3. STATUS OF INVASIVE TREE SPECIES IN SOUTHERN AFRICA

3.1 SOUTH AFRICA

3.1.1 Background

South Africa is fairly wooded, with 7 percent of the land area under forest cover and a further 27 percent under some form of woodland. Other vegetation types include moist savannah, ‘bushveld’ savannah and arid savannah. Natural forests and woodlands, which have predominantly slow growing tree species, would not have met increasing requirements for timber, mine props, tannin, charcoal, etc., for South Africa. Introduction and subsequent commercial production of selected exotic, fast growing tree species was considered an alternative to meet the demand.

About 750 tree species and around 8 000 shrubby, succulent and herbaceous species are recorded as having been introduced into South Africa (van Wilgen et al., 2001). Of these 8 750 species, 161 are now regarded as invasive, 68 percent of these invasive species (110 species) are classified as being woody, and the number may go up as the status of recent introductions becomes clearer. The earliest record of introduced alien tree species (pines) invading natural vegetation in South Africa is from about 1855, when Pinus halapensis was noted to be spreading into the fynbos biome (Richardson and Higgins, 1998). However, Poynton (1979a) referred to Pinus pinea as among the very first of all exotic conifers to become established in the Cape. It was also one of the first introduced species to become naturalized in its new environment, where it was reported to be reproducing spontaneously and freely from seed in the Cape Peninsula. P. pinea reached South Africa in about the middle of the eighteenth century, while P. halapensis was introduced into South Africa in about 1830, which makes the former probably the first to become naturalized. Further observations on the natural spread of other species, such as Pinus pinaster, were made in about 1928 (Kruger, 1977). Since then, significant progress has been made, particularly in the last quarter century, in the study of invasive tree species, especially introduced or alien species in South Africa (Kruger, 1977; Stubbings, 1977; le Roux, 1981; Wells et al., 1986; Richardson et al., 1994, 1997). In the only recorded case of an indigenous species as an invader species, Theron (1978) noted Acacia ataxacantha as an aggressive invader species in eastern South Africa.

Introduction of tree species in South Africa dates back to the middle of the seventeenth century, when species were introduced for a range of purposes that included timber, tannins, oils, firewood, ornamentals, stabilizing sand dunes, windbreak barriers, hedges, soil conservation and shade (Troup, 1932; Streets, 1962; Poynton, 1979a, b). Among the species introduced for commercial purposes, softwoods, mostly Pinus spp., are currently planted on 707 200 ha, while hardwoods (wattle, eucalypts and poplar) occupy 623 738 ha. On a species basis, Pinus patula, P. elliottii, P. radiata, P. taeda, and P. pinaster constitute 47.7, 28.3, 9.3, 5.5 and 3.0 percent respectively of the Pinus spp. area, with the remaining 6.2 percent being minor species and hybrids. Among the hardwoods, Eucalyptus grandis, other gums (mostly hybrids), Acacia mearnsii and poplars occupy 45.0, 36.1, 17.1 and 0.3 percent respectively of the total area of hardwoods.

Some of the introduced tree species (quickly naturalized in the new environment) are reproducing consistently and have sustained populations over many life cycles without direct human intervention. Furthermore, some of these naturalized species have become invasive in the new environment, where, without their attendant natural enemies (mostly left behind in the country of origin), they are able to survive, reproduce and spread unaided and at alarming rates across the landscape (Richardson et al., 2000a; van Wilgen et al., 2001).

3.1.2 Invasive alien tree species and extent of invasion

The total area invaded by alien trees in South Africa is over 100 000 km², which is over 8 percent of the country’s total area (van Wilgen et al., 2001). These invasions are mostly concentrated in wetter regions of the country or sometimes along river systems (perennial, seasonal or ephemeral). When split into the major biomes, the fynbos (a Mediterranean-type shrub land) is the most invaded biome by Pinus, Acacia and Hakea species in the mountains, lowlands and along all major river systems (Richardson et al., 1997; Cowling et al., 1999; Le Maitre et al., 2000; van Wilgen et al., 2001). The forest biome is also heavily invaded, but the extent has not yet been quantified (Richardson et al., 1997). The forest biome is invaded by mainly Acacia cyclops, Acacia mearnsii, Acacia saligna, Eucalyptus spp., Melia azederach, Pinus spp., Psidium guajava, Sesbania punicea and Solanum mauritianum. The grasslands and savannah biomes are also said to be extensively invaded by a range of species that include Acacia spp., Melia azedarach and Jacaranda mimosifolia, which invade the open land, river banks and beds. The Nama karoo (semi-desert shrub land of summer rainfall) is probably the fourth most invaded biome, where Prosopis species have invaded at least 18 000 km² of the low lying alluvial plains and seasonal and ephemeral water courses. The succulent karoo (semi-desert shrub land with winter rainfall) are also heavily invaded by Prosopis spp.

Richardson et al. (1994) gave a rough ranking of different vegetation types in terms of their vulnerability to invasion by pines as:

Forest < shrubland < grassland < < dunes < bare ground

Very degraded or fragile environments (bare ground, dunes) actually benefit from pine colonization, since the trees contribute to habitat restoration and accelerate the natural dynamics of environmental reconstitution.

Table 1 shows the major invasive tree species in South Africa, the original purpose of introduction, the areas affected and their relative invasiveness. For commercial plantation species, the area under plantation for each species is also shown for comparison purposes. The year of introduction is also shown for comparing the effect of time on affected areas. Some of these factors have been used to explain the extent of invasions in a model that incorporates information on the species’ attributes, residence time, extent of planting, ground cover characteristics, locality (latitude), disturbance regime, and the resident biota in the receiving environment (Richardson, 1998).

Table 1. Invasive alien tree species, area affected and their degree of invasiveness in South Africa

Notes:

(1) Purpose of introduction: amen. = amenity use; orn. = ornamental use; shelt. = shelterbelt use; charc. = charcoal production; fwood = fuelwood production.

(2) Category – these are the three categories used in Republic of South Africa legislation. See Appendix for regulations for each category.

(3) Index of invasiveness: 5 = highly invasive; 1 = not invasive.

Sources: Hall and Boucher, 1977; Duggan and Henderson, 1981; Richardson et al., 1994; Dept. of Agriculture, Regulation No. 15, 2001.

There are more than a dozen other tree species and shrubs that have been declared weeds. However, most are localized species or their importance is currently not as great as those listed in Table 1. Nevertheless, many of them have the potential to spread quickly if allowed to establish in new, suitable habitats, and so should be considered dangerous. The declared weeds are Cestrum aurantiacum, C. laevigatum, Chromoleaena odorata (considered by some to be the No.1 alien weed and threat to biological diversity in eastern South Africa), Cinnamomum camphora, Cytisus scoparius, C. monspessulanus, Eugenia uniflora, Lantana camara (massive priority problem weed in South Africa), Leptospermum laevigatum, Litsea glutinosa, Nerium oleander, Nicotiana glauca, Opuntia spp. (in drier regions), Paraserianthes lophantha, Pittosporum undulatum, Rhus succedanea, Rosa rubiginosa, Rubus spp. (a major biological diversity threat as difficult to control and known to hybridize with native species), Sesbania punicea (becoming less important as a result of successful biological control), Schinus terebinthifolius, Solanum mauritianum (a major and widespread weed), Spartium junceum, Tamarix chinensis, T. ramosissima, Tecoma stans, Thevetia peruviana and Triplaris americana. The tree species and shrubs declared as invaders include Ailanthus altissima, Eriobotrya japonica, Gleditsia triacanthos, Ligustrum ovalifolium, Lonicera japonica, Metrosideros excelsa, Mimosa pigra, Myoporum tenuifolium, Pyracantha angustifolia, P. crenulata, Phytolacca dioica, Robinia pseudoacacia, Salix babylonica, S. fragilis, Syzygium cumini, S. jambos and Tipuana tipu.

3.1.3 Environmental and economic impact of invasive tree species

Invasive alien tree species have been shown to cause both environmental and economic impacts in South Africa. They have been shown to have a negative effect on all components of biological diversity, from genes to whole ecosystems. In South Africa, invasive alien tree species and shrubs have been shown to have the following negative effects:

 reduction in stream flow and available water;

 loss of potentially productive land;

 loss of grazing potential;

 poisoning of humans and livestock (e.g. Melia azedarach and Lantana camara);

 increasing costs of fire protection and increasing damage in wildfires;

 increasing soil erosion following fires in heavily invaded areas;

 siltation of dams;

 changing soil nutrient status;

 loss of biological diversity and threat to native plant species;

 changing biomass of ecosystems;

 changing habitat suitability for native animal species; and

 hybridization with local related genera (e.g. Rubus sp.), thus exchanging genes.

South Africa has made significant strides in describing and quantifying the economic and environmental impact of alien invader tree species in its various ecological regions (Le Maitre et al., 1996; Chapman and Versfeld, 1995; Prinsloo and Scott, 1999; Holmes and Marais, 2000; Le Maitre et al., 2000; van Wilgen et al., 2001). Table 2 shows some of the documented environmental impacts caused by different invasive alien tree species.

In the fynbos biome, Richardson et al. (1989) estimated that invasions could reduce species richness by between 45 and 67 percent. Other studies have provided documented evidence of modified nutrient regimes due to either increased nitrogen fixation or increased decomposing biomass (Musil and Midgley, 1990; Witkowski, 1991; Stock and Allsopp, 1992; Musil, 1993; Yelenik, 2000) and impacts on seed banks of native fynbos species associated with Acacia saligna invasions (Holmes and Cowling, 1997a, b).

The impact of invasive alien tree species on water sources in South Africa has been studied. The results indicate a reduction in steam flows of between 4.7 and 13.0 percent (Dye, 1996; Le Maitre et al., 1996; Prinsloo and Scott, 1999, Le Maitre et al., 2000). A nationwide study revealed that invasive alien plants were using 6.7 percent of the total mean annual surface runoff, or 9.95 percent of the utilizable surface runoff, on the basis of modelled estimates (Le Maitre et al., 2000). These figures are crude estimates since there are large variations even between very similar adjacent watersheds, but are considered to be the best possible estimate on a national scale (Le Maitre pers. comm.). The invasive alien species have also been ranked according to their water use (Le Maitre et al., 2000). In decreasing order of water use, they are Acacia mearnsii, A. cyclops, A. dealbata, Pinus spp., Eucalyptus spp., Prosopis spp., A. saligna, Melia azedarach, Solanum mauritianum, Lantana camara, Chromolaena odorata, Hakea spp., Populus spp., Jacaranda mimosifolia, Sesbania punicea, Rubus spp., A. longifolia, Psidium guajava, Caesalpinea decapetala, Salix spp., A. melanoxylon, A. decurrens and Quercus robur. Recent studies show however that steam-flow reduction by forest trees is not a constant for any tree species, and varies considerably in both time and space (Anon. page 29).

Table 2. Some of the notable environmental impacts of invasive alien trees in South Africa and the biomes affected

Source: van Wilgen et al., 2001.

3.1.4 Control of invasive trees

Early studies on the impact of invasive alien trees in South Africa concentrated on environmental impacts, such as effects on native species, loss of ecological services and biological diversity. Some studies (van Wilgen et al., 1992, 1996, 1997; Le Maitre et al., 1996, 2000; Chapman and Versfeld, 1998) quantified the impact of invasive alien tree species on water resources. These studies demonstrated that invasive alien trees were consumptive water users, and, based on mathematical modelling, it was estimated that they were using as much as 6.7 percent of the country’s total run-off. These studies convinced the Government of South Africa to take the issue of invasive trees seriously in a country where water is considered to be a limited resource, as average rainfall is only 490 mm per annum, although the invasive aliens problem is primarily one of higher-rainfall areas. The work led to the establishment of the ‘Working for Water’ programme that aims to control invasive alien plants to protect water resources and ensure the security of water supply (van Wilgen et al., 1998, 2001). Besides enhancing water supply, other benefits of controlling invasive trees include stemming loss of biological diversity, reducing fire hazard, stabilizing catchment areas and modifying erosion impacts, with social benefits such as job creation in the labour-intensive clearing programmes (Le Maitre et al., 2001). The government also enacted laws that govern the marketing of seed and growing of some species declared as weeds or invaders in South Africa (Government Gazette, 2001).

There are four main immediate methods used in South Africa to control invasive alien tree species:

 mechanical control (hand pulling, slashing, rotor tilling, mowing, ring-barking and felling);

 chemical control (spraying young saplings, frilling and painting, chop-and-squirt treatments, and injecting chemicals into large trees);

 use of fire in controlled burning; and

 a combination of mechanical and chemical control methods in an integrative manner, coupled with biological control where suitable.

Biological control is being developed as a long-term programme, using insects, fungi or other biological control agents, inter alia to destroy seeds of the invasive trees. The biological control programme has successfully released an insect that attacks seeds of Acacia mearnsii a seed-feeding weevil (Melanterius maculatus) in most of the provinces of South Africa (Pieterse and Boucher, 1997). The role of biological control as one element in an integrated control programme in South Africa is discussed by Little et al. (in press).

There are proposals to legislate the screening of new species for their potential invasiveness before introduction (Van Wilgen et al., 2000). Without a protocol for screening potential invasive species before their introduction, the problem becomes a vicious cycle in which one set of problems is replaced by another as new species enter the country taking over from those brought under control (Richardson et al., 1990; Rejmánek and Richardson, 1996; Van Wilgen et al., 2000). Researchers are also looking into the possibility of producing seedless clones of commercially grown pines (Richardson, 1998).

The following are some important conclusions and developments in the control of alien invasive tree species in South Africa:

 The cost of controlling alien plant invasions in South Africa has been estimated to be around US$ 1 200 million, or US$ 60 million per year for the estimated period of 20 years that it will take to deal with the problem (Chapman and Versfeld, 1998), although some authorities challenge the shortness of the 20-year timeframe and the consequent financial calculations, and others contest the values used in costing the operation (Dave Dobson, SAWGU, pers. comm.).

 The potential reduction in value of the fynbos biome (1 million ha) due to invasion (based on six components: water production, wild flower harvesting, hiker visits, ecotourism, endemic species and genetic storage) was estimated at US$ 11 750 million annually (Higgins et al., 1997).

 The net present cost of invasions by black wattle (Acacia mearnsii) in South Africa has been estimated to amount to US$ 1 400 million, although some assumptions of the model used^[1] are debated.

 The plantation forestry industry, which is often thought to be the major source of infestation for invasions, contributes US$ 300 million, or 2 percent of the Gross Domestic product (GDP), and employs 100 000 people. The downstream forest-based industries contribute a further US$ 1 600 million, much of it in export earnings. However, in trying to attribute blame, it must be remembered that the forest industry was often supported by government in its introduction activities, and also that probably half of the problem species arrived as ornamentals.

 Thirty-eight percent of the area invaded by woody alien species in South Africa is occupied by species used in commercial forestry, although not necessarily planted for commercial logging, as the commercial species were also used by official agencies and land users as a source of seedlings for fuelwood, shelter belts, windbreaks, woodlots, etc.

 The Government of South Africa committed US$ 100 million between 1995 and 2000 to the ‘Working for Water’ Programme for the control of these invasive alien species.

 The control or eradication programme, which is mostly manual and chemical, has created significant employment opportunities for poor communities. In 1998, about 40 000 people were employed by the ‘Working for Water’ Programme to clear invasive alien tree species.

 In South Africa, biological control is envisaged as a long-term option for controlling further invasions and re-invasions. The total cost of biological control research initiatives between 1997 and 2000 was US$ 3 million.

 Using biological control to clear invasive alien tree species and shrubs could cost US$ 400 million over 20 years, or US$ 20 million per year, a cost considered manageable for a developing country like South Africa. However, biocontrol is currently only applicable to a very limited range of tree species, and extreme care has to be taken to ensure that there are no unwanted side-effects associated with the introduction of alien biocontrol agents. This presupposes considerable technical capacity, which is in limited supply. The provisions of CBD must also be adhered to in this context.

3.1.5 Awareness and potential conflicts of interest

Judging by the number of publications on invasive alien tree species in South Africa, awareness of the negative impacts of invasive alien tree species must be very high. The government has put in place regulations governing the marketing of seed and planting of species declared as invaders (Government Gazette, 2001). The government has also produced campaign materials in the form of easily readable brochures, flyers, pamphlets, etc., on the negative impacts of invasive alien tree species. One such brochure is entitled The Environmental Impacts of Invading Alien Plants in South Africa and is an easily readable brochure aimed at the lay person. The government also set up the ‘Working for Water’ programme that aims to eradicate and control invasive alien tree species in South Africa to minimize water lost to these undesirable plants.

Legislation already exists within South Africa which requires an Environmental Impact Assessment for listed activities (National Environmental Management Act), and the introduction of new species is classed as a listed activity.

Many land owners (other than timber companies) are making use of alien commercial (category 2) species that have various desirable attributes other than simple commercial value. These include shade, shelter for livestock, windbreaks, fuelwood and structure on river banks (to diversify habitat for angling). These are used in areas where the country has little in the way of alternative native species to provide a similar function.

It should also be borne in mind that many invasions have had their origin in non-commercial activities.

There was lobbying and counter-lobbying on a proposal to declare one of the commercial species (Acacia mearnsii) a weed (Stubbing, 1977; Pieterse and Boucher, 1997). The potential rift between commercial users of invasive tree species and environmentalists has been avoided by the declaration of most of the economic species as invaders rather than weeds. If declared weeds, the law would require that they be eliminated automatically. One area of conflict has been the start of a biological control programme for Acacia mearnsii (Pieterse and Boucher, 1997). The release of the seed-feeding weevil (Melanterius maculatus) for the biocontrol of Acacia mearnsii was done in most of the provinces of South Africa, with the exception of KwaZulu-Natal and Mpumalanga provinces, where there is a conflict of interest with wattle growers (PPRI, 2002). The wattle growers in KwaZulu Natal, with most of the South African commercial wattle plantations, rely on seed in the soils for about half of the regeneration, and have negotiated with PPRI to release initially in non-commercial areas to allow growers to build up seed banks sufficient to supply the entire industry for line-sowing purposes (Dave Dobson, SAWGU, pers. comm.).

3.2 ZIMBABWE

3.2.1 Background

Zimbabwe is still fairly wooded, with 66 percent of the country’s land area being under some form of woodland cover (Nyoka and Musokonyi, 2002). Only 27 percent is under cultivation, with the remainder being under other forms, such as grasslands, exotic plantations and settlements. The country has two major phyto-regions namely Flora Zambesiaca and the afromontane. The Flora Zambesiaca comprises five major woodland types, namely Miombo, Mopane, Teak, Acacia and Terminalia-Combretum. These woodlands are believed to contain up to 8 500 species, with 4 600 being endemic. The major genera constituting the endemic species include Bolusanthus, Cleistochlamys, Colophospermum, Diplorhynchus, Pseudolachnostylis and Viridivia. The afromontane phyto-region is scattered along the eastern highlands, which forms the country’s eastern border with Mozambique. Most of the exotic plantations of pines, gums and wattle are in the eastern highlands. The woodlands provide commercial timber, mainly for furniture, local construction poles, fuelwood, fruits, livestock browse and fodder, medicines and shelter for other fauna and flora.

The commercial forest plantations cover an area of 118 000 ha, of which 68 percent is planted with pines. Among the pines, Pinus patula constitutes 61 percent; P. taeda, 22.8 percent, P. elliottii, 10 percent; P. kesiya, 2 percent; and P. oocarpa, 0.2 percent. The remaining 32 percent is under hardwood, namely Eucalyptus grandis, E. cloeziana and Acacia mearnsii. Acacia mearnsii occupies an area of 11 400 ha, which is 30.2 percent of the area planted with hardwoods. All the species used in commercial forestry are exotics as the country’s only softwood conifer, Widdringtonia nodiflora, which occurs in small quantities, was found to be slow growing. Besides species introduction for commercial forestry, there were also significant introduction of species for other purposes such as fruit, amenities, ornamental, windbreaks, shade, fodder and agroforestry. There were also accidental introductions in which some tree species were introduced without defined purpose. Some of these introduced species naturalized in their new environment, and some are invading new areas.

The problem posed by these invasive alien tree species in Zimbabwe was realized by the Nyanga National Park as far back as the 1980s. The National Park estates are either adjacent to commercial forestry plantations (the major source of infestation) or had their own plantings in the early 1920s. The exotic species were planted as a deliberate policy to provide fuelwood and construction timber and to beautify the parks with ornamentals such as jacarandas and syringas. The syringa (Melia azedarach) was observed to be regenerating on its own in the 1950s (SRFC, 1956). Also, more invasions have been observed inside forest estates in pockets usually left for conservation purposes, along watercourses and outside timber estates. Despite the early awareness, there has been no institutionalized research on invasive tree species, although there has bee some research by individuals.

3.2.2 Invasive alien tree species and extent of invasion

Unlike in South Africa, where a lot of work has been done in terms of identifying major invasive tree species and their distribution, very little similar work has been done in Zimbabwe. The list of invasive tree species in Zimbabwe was developed from a survey that was carried out through questionnaires among major timber companies, environmentalists, forest consultants, non-governmental organizations (NGOs), etc. This information was complemented by information gathered from physical survey of some of the areas.

The major invasive alien species identified belong to Pinus and Acacia genera. The other less important, but nevertheless invasive genera, are Populus, Callistris, Jacaranda, Melia, Psidium and Ziziphus. Other species, such as Bauhinia spp., Senna spp., Callistris spp., Casuarina spp., Grevillea spp., Prunus spp., Albizia spp., Morus spp. and Toona spp., are generally invasive in very localized areas. Species of the genus Eucalyptus are also very prominent as invaders in the Vumba. Table 3 shows the species that were identified as invasive through the survey. Also included in the table is information on ecosystems invaded and the period of introduction of the species, if known.

Table 3. Major invasive alien tree species in Zimbabwe, their use, ecosystem invaded, period of introduction and their degree of invasiveness

Among the pines, Pinus patula is the most aggressive invader of afromontane forests and grasslands and miombo woodlands in localities above 1 600 masl, where it is able to produce viable seed. Among the acacias, Acacia mearnsii is the most aggressive invader of stream banks, forest margins and miombo woodlands above 1 600 masl in the mist belts of the eastern highlands of Zimbabwe, and together with A. melanoxylon, is an aggressive invader of the montane grasslands. The montane grassland ecosystem is rapidly disappearing under a blanket of invaders and is generally considered to be the most threatened habitat or landscape in the area. In parts of the Vumba area, Prunus cerasoides, Grevillea robusta and Callistemon sp. are also reported to be regenerating naturally, with P. cerasoides an aggressive localized invader in the Essex Road area. The other important invasive species are Pinus radiata, A. melanoxylon, A. dealbata, A. elata, Cupressus lusitanica and Eucalyptus globulus, all invading the eastern highlands. The other localized invasive tree species in the eastern highlands are A. podalyriifolia in the La Rochelle botanical gardens, Prunus cerasoides in the Troutbeck area of Nyanga, and Albizia procera and Cyphomandra betacea in Chipinge.

Populus × canescens, first introduced and planted at Hope Fountain Mission near Bulawayo by Reverend Helm in 1874/1875 as an ornamental, is the first known recorded introduction of a tree species in Zimbabwe, as this was some 16 years before the occupation of the country by European settlers. The species is a serious invader of river banks and river beds. It has invaded the upper reaches of Nyangombe (Nyanga district), Odzani (Mutasa district) and Tandaai (Cashel) rivers, streams near Nyazura, all in the eastern part of Zimbabwe, and the Mukuvisi river system in Harare and vleis in and around Harare and Ruwa, all in the highveld. The species spreads by suckering.

The most invasive species countrywide, Jacaranda mimosifolia and Melia azedarach, have also been recorded as invaders in Matopos, Hwange and Kyle National Parks. Natural regeneration of M. azedarach was observed in about 1956, some 55 years after its introduction (SRFC, 1956). Among fruit tree species, Ziziphus mauritiana and Psidium spp. are now known to be aggressive invaders in Zimbabwe. Z. mauritiana is believed to have been introduced by either Portuguese or Arab traders way back in the fourteenth century from the Indian subcontinent. It invaded the whole of the Zambezi valley, where it remained confined until the last two decades, when it started invading open spaces and roadsides in urban areas. It is spread mainly by humans, animals and birds, that consume the fruit. Its fruiting in the middle and highveld^[2] is very irregular due to lower temperatures, which appear to be unfavourable for its flowering. There could be strong fears of hybridization (genetic pollution) with other indigenous relatives of this species, such as Ziziphus mucronata. Among the guavas, two species (Psidium guajava and Psidium cattleianum) are known to invade the wetter and warm, lowland forest areas of the eastern highlands (Vumba, Chimanimani and Chipinge), the middleveld around the Morgenster Mission-Charumbira and Murinye areas in Masvingo.

In the central part of the country, the most notable invasive tree species are Pinus roxburghii, Bauhinia spp., Toona ciliata, Populus x canescens, Cotoneaster pannosa, Homalanthus populifolius and Caesalpinea decapetala. Specifically, Homalanthus populifolius, Toona ciliata and Bauhinia spp. are widespread in Harare, while Caesalpinea decapetala is said to be invading large areas in Arcturus. Senna didymobotrya is widespread throughout the highveld (L.J. Mullin, pers. comm.).

The extent of invasive alien tree species in Zimbabwe is difficult to estimate as there has not been a thorough survey carried out to determine the extent of spread. It would, however, appear that the National Parks (Nyanga and Chimanimani) and botanical gardens (La Rochelle and Vumba) in the eastern highlands of Zimbabwe have been the most obviously affected by these alien invasive tree species, but there has also been considerable invasion of the montane grassland ecosystem on private land holdings in the Vumba. The Nyanga National Park (47 150 ha) is said to have 20 percent of its area affected by invasive alien tree species. Based on the two most invasive tree species, Pinus patula and Acacia mearnsii, the invaded or potentially invadable area in Zimbabwe is about 200 000 ha, most of which is in the eastern highlands of Zimbabwe. The total area invaded, including those of localized and widespread species, although difficult to estimate, could be up to 350 000 ha.

3.2.3 Environmental and economic impact of invasive tree species

There are no known systematic studies on the environmental and economic impact of invasive alien tree species in Zimbabwe, but indirect evidence (old paintings and photographs of the area) indicates considerable impact on the biological diversity of the Vumba grasslands as a result of transformation by invaders (Dr Colin Saunders, TEAM VUMBA, pers. comm.).

Results from research in northeastern Zimbabwe in another context indicate that woodlots of Eucalyptus camaldulensis, established on areas that were formerly fields and grazing areas in the savannah-like conditions that exist in southern Africa, do not cause loss of species richness or soil nutrients. In heavily deforested areas, Eucalyptus woodlots might even contribute positively to the protection and preservation of indigenous tree species and other flora (Tyynela, 2001).

3.2.4 Control of invasive tree species

There is currently no official policy or legislation concerning invasive alien tree species. Despite this, there are active control programmes for invasive alien tree species in Zimbabwe. The programmes are largely individual efforts of private companies, environmentalists and National Parks. The National Parks (Nyanga and Chimanimani) estimated their annual budget for control of invasive tree species to be about Z$ 200 000 (US$ 3 600). The three big forestry companies commit some of their labour and equipment for a full day every year to help control invasive trees in National Parks and surrounding communal and state land, over and above their routine control along water courses, vleis and conservation areas in their own estates. The companies estimate that the programme of controlling invasive alien trees, which began four years ago, is costing them about Z$ 5.5 million (US$ 100 000) annually. The actual figures are difficult to obtain as control of invasive alien trees is often combined with other routine silvicultural operations, particularly if the trees are invading streams, vleis and roadsides in the compartments they would be working on already.

The main method of control is mechanical and controlled burning. The control teams either fell the trees, ring bark the trees or uproot saplings. Chemical control is used to destroy stumps of gums to stop further coppicing. In inaccessible areas, one of the forestry companies is planning to engage members of the mountaineering club to assist in clearing invasive alien tree species in the steep and inaccessible areas. Most of these inaccessible areas would be left as conservation areas because of their species richness.

Zimbabwe has some excellent legislation governing the control and eradication of noxious weeds but there is no policy or legislation governing invasive alien tree species. It will be important for environmentalists and NGOs to lobby the government to draw up policies and enact laws for control of invasive alien tree species in line with the Convention on Biological Diversity (CBD), to which Zimbabwe is a signatory.

Because this information on Zimbabwe derives mostly from rapid rural appraisals, anecdotal communications, estimates and visual observations, it is almost certainly necessary that, besides lobbying the government to control invasive alien tree species, there is a need to:

 investigate the true extent of the problem of invasive tree species in Zimbabwe;

 investigate and quantify the actual environmental damage caused by invasive tree species;

 investigate and quantify the actual economic impact caused by invasive tree species;

 draw up an implementable programme to eradicate or control invasive tree species; and

 solicit donor assistance for financial and material resources needed for a control programme.

3.2.5 Awareness and potential conflicts of interest

Although very little research has been done on aspects of invasive alien tree species in Zimbabwe, the potential problems posed by alien invasive tree species in general is widely understood by all forestry companies, the Department of National Parks and Wildlife Management and a significant number of local environmentalists and NGOs. In probably one of the few recent cases of research into invasive alien tree species, Tafangombe (2001) investigated the relationship between some geographic and soil factors and the spatial distribution of invasive tree species in the Nyanga National Park.

The awareness and appreciation of the problems caused by invasive tree species are beginning to grow among ordinary people. Invasive alien tree species in Zimbabwe are often referred to as ‘self-seeded trees’. Some local environmentalists (The Nyanga Rural District Council Intensive Conservation Area Committee) have produced flyers and pamphlets for distribution in the awareness campaign. The group actively participates in the control of invasive species in Nyanga, but is severely handicapped by lack of operational funds. On selected days, members of the group assign their gardeners to help control invasive trees (Mrs Mary Clarke, pers. comm.).

Forest companies, blamed for some of the infestations emanating from their estates, have since taken an active lead in the control of invasive trees. Some ordinary people surrounding the invaded areas appear unconcerned and unaware of the environmental threats posed by these species. In some instances, they have questioned staff of forestry companies assigned to clear the invasive trees on communities’ properties surrounding the plantations. For the communities, the free-growing trees are useful for the provision of firewood, construction and other local needs. The efforts of forestry companies therefore may not be successful unless awareness is raised among communities bordering the forests. Also, efforts for eradicating or controlling invasive species will also need to be supported by the local community, who still grow some of these invasive species as ornamentals in their gardens. The Nyanga Rural District Council Intensive Conservation Area committee is educating its members to use alternative species for ornamental purposes that are non-invasive. What they lack, however, is information on these alternative species (Mrs Mary Clarke, pers. comm.).

A local NGO in the Vumba area, TEAM VUMBA (The Environmental Association for Management of Vumba’s Unique Mountain Biodiversity Areas) is actively engaged in combating the spread of alien invasive plants in the Vumba Mountains (Dr Colin R. Saunders, TEAM VUMBA, pers. comm.)

Some leading foresters in some of the forest companies, and environmentalists, are of the view that policy or legislation should be put in place to control the growing of some invasive species. There is however no consensus on the ‘polluter pays’ concept, as some view it as too punitive, and that it does not take cognizance of the social and economic contribution of commercial forestry. Private companies claim that they are already playing their part, but private citizens, who probably are contributing significantly to these invasions (from their gardens), may not be aware. Some argue that since the Government of Zimbabwe is a signatory to the CBD and that these invasive tree species are especially cited as a major threat to biological diversity, it must therefore take the lead through its departments, such as the Forestry Commission and the Department of Agriculture and Natural Resources.

3.3 ZAMBIA

3.3.1 Background

The vegetation of Zambia is divided into four main types, namely closed forests, woodlands (or open forests), termitaria and grasslands (Sekeli, 2000). The closed forest is further subdivided into dry evergreen, dry deciduous, montane, swamp and riparian forests. The woodlands (or open forests) and the termitaria forests are also further subdivided into miombo, Kalahari, mopane and munga forests. In terms of species diversity, Zambia is estimated to have in excess of 5 000 plant species, of which half are trees and shrubs (Fanshawe, 1971; Sekeli, 2000).

Zambia has 57 000 ha of planted forests, of which the dominant species are two pines, Pinus kesiya and P. oocarpa, and two eucalypts, Eucalyptus grandis and E. cloeziana. Other plantation species occurring on a smaller scale include P. merkusii, P. michoacana, E. tereticornis, E. camaldulensis, and a hybrid between E. tereticornis and E. camaldulensis (Sekeli, 2000). At least 80 Eucalyptus spp. and 35 Pinus spp. are recorded as having been introduced into Zambia in a variety of species trials (Poynton, 1979a, b; Sekeli, 2000).

3.3.2 Invasive tree species and extent of invasion

Most plantation forestry species are regarded as non-invasive (Malaya, pers. comm.), and most species that have been introduced as ornamentals are also regarded as non-invasive. There is documented evidence that species such as Pinus patula and Acacia mearnsii, the most invasive tree species in South Africa and Zimbabwe, are unsuccessful in Zambia due to environmental limitations. Pinus kesiya, the fourth most important pine in Zimbabwe, has been cited as only moderately invasive in Zimbabwe, but is considered non-invasive in Zambia. Species such as Jacaranda mimosifolia and Melia azedarach, widely reported as invasive in Zimbabwe and South Africa, are not known to be invasive in Zambia. One fruit tree species, Ziziphus mauritiana, introduced some time ago, is the only fruit tree species currently considered invasive. However, the species is now widely viewed as ‘native’ by the average person and it has successfully naturalized and invaded new areas. The fruits of this species, as in Zimbabwe, are widely consumed and hence are widespread in Zambia. It is mainly confined to roadsides and open spaces in urban areas.

Could most of the tree species considered invasive elsewhere in the region be non-invasive in Zambia? It may be that the invasions are still localized and so the true status of each species is not yet clear. In addition, the geographical environment of Zambia differs significantly from South Africa and Zimbabwe. Since most of the species are not considered invasive, there has been no systematic survey, and so there is no overview available of the area which may be affected by invasive tree species.

3.3.3 Environmental and economic impacts of invasive tree species

Because most species are currently considered non-invasive in Zambia, no systematic assessment has been made of the possible impact, if any, of invasive species in Zambia.

3.3.4 Control of invasive tree species

There is no control programme for Ziziphus mauritiana, the only species considered invasive in Zambia.

3.3.5 Awareness and potential conflicts of interest

Awareness is difficult to judge in a situation where the only species technically considered invasive is popularly regarded as being useful and not a problem.