Pinus patula is a Central American closed cone pine, which is restricted to Mexico, growing on the Sierra Madre Oriental in the eastern part of the country. Its latitudinal range of distribution is from 16°N to 24°N, between altitudes of 1 500 and 3 100 masl (Wormald, 1975; Dvorak et al., 2000). It is generally restricted to moist temperate to subhumid temperate climates, although it has also been recorded in subtropical climates. The annual precipitation in the form of rain varies from 1 000 mm to 2 500 mm, with most of the rain falling between June and October. The dry season period (with monthly precipitation of less than 40 mm) varies from none at some sites, to up to five months at others, between the months of December and May. The average temperature at Pinus patula sites ranges from 10(to 18°C. The soils are usually deep, well-drained and fertile clay soils.
Pinus patula was first introduced into South Africa in 1907. Around 1920, the species was also introduced into Zimbabwe (Poynton, 1979a; Barrett and Mullin, 1968). The species was later introduced into Zambia, where it failed, but the exact year of introduction is not known (Poynton, 1979a). P. patula became a major plantation species in Zimbabwe when, in 1934, it displaced Pinus radiata as principal coniferous species following severe fungal attacks (Diplodia sp. and Dithistroma sp.) on P. radiata. By 1970, a total of 36 000 ha were under P. patula, representing 74 percent of the area under pines in Zimbabwe. Today, P. patula is grown on about 50 000 ha, i.e. some 60 percent of the area planted to pines in Zimbabwe. In South Africa, the area of P. patula plantations was 223 600 ha in 1970, and in 2001 was still the principal coniferous species, growing on about 337 000 ha, which is almost half of the area planted with pine. South Africa has the largest area planted with this species in the world, and Zimbabwe could be ranked third or fourth. The predominant areas of production are cool, mist-belts and wet high altitudes (1 500 to 1 900 masl), summer rainfall environment of the eastern districts of Zimbabwe, and also the cool, mist-belts and moist summer rainfall areas in Mpumalanga and Kwazulu-Natal provinces, as well as on a smaller scale in Eastern Cape province. The species thrives on a wide variety of soils, including those derived from dolerite, granite, dolomite, quartzite and sandstone.
Pinus patula is a precocious species, flowering as early as two years (Barrett and Mullin, 1968). In Zimbabwe, female flowers appear by three years of age, while male flowers appear a year later, with viable seed being set by the fifth year. The species produces abundant cones by eight years old. Flowering, cone production and seed set is poor at altitudes below 900 masl in South Africa and at altitudes below 1 200 masl in Zimbabwe (Barrett and Mullin, 1968; Denison,1973; Barnes and Mullin, 1974). The cones of P. patula are serotinous and may remain unopened for one to two years after ripening. The seed may remain in the cone for seven years with loss of viability of less than 40 percent.
Pinus patula has invaded an area estimated at 17 600 ha in South Africa (Richardson et al., 1994), while in Zimbabwe the area could be over 100 000 ha. The species is an aggressive invader in medium- to high-altitude areas (above 1 500 masl in Zimbabwe and above 1 200 masl in South Africa) where it flowers and seeds well. The species has invaded afromontane forests, miombo woodlands, and grasslands. Although the area invaded by this species in South Africa is comparatively small, the species nonetheless is considered a serious potential invader. The area quoted in South Africa is low mainly because, although P. patula is currently planted in over 337 000 ha (see Table 1), much of the area was planted in the late 1960s to early 1970s, which coincided with greater awareness of problems posed by invasive species, as well as the start of deliberate control programmes against alien tree species invasion (Le Maitre, pers. comm.). It is therefore likely that P. patula was not allowed to reach its full potential for invasion. The species has all the attributes of a successful invader in both South Africa and Zimbabwe (Richardson et al., 1994; Rejmánek, 1996; Richardson and Higgins, 1998).
In South Africa and Zimbabwe, the species flowers and produces seeds within five years (i.e. short juvenile phase), is a prolific seeder every year and its seed averages only 0.127 seeds per milligram. The cone crop is nearly always good every year and the serotinous cones give the seed an extended storage life. The seed, which is winged, is blown by wind for several kilometres. Although the species was introduced in South Africa in 1830, it was only planted extensively in the 1970s, i.e. its residence period on an extensive scale is less than 50 years. In Zimbabwe, the species was extensively planted from around 1934, but a deliberate effective control programme only began in the late 1990s. All these factors and attributes have probably contributed to making P. patula one of the most invasive or potentially invasive Pinus species in southern Africa.
Figures for the contribution of Pinus patula in isolation to the GDP of both Zimbabwe and South Africa are not readily available. Planted forests total about 118 000 ha in Zimbabwe and 1 400 000 ha in South Africa. The area planted with P. patula is about 49 000 ha in Zimbabwe and 337 000 ha in South Africa (see Tables 1 and 3). Forestry in these two countries contributes 3 and 2 percent respectively to national GDP. Everything being equal, P. patula then roughly contributes 1.2 percent to the GDP of Zimbabwe and 0.5 percent to South Africas GDP. This a crude estimate, but good enough to indicate the economic importance of commercial P. patula plantings.
Pinus patula was recently declared an invader species in South Africa. The species was also declared a transformer species (Government Gazette, 2001). This classification may not have created serious conflict of interest with those commercially exploiting the species for timber and pulp. According to new regulations, as an invader species, those growing it are expected to follow the published regulations to minimize further infestations and to bring under control all old infestations or invasions. Indications are that biological control will not be considered as a control option in the near future, probably due to the conflict which arose between environmentalists and growers when biological control was used against Acacia mearnsii (Le Maitre, pers. comm.).
The majority of the control programme of old infestations in South Africa and Zimbabwe is by mechanical and chemical control. In Zimbabwe, some private companies are already experimenting with alternative species that are less invasive (van der Lingen, pers. comm.). Such species include Pinus tecunumanii, which is a known shy flowerer and seeder (Nyoka and Tongoona, 2000). In South Africa, there are reports that research is underway to develop sterile clones of Pinus patula (Richardson, 1998). The cost of controlling P. patula alone in both South Africa and Zimbabwe has not been estimated. The classification of P. patula as an invader and transformer species, together with the regulations in South Africa, is expected to expedite the eradication and control of this species in areas already affected, and is also expected to reduce the spread of the species to new areas.
The black wattle, Acacia mearnsii De Wild. (Syn. A. mollissima Willd.; Acacia decurrens var. mollis Lindl.) is a native of Australia (Tasmania, Victoria, Queensland, South Australia and New South Wales). The natural range of distribution is bound by latitudes 34°S and 44°S, and altitudes from sea level to 885 masl (Sherry, 1971). The annual rainfall, which ranges from 470 to 1 500 mm, shows no particular pattern.
In southern Africa, A. mearnsii flowers from late July to October. The period from fertilization to pod ripening is usually 14 months. A pod normally contains 1 to 14 seeds, but the average is 7 seeds. There are between 48 600 to 70 200 seeds per kilogram. The tree has a very short juvenile period as it begins to flower at about 20 months, and ripe seed is occasionally found on 3-year old trees. From the fifth or sixth year onwards, annual seed crops are normally copious. By contrast, in its native range, good seed years are relatively infrequent, and often no seed at all may be produced in some years, mainly because of the action of insect pests. Unlike the African acacias, that are essentially pioneer species, i.e. pioneering succession from grasslands to savannah or from savannah to drier-type forests, the black wattle occupies a higher position in that it is present as a normal component of the climax high forest in its native habitat (Sherry, 1971). The seed is spread by running water, and heat from fire improves seed germination. The species has all the three attributes of a successful invader: short juvenile phase; good annual seed crop; and small seed size.
The species reached southern Africa in about 1864, when it was introduced into South Africa. The purpose of introduction of this species was for shade, windbreaks and fuelwood, and later for its tannin bark (Troup, 1932; Streets, 1962). The species was extensively planted as a shade tree and shelterbelt, before the first commercial plantations for tannin bark in the last quarter of the nineteenth century. One hundred years after first introduction (1964), there were close to 324 000 ha of A. mearnsii in South Africa (Sherry, 1971). The area then progressively declined until it reached the current official level of about 107 000 ha (DWAF, 2001), although unofficial estimates quote 132 000 ha. A. mearnsii was introduced into Zimbabwe in about 1902 (Troup, 1932; Streets, 1962; Barrett and Mullin, 1968; Sherry, 1971). The first planting of the species in Zimbabwe was in what is now Nyanga and Matopos National Parks, then owned by Cecil John Rhodes. This was followed by commercial plantation of about 110 ha planted in 1923. Large-scale commercial plantation forestry began in 1946, and in 1950, a total of 8 737 ha were recorded throughout the country, but with 98 percent of the area being in the eastern districts of Nyanga, Chimanimani, Makoni and Mutasa. By 1962, the area planted had reached 26 000 ha, all in the eastern districts, with only 85 ha in the central part of the country (Sherry, 1971; Lyut et al., 1986). Today, the area under A. mearnsii has shrunk to 11 400 ha, all confined to the southern part of the eastern highlands of Zimbabwe (van der Lingen, pers. comm.). A. mearnsii was introduced into Zambia between 1950 and 1954, but the actual year is not known. However, the species failed to establish.
Acacia mearnsii is now considered to be a major invasive tree species in South Africa, where it is estimated (based on somewhat subjective data) to have invaded some 2 500 000 ha. Figures for Zimbabwe cannot be ascertained as yet, but potentially over 200 000 ha is affected, most of it in the mist belt of the eastern highlands. The species does not seem to produce viable seed at lower altitudes, which explains why most of the other plantings in the country, which were not in mist belts, never became invasive in Zimbabwe. In South Africa and Zimbabwe, the species is the most important invader of riparian habitats, roadside and veldt, grasslands, forest edges and woodlands. It is believed to be spread by running water, which probably explains its distribution as an invader of stream banks (van der Lingen, pers. comm.).
The source of infestation in both countries is commercial plantations and neglected jungle stands of the species established from earlier invasions. The species was declared an invader species in South Africa in 1984, after years of protracted debate on proposals for the species to be declared a weed. The species is rated as the number eleven invader in the fynbos biome, fifth in Natal and sixth in Transvaal (Pieterse and Boucher, 1997). The species is also considered to be the major invader in the mist belt of the eastern highlands of Zimbabwe.
Currently, the commercial black wattle industry contribute some R 800 million to South Africas GDP and employs over 22 000 people. In Zimbabwe, earnings are estimated at US$ 3 million and provides employment for some 2 000 people (van der Lingen, pers. comm.). Its uncoordinated control could create a potential conflict of interest.
The classification of A. mearnsii as an invader species in South Africa and not as a weed obviously brought relief to those who grow it as an industrial species. As an invader species, those wishing to grow this species are supposed to have a permit and are expected to ensure that the species is kept under control and prevented from escaping by following stringent regulations set up for invader tree species (Government Gazette, 2001).
Theron (1978) described the most effective methods of controlling Acacia species. The area infested with seed should be burnt with fire. The heat, which stimulates germination, will ensure the germination of practically all the seeds. After germination, the area is then sprayed with herbicides such as 2,4,5-Trichlorophenoxy-acetic acid (2,4,5-T) or Glyphosate (Roundup®), while any seedlings that survive may be pulled out by hand or burned again. Where feasible, Theron (1978) suggested the use of heavy-duty rotary slashers. If the area has large trees that are not easy to slash, such trees can be ring-barked or also sprayed with 2,4,5-T. Besides chemical control using herbicides, biological control of A. mearnsii is now possible in South Africa, where a seed-feeding insect and a fungus (mycoherbicide) have been identified and are now available. The insect, a seed-feeding weevil (Melanterius maculatus), was imported from Australia (PPRI, 2002). The weevil has already been released in South Africa, except in KwaZulu-Natal and Mpumalanga provinces, where there is a conflict of interest with the wattle growing industry. Two gall-forming midge species, Dasineura sp. and Asphondylia sp., also from Australia, are still being evaluated in quarantine to supplement the seed-feeding weevil in reducing establishment (PPRI, 2002). A locally occurring fungus, Cylindrobasidium laeve, which kills freshly cut stumps of A. mearnsii, was registered as a mycoherbicide (Stumpout®). The cost of controlling A. mearnsii, which at a rough estimate has invaded over 2 500 000 ha in South Africa, is about US$ 1 600 million. The cost has not yet been estimated for Zimbabwe.
 P. patula Schiede ex
Schect.& Cham var. patula; P. patula Schiede ex Schect.&
Cham var. longipendunculata Loock ex Martinez. The common name in English
is Patula pine or Mexican weeping pine, and in Spanish, Pino