THE STATUS OF INVASIVE ALIEN FOREST TREES SPECIES IN SOUTHERN AFRICA^6,7,8

by
Betserai Isaac Nyoka⁹

INTRODUCTION

With the actual and projected development of plantation forestry, the issue of potential invasiveness is of growing concern (see the overall paper on forest reproductive material in this issue of Forest Genetic Resources). FAO has initiated a corporate-wide work on biosecurity in food and agriculture, which includes, from a forestry perspective, the issue of invasive forest trees. This paper is based on a report commissioned by FAO entitled `A case study on the status of invasive forest trees species in Southern Africa''. The specific objective of the study was to review available literature on the importance of invasive tree species, their impact on biodiversity and on economic development in the three countries of Zambia, Zimbabwe and South Africa.

Documented introduction of tree species in southern Africa dates back to the middle of the 17th century when the first tree species were introduced in South Africa (Troup, 1932; Streets, 1962). In Zimbabwe, the earliest recorded case of forest tree introduction was about 1874, some 16 years before the occupation of the country by European settlers (SRFC, 1956). As many as 750 tree species are recorded as having been introduced in southern Africa (van Wilgen et al., 2001).

The plantation forestry sector, the source of timber and tannin bark is entirely based on exotic tree species. The economic and social contribution of this sector to the southern African countries is shown in Table 1. Besides the direct and tangible economic benefits derived from exotic trees species used in commercial forestry, exotic trees species have also been used to provide firewood in areas deficient of native tree species, fodder, fruits, windbreak, shade, etc.

INVASIVE TREE SPECIES AND EXTENT OF INVASIONS

The introduction and subsequent use of exotic tree species in southern Africa has not been without cost. Some of the species introduced naturalized and became invasive, causing immense environmental damage. Table 1 shows some of the major invasive tree species in southern Africa. It is apparent that exotic tree species from a cross section of uses are all contributing to the invasions. The total area invaded by alien trees in South Africa is about 100 739 km² which is 8.07 percent of the country's total area (van Wilgen et al., 2001). In Zimbabwe, the total area invaded is not known but estimates put the area at more than 450 000 ha. All the major ecosystems in South Africa and Zimbabwe have been affected. Zambia is considered safe although lack of awareness could be contributing to this notion. This scenario may be attributable to the fact that South Africa has had the highest number of species introduced and the longest period of tree species introduction of over 300 years. Thirteen Acacia species (A. baileyana, A. cyclops, A. dealbata, A. decurrens, A. elata, A. implexa, A. longifolia, A. mearnsii, A. melanoxylon, A. paradoxa, A. podalyriifolia, A. pycnantha and A. saligna) have all been declared invader species or weeds in South Africa while six of the same species (A. decurrens, A. elata, A. mearnsii, A. melanoxylon and A. podalyriifolia) are known to be in invaders in Zimbabwe. Nine Pinus species (P. canariensis, P. elliottii, P. halapensis, P. patula, P. pinaster, P. pinea, P. radiata, P. roxburghii, P. taeda) have been declared invaders in South Africa and 6 Pinus species (P. elliottii, P. kesiya, P. patula, P. radiata, P. taeda, P. roxburghii,) have been identified as invaders of varying degrees in Zimbabwe. The other invader tree species occurring in at least two countries are Populus canescens, Psidium spp., Melia azedarach, Jacaranda mimosifolia, Albizia procera, Grevillea robusta, Bauhinia spp., Senna spp. and Caesalpinea decapetala and Cedrela toona all in South Africa and Zimbabwe and Ziziphus mauritiana in both Zimbabwe and Zambia.

Besides Z. mauritiana, most exotic tree species in Zambia are regarded as noninvasive. There is documented evidence that species such as P. patula and A. mearnsii the most invasive tree species in South Africa and Zimbabwe were unsuccessful in Zambia due to environmental limitations.

Table 1: Summary of forestry statistics and invasive alien woody species in three southern African countries.

na = information not available or not an issue; *figures are estimates.

ENVIRONMENTAL DAMAGE CAUSED BY INVASIVE TREES AND THEIR CONTROL

Documented environmental damages caused by invasive alien tree species in southern Africa include reduction in stream flow, change in soil nutrient status, reduction in species richness, increase in biomass of some ecosystems and genetic pollution (Van Wilgen et al., 2001).

The control of alien invasive species in South Africa began in the 1940s and in the 1980s in Zimbabwe. Early efforts were largely uncoordinated and erratic, and as a result did little to stem their spread. Today, the main methods of control are mechanical, chemical, fire and biological control.

Control programmes of invasive alien tree species in Zimbabwe and South Africa spend respectively US$100 000 and US$20 million annually.

CONCLUSION

A wide range of trees species were introduced in southern Africa (South Africa, Zambia and Zimbabwe) for a very wide range of purposes. Unfortunately some of these species have naturalized and have become invasive threatening the biodiversity of the region. South Africa is now the worst affected by invasive tree species followed by Zimbabwe.

The cost of managing invasive alien tree species has to be weighed against the economic and social benefits derived from them. The wattle industry, based on A. mearnsii for example, a major invasive tree species in South Africa and Zimbabwe contribute respectively US$75 million and US$3 million annually. P. patula, another major invasive alien tree species in South Africa and Zimbabwe is planted to an area of 337 337 ha and 49 141 ha respectively and forms multi-million dollar timber resource for pulp and paper industries in the two countries.

There is therefore, a need to look at both the advantages and disadvantages associated with each species and decisions need to be taken, species by species rather than a wholesome approach in order to best balance contradictory requirements.

A more global review on the issue of invasive forest trees, worldwide, is being initiated by CABI for FAO. The preliminary findings of the review will be outlined in the next issue of Forest Genetic Resources.

REFERENCES

SRFC. 1956. Exotic Forest Trees in the British Commonwealth. Southern Rhodesia Forestry Commission. 37 pp.

Streets R. J. 1962. Exotic Forest Trees in the British Commonwealth. Oxford University Press, Oxford. U.K. 750 pp.

Troup, R. S. 1932. Exotic Forest Trees in the British Empire. Claredon Press, Oxford. U.K.

Van Wilgen, B. W., Richardson, D. M. Le Maitre, D. C., Marais C. & Magadlela. 2001. The Economic Consequences of alien plant invasions: Examples of Impacts and approaches to sustainable management in South Africa. Environment, Development and Sustainability 3: 145-168.

⁶ Received June 2002. Original language: English.
⁷ Paper based on FAO report `A case study on the status of invasive forest trees species in Southern Africa'. Forest Resources Working Papers . Forest Resources Development Service, Forest Resources Division, FAO, Rome. (in press 2002).
⁸ Paper presented at a workshop on "Prevention and Management of Invasive Alien Species: Forging Cooperation throughout Southern Africa", 10-12 June 2002, Lusaka, Zambia
⁹ Forest Research Centre, P. O. Box HG 595, Highlands, Harare, Zimbabwe