In short: a dense stand of grasses, being fast growers and very efficient energy converters, is able to take up and utilise most of the rainwater as it seeps into the ground. Thus very little water actually reaches the deeper soil layers where many woody plants have their roots (Timberlake & Calvert 1993).

Grasses are well adapted to the erratic climate conditions in southern Africa: annual grasses can produce seeds within a short period of time (some within a fortnight!), specialising also in producing large numbers of seeds. Perennial grasses on the other hand invest in a strong root system. With this root system these grasses are able to survive dry years very well (O'Connor 1991a; O'Connor 1994; O'Connor 1995).

Similarly, grasses are also well adapted to survive grazing. However, the combination of heavy grazing and a drought has been proven to be fatal to these perennial grasses (O'Connor 1991a; O'Connor 1995). Thus a fairly dense perennial grass cover can easily be destroyed by overgrazing combined with drought.

As soon as the grass cover is reduced, two things happen:

i. Less water is used by the grasses, so more of the infiltrating water reaches the deeper soil layers and thus the roots of the woody plants. These then can establish and grow faster.

ii. Decreased plant cover, especially grass cover, will lead to increased soil erosion and soil compaction, in itself leading also to more run-off (Stocking 1988; Lal 1994; Strohbach 2000a). This leads to a reduced ability of grass seedlings to establish (O'Connor 1991b; O'Connor 1996), leading again to a further reduction in the grass cover.

Trees and shrubs are generally not affected by the reduced infiltration rate after the initial establishment boost. This is due to the funnel effect - they collect water in their branches and funnel this along the trunk into the soil and through channelling right down to their root level.

Bester (1998/99) gives an overview of the state of bush encroachment in Namibia, whilst Coetzee (1999) and Strohbach (2000a) give an overview of the erosion status in the country.

Okahandja district

Relevé data has been collected in 1956 by Prof. O.H. Volk on a farm in the Okahandja district. With fairly accurate locality description, this data allowed comparison with the present day situation (Strohbach & Sheuyange 1999). For the purpose of this paper, only 3 representative plots are compared:

Volk Plot 720

Volk’s Plot 720 was initially surveyed on 12 March 1956, and resurveyed on 14 April 1999. This plot represents the typical sandy plains of the CPL2 (de Pauw et al. 1998/99) in the Okahandja district. No drastic change in shrub composition can be detected from the given abundance ratings, although Volk states “the view is unobstructed” (i.e. little shrub). This cannot be said today. The drastic change is in the grass cover: Volk did not observe any annual grass species, whereas he estimated the perennial grass cover to be near 100 %. At present the

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perennial grass cover is less than 20 %, with an annual grass cover of about 15 % (Figures 20 and 21).

Figure 20: Summarised vegetation composition of Volk plot 720.

Figure 21: Photo of Volk plot 720 taken on 14 April 1999.

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Volk Plot 739

Volk Plot 739 has initially been surveyed on 15 March 1956, and resurveyed on 15 April 1999. The plot is situated on alluvial soil (highly erodable) next to the banks of a river. This plot changed drastically: The tree cover nearly doubled, whereas the dwarfshrub and grass cover diminished from over 80 % to less than 10 %. The grass cover, which was dominated by Monelytrum luederitzianum, has been totally replaced by Enneapogon desvauxii, which affords virtually no protection to the ground. Even browse species like Grewia flava, which have been recorded during 1956, are absent at present.

Volk Plot 757

Volk’s plot 757 was initially surveyed on 23 March 1956, and resurveyed on 20 April 1999. This plot represents the shallow Omuramba’s on the farm – a shallow, fairly wide, fairly slow-flowing water course typical of the CPL2. He describes the vegetation as “nearly without trees” – as a matter of fact, he did not list any tree or shrub species in this relevé. The opposite is true today (Figures 24 and 25): the vegetation cover of just over 50 % is made up by Acacia

mellifera subsp. detinens (50 %) and some Acacia tortillis as well as Acacia reficiens. The grass cover of over 80 % (half of which were perennial grasses) has been reduced to about 5 % - all of them annual species. The shallow omuramba has been eroded to form a river proper.

Looking at the general trend, three tree / shrub species and one dwarfshrub species can be highlighted as encroaching on the farm, being Acacia mellifera subsp. detinens, Acacia tortilis

subsp. heterecantha, Dichrostachys cinerea and (as dwarfshrub) Monechma genistifolia subsp. genistifolia (Figures 26, 27, and 28). Of concern is the general reduction of browse species like Grewia flava and Tarchonanthus camphorathus. But not only the woody species show a change – all grass species show a reduced abundance. Two species, Anthephora pubescens and Brachiaria nigropedata, could not be found on the entire farm (13 000 ha) at all during 1999 (or the present (2000) season) (Figure 29 and 30).

This reduction in vegetation cover leads to increased soil erosion as well as soil surface compaction combined with clay bubbling (“Schaumböden” – as described by Volk & Geyger 1970) (Figure 31), which again leads to poor seedling emergence and thus a further reduction in vegetation cover. The results of this vicious circle can be seen in Figures 32 to 34. Volk described a soil profile on the farm as having mottling at 70 cm and plinthite (iron-manganese concretions) at 120 cm. This same mottling has been found at 48 cm, whilst the plinthite is presently at 80 cm. This means that between 22 and 40 cm of soil has been lost from the site during the past 44 years – between 5 and 10 mm per annum!

Windhoek district

Kellner (1986) made a description of the vegetation of the Daan Viljoen Nature Reserve, as well as two farms in the Windhoek district, within the ESC2 (Khomas Hochland) (de Pauw et

al. 1998/99). As the data is not as complete with locality descriptions as that of Volk, only a comparison based on the average abundancies between the data from 1985 and the present day situation is possible.

Field work for the vegetation survey project during this growing season included some surveys in the Oas-Oanob conservancy. Standard Braun-Blanquet procedures have been used during the compilation of the relevés (Strohbach 2000b). The average abundancies of the various grass species was calculated for all relevés within the community (Figures 34 and 35).

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Figure 22: Summarised vegetation composition of Volk plot 739.

Figure 23: Photo of Volk plot 739 taken on 7 August 1999.

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Figure 24: Summarised vegetation composition of Volk plot 757.

Figure 25: Photo of Volk plot 757 as seen on 20 April 1999.

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Figure 26: Change in the cover of some selected tree species in the Okahandja district (average of all relevés).

Figure 27: Change in the cover of some selected shrub species in the Okahandja district (average of all relevés).

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Figure 28: Change in cover of some selected dwarfshrub species in the Okahandja district (average of all relevés).

Figure 29: Change in cover of some selected perennial grass species in the Okahandja district (average of all relevés).

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Figure 30: Change in cover of some selected annual grass species in the Okahandja district (average of all relevés).

Figure 31: Clay bubbling below the surface crust leads to decreased water infiltration.

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Figure 32: Sheet erosion is widespread…

Figure 33: … sometimes even leading to gully erosion.

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Most perennial grasses show a reduced crown cover in the present season compared to 1985. The most notable are Brachiraria nigropedata, Cenchrus cilliaris and Eragrostis nindens, which show a reduction of 50 % or more in abundance. In the case of Brachiaria nigropedata, the reduction is from 10 % crown cover to less than 2 % crown cover – a sure sing that the grass is threatened. This species has been described as a Decreaser by Joubert (1997), meaning that it will decrease in abundance with increased grazing pressure (Vorster 1982; Fourie & du Toit 1983; Bosch & Janse van Rensburg 1987; Trollope et al. 1989; Trollope 1990; Hurt & Bosch 1991). Eragrostis nindensis has been described as an Increaser II by Joubert (1997), often attaining 60 % of the species composition (on a frequency base). The decrease from ca 11 % to just over 5 % also indicates a degrading veld condition. Stipagrostis uniplumis, described by Joubert (1997) as an Increaser III, has actually doubled in abundance from 1985 to 2000, supporting the the data of Joubert. Wether it is really an Increaser III (i.e. a plant increasing under extreme conditions of overgrazing), remains however doubtful.

Figure 34: Change in cover of some selected perennial grass species in the Khomas Hochland (average for all relevés).

Figure 35: Change in cover of some selected annual grass species in the Khomas Hochland (average for all relevés).

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Most annual species also show a slight increase (Figure 35), which is to be expected, as these are described by Joubert (1997) as Increasers II or III. An extraordinary increase from virtually absent to 35 % crown cover is seen with Aristida adscensionis. This ubiquist has an extremely wide ecological amplitude, and has been favored in the past season by the extraordinary high rainfall, as well as the fact that the competing perennial grasses have been reduced in abundance.

This degradation trend can be ascribed to overgrazing by especially game. No conclusion can yet be made weather this severe degradation is local on the particular farm, or more widespread.

The need for long-term monitoring

Vegetation dynamics, especially vegetation degradation, can be studied in two ways: comparing various stages of degradation close together at once (Bosch & Janse van Rensburg 1987; Bosch et al. 1987) or comparing observations from the same site from numerous years.

The main advantage for the first approach is that a fairly good indication of the dynamic trends can be obtained within a short period of time. However, very often the trends found are erratic due to various forms and intensities of the driving factor (e.g. grazing intensity, grazing history, climatic history) or varying habitat factors and are not all that well comparable. The mayor concern is however the fact that the type and especially the intensity of the driving factor is in most cases not known. Should wide-spread degradation be present, it can also well happen what no examples of the original ‘climax’ vegetation can be found, thus giving an incomplete picture of the degradation gradients. Strohbach (1992), Joubert (1997) and Strohbach (1999) give examples of the application of this method in various parts of Namibia.

In the second approach both the changes as well as the driving forces for these changes can be monitored. The mayor problem however is the fact that such monitoring has to be done over numerous years – changes often only become apparent after 5 to 20 years. This long-term nature of the research makes it both impractical and expensive for many research organisations – yet, as can be seen from the two examples in the commercial farming area above, are very necessary for insight in the state of the environment. This is also confirmed by Ward et al. (1998), who studied degradation in communal Otjimbingwe and the surrounding commercial farms. They found little difference between these two pastoral systems, ascribing it partially to the fact that observations where done over a limited period of time only.

An initiative has been launched, subject to the approval of funding, by a number of German universities and research institutes, in collaboration with southern African partners, to start with long-term monitoring at sites along a climatic gradient (BIOTA 2000). In principle, a number of bio-observatories will be set up along the rainfall gradient from the winter rainfall area in the Cape, to the arid zone between the Richtersveld and southern Namibia (which also represents the change-over between winter- and summer rainfall) up to the higher rainfall areas of the Kavango. The bio-observatories will consist of a 1 km2, which will be subdivided into 100 ha squares. In the square kilometre observations will be made regarding the species diversity, whilst species abundance and dynamics will be studied in greater detail within some of the ha squares. Species groups to be studied include cyanobacteria, fungi, insects, small mammals, the vegetation, etc. These studies will be aided with remote sensing, using also historic data as far as available as well as soil surveys. The changes in soil properties (due to e.g. erosion) will also be studied. The bio-observatories are to be placed in pairs across a land-use boundary, e.g. commercial vs. communal, farming land vs. nature conservation areas, conventional rotational grazing vs. the high intensity / short duration rotational system as practised by the followers of

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the “Holistic Resource Management” movement (Savory 1988). The studies on the biological diversity is to be complemented by socio-economic studies on the contrasting land-use practises at each observatory pair. Within Namibia, 3 primary observatories (were all disciplines are involved) as well as 6 secondary observatories are to be placed.

Although only at a limited number of sites, this project is a start towards desperately-needed, well documented, long-term monitoring sites. As such, this project will form an integral part of the Nambian Long-Term Ecological Research Network (Na-LTER 2000).

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