A. hockii is a widespread acacia shrub or small tree found in moist savannah landscapes of sub-Saharan Africa. In East Africa particularly, it is one of the most common small Acacia species, and was noted as invasive in parts of...
A. hockii is a widespread acacia shrub or small tree found in moist savannah landscapes of sub-Saharan Africa. In East Africa particularly, it is one of the most common small Acacia species, and was noted as invasive in parts of Uganda more than 50 years ago. It appears to be a typical ‘native invader’, increasing in plant density dramatically following certain changes to land management such as altering the fire regime or grazing system and/or changing climates. There are no records of intentional or accidental introduction of A. hockii to other countries, though there is a risk of invasion in other parts of its native range. Chemical control was shown to be effective but uneconomic, and cultural techniques may prove to be more cost effective.
Although there have been some taxonomical issues regarding A. hockii in the past, it appears that its limits are now generally accepted. However, it is still incorrectly included as a synonym of the morphologically similar A. seyal Del. in some publications and databases (such as FAO, 2013 and ICRAF, 2013), and that continues to cause confusion. Although the two are closely related, it seems preferable to maintain them as distinct species.
In their monograph on A. seyal, Hall and McAllan (1993) stated that the then current view of A. seyal was essentially consistent with that put forward by Hutchinson and Dalziel (1958), when it was indicated for the first time that both A. seyal and A. hockii were widespread in tropical Africa. Hall and McAllan (1993) added that as distinctions between A. seyal and A. hockii became better appreciated, varieties originally described under the name A. seyal were adopted as synonyms of A. hockii.
A treatment of African Acacia species by Vassal (1972) placed A. hockii as one of 18 species in subsection Uniseriae, and in a subgroup of ten species with dehiscent pods and capitate inflorescences, also including A. ehrenbergiana, A. karroo, A. kirkii. A. macrothyrsa and A. xanthophloea.
Vachellia hockii is also treated as a synonym here, being part of the reclassification of the entire Acacia genus that is proposed by some taxonomists but not universally accepted, though it has been adopted by some databases (such as USDA-ARS, 2013). The type specimen is from Katanga, DR Congo. Further taxonomic revision may be expected.
A. hockii is a multi-stemmed shrub 2-4 m tall, or a small tree to 6-7 m tall with an open crown occasionally 9 m wide, with 12 m also recorded in exceptional individuals. Bark is red-brown to greenish or greenish-brown, rarely pale-yellow, peeling off in papery layers, brown when not burned or grey brown and in plates where burned, yellow underneath. Inner bark is not powdery (as it is in A. seyal). Young twigs and branchlets puberulous to densely puberulous, rarely glabrous, with sessile glands, reddish, sometimes numerous, usually elongate and slender. The thorns are spinescent stipules, short (up to 2 cm long, rarely 4 cm), straight, suberect or spreading, subulate or flattened on the upper side, with ‘ant-galls’ and other prickles absent. Leaves have (1)2-11 pairs of pinnae; each with 9-29 pairs of leaflets, 2.0-6.5 mm long and 0.5-1.2 mm wide, usually densely ciliolate but sometimes glabrous, obtuse to acute but not spinulose-mucronate at the apex, with lateral nerves invisible beneath. Leaves often have a gland on the petiole and between the top 1(3) pairs of pinnae. Flowers are bright yellow or orange, in axillary, pedunculate heads 5-12 mm in diameter. The pods are reddish brown, narrow, straight or crescent shaped, puberulous and (4)5-14 cm long and 0.3-0.8 cm broad. Seeds are olive-brown, 5-7 x 3-4 mm, smooth, elliptic and compressed.
A. hockii is native to many dry areas in tropical Africa south of the Sahel, to eastern and southern Africa, and also Yemen (ILDIS, 2013). It is present in Saudi Arabia (GBIF, 2013) and likely to be native there also. In West Africa it occurs in the moister savannah regions of the Guinea zone. There are no records of intentional or accidental introduction of A. hockii to other countries.
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
A. hockii is not a new invader. Harrington (1968) noted that it was the most widespread small Acacia species in East Africa, and 45 years ago it already occupied 77,700 square kilometres (30,000 square miles) of potentially high quality rangeland in Uganda. It has proved particularly invasive in south-western Uganda. In the Lake Mburo National Park, Mbarara District, NARO (2004) observed that A. hockii had become a major problem since the mid-1990s. In East Africa, it tends to invade overgrazed grasslands and evergreen thickets along with other species, including A. drepanolobium, A. kirkii and A. seyal.
Intentional introduction of A. hockii appears unlikely, as it is rarely found on lists of useful plants and is not identified as a plant for reforestation purposes, as are many other Acacia species. It is, however, listed by Parker et al. (2007) as a weed with the potential to invade the USA, and its introduction is prohibited in Australia.
A. hockii occupies a wide range both of habitat and altitude, from sea level to at least 2,400 m altitude. In West Africa, A. hockii is found in moister savannah regions of the Guinea zone. In East Africa it is found in deciduous woodland, wooded grassland and deciduous and semi-evergreen bushland, thickets and scrub. It can become common on sloping or rocky ground, and is often associated with poor soils, especially sandy soils, where it often becomes the dominant shrub.
There is little specific information available in the literature regarding the biology of A. hockii, though it may be considered similar to that of the closely related A. seyal (see Hall and McAllan, 1993). A. hockii can withstand intensive browsing and is particularly fire tolerant, and tends to resprout vigorously after cutting or burning. Harker (1959) includes a description of A. hockii growth and development, and the effects of heat on seed germination.
Different chromosome numbers have been reported, with Hamant et al. (1975) suggesting 2n=52, whereas Renaud et al. (1983) indicated that it was 2n=26. However, as with other African Acacia species, it is possible that more than one ploidy level exists in A. hockii.
A. hockii is a principal component of several plant associations. In Ethiopia, Heteropogon contortus-Acacia hockii was one of seven plant community types identified in Gamo Gofa Zone in the south (Soromessa et al., 2004), and Tikssa et al. (2010) noted Tamarix nilotica-Acacia hockii as one of the seven community types identified in the Awash valley. In Tanzania, A. hockii is common in Brachystegia woodland. In the Serengeti it can be one of the dominant species in Acacia-Commiphora wooded grassland and in the open understory in Combretum-Terminalia secondary wooded grassland. In the Maruzi Hills Forest Reserve in Uganda, species with the highest proportional abundance were Combretum molle (23%), A. hockii (17.7%), Combretum collinum (16.1%), Grewia mollis (6.5%) and Lannea barteri (6.5%).
In East Africa A. hockii tends to invade overgrazed grasslands and evergreen thickets along with other species, including A. drepanolobium, A. kirkii and A. seyal.
A. hockii is a tropical species, preferring moist savannah regions, and so is generally absent in the true Sahelian zone and other drier savannahs where other Acacia species generally dominate. It can tolerate very high temperatures but is intolerant of frost. It also tolerates poor soils.
Pods and seeds form part of the diet of native species and livestock (Gwyne, 1969), and elephants, antelope and chimpanzees seek out the dry pods (Wickens et al., 1995). Seed dispersal is likely to be aided by these animals. Seeds might also be dispersed by water where plants exists in areas liable to flooding.
Intentional introduction of A. hockii appears unlikely, as it is rarely found on lists of useful plants and is not identified as a plant for reforestation purposes, as are many other Acacia species.
Invasion of A. hockii in natural and semi-managed grasslands can drastically reduce stocking density by impeding access to livestock and by reducing the presence and growth of palatable forage species due to competition for light and water.
A. hockii also harbours the unpalatable grass Cymbopogon afronardus [Cymbopogon nardus] by restricting grazing and trampling beneath it. The costs of removing A. hockii and recouped costs from increased cattle production are discussed by Harrington (1970).
The species has a wide range of uses throughout its native range as a source of fuelwood, exudate gum, resins, bee forage, bark fibre and fodder, as well as having various medicinal uses (Burkill, 1985).
The wood has moisture content of 12% and air-dry wood density of 0.58-0.64 g/cm3 (Bolza and Keating, 1972). The timber is hard and straight grained, but its small bole size, susceptibility to termites and decay and availability of other species means that it is not reported as a source of sawn timber. Small wood is used in rustic construction for homes and shade housing for cattle, and thorny branches are used for cattle pens and fencing. Where common, it is often used as a source of firewood, and the high fixed carbon in A. hockii makes it ideal for charcoal making (pyrolysis) (Musinguzi et al, 2012).
A. hockii produces tannins in comparable amounts to the black wattle A. mearnsii, but the protein precipitating capacity of A. hockii is different, suggesting that poor quality leather would be produced. However, mixtures of extracts from different native Acacia species resulted in protein precipitating profiles similar to that of wattle extract (Mugedo and Waterman, 1992). The bark also yields a fibre for rope making and basketry in Tanzania.
Leaves form part of the diet of native species and livestock, as do pods and seeds (Gwyne, 1969). Foliage and pods are browsed by goats, flowers are eaten by baboons, seed by chimpanzees, and elephants and antelope seek out the dry pods and new leaves after fires (Wickens et al., 1995). Chemical and mineral composition suggest that A. hockii has low nutritive value compared to 15 indigenous Kenyan browse plants selected by Ondiek et al. (2010), and medium palatability amongst 40 species tested in Ethiopia (Kaitho et al., 1996), with high levels of tannins in the leaves (Kaitho et al., 1998).
The flowers are sweet scented and are a good source of bee forage (Wickens et al., 1995). A. hockii leaves are also reported as a feed for the African wild silkmoth Gonometa postica (Lepidoptera: Lasiocampidae) (Ngoka et al., 2007), though it is not known whether there is any commercial production.
The inside of the bark is edible as a famine food. Inner bark fibre is also chewed for its juice, which has a sweet taste, and the Maasai chew the white inner bark to slake thirst.
The species also yields an edible, translucent, reddish exudate gum. Its chemical composition was reported in Anderson et al. (1984). Wickens et al. (1995) stated that it is marketed as African gum arabic but is inferior to true gum arabic from A. senegal. It is used by the Mbeere tribe in Kenya as an adhesive.
Many medicinal uses are reported, including as a pain killer, vermifuge and to treat stomach troubles, dropsy, swellings, oedema and gout (Burkill, 1985). In Kenya it has been used against malaria, abdominal pains and applied to abscesses, and in Tanzania, boiled bark is given to children with fever, and a root decoction is taken for hookworm, and for the treatment of tuberculosis and related ailments in Uganda (Tabuti et al., 2010).
A. hockii may be confused with the closely related A. seyal, as both share similarly coloured peeling bark, tree form, size and general morphology. Means for their separation in the field include the presence of powdery bark in A. seyal which is not present in A. hockii, and the pods of A. seyal, which are glabrous as opposed to those of A. hockii which are usually puberulous. A. hockii twigs are usually more elongate, slender and puberulous than those of A. seyal, and have reddish or brownish bark which does not peel to expose the inner layer, a characteristic of A. seyal. A. hockii also prefers moister savannah zones as compared to A. seyal, which tolerates drier sites (Burkill, 1985).
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Cultural Control and Sanitary Measures
Harrington (1974) experimented with the use of fire for controlling dense stands of A. hockii approximately 3.5 m high in Uganda, on a savannah pasture dominated by Cymbopogon afronardus, Hyparrhenia filipendula and Themeda triandra. Treatments were fire protection, early and late triennial burns, early and late annual burns and two burns per year, applied to four grazing managements. Clear differences were apparent in both the grass sward and the structure of the shrub layer after three years. With no fire, the A. hockii stand became a tangle of interlocked branches which restricted cattle movement. Early annual burning and two burns per year both produced relatively open grassland with stunted bushes. Late annual burning caused a denser bush growth than early burning, but as the bushes were mainly single stemmed with few low branches, cattle movement was not greatly impeded.
In Cameroon, Peltier and Eyog-Matig (1989) found that the rate of production of A. hockii, Dichrostachys glomerata and Piliostigma was 0.15 and 0.18 m3/ha for unbrowsed but burned stands (fires early or late in the dry season, respectively), and 0.20 m3/ha for browsed plots and 0.41 m3/ha on burned plots. Browsing improved wood production by reducing competition from herbaceous vegetation and the restriction of fire is seen as crucial to successful regeneration of these woody species.
Manual weeding with a hoe is a widely used control method in Uganda, but it takes up to 50-75% of farmers' time, and with labour being scarce and very expensive, weeding is often delayed by a month or more, and use of oxen and tractor-mounted equipment is impracticable in many situations (Wetala, 1978). Small trees can regrow their biomass in 3-4 years, and if wood harvesting is reduced to below four years, this can help to prevent rapid invasion of A. hockii and help the recolonisation of natural pastures.
The earliest record of chemical control is Harker (1959), who reported that a 2:1 mixture of 2, 4-D and 2,4,5-T in dieseline gave only 25% control after 6 months when applied to cut stems, though mortality increased to 88% when trees were burnt after spraying. Application to tree trunks 6-10 cm in diameter was ineffectual, but gave good control on regrowth from cut stumps. However, 2,4,5-T is now banned and should no longer be used.
In western Uganda, foliar applications of picloram gave more effective control of A. hockii than picloram granules or foliar treatment with 2, 4-D + picloram (Anon., 1968; Harrington, 1968). Harrington (1968) also found that A. hockii was more susceptible to applications applied in February, August, September and October than in other months, when applications killed 30-80% of trees, though with no apparent advantage in spraying at any particular time of day
Harrington (1970) found that applications of picloram to cut stumps of 20 cm diameter was more efficient than foliar applications, and observed that vigourous regrowth 12 months after treatment had died back after 19 months. A stump treatment of a 4:1 mixture of 2, 4-D and picloram (much cheaper than pure picloram at the time) killed 100% of plants (Harrington, 1970).
In trials established in 1968 in Uganda, picloram or a 4:1 mixture of 2,4-D + picloram were applied to A. hockii bushes by (a) a technique that simulated aerial spraying, and (b) spraying directly on the foliage (Harrington, 1972). Results in November 1969 showed that (a) was unsatisfactory but (b) killed more than 70% of plants, although only at the higher herbicide concentrations. However, the last result was poorer than results in previous trials, possibly due to moisture stress inhibiting herbicide translocation. Supplementary application of picloram granules improved the mortality rate but not enough to warrant the expense. The 2,4-D + picloram mixture gave a poorer mortality rate than picloram alone even when the solutions contained the same amount of picloram. Previous burning of treated areas affected the results, but also made the movement of spraying teams much easier (Harrington, 1972).
In experiments carried out at Ankole, Uganda (Harrington, 1973a), A. hockii bushes less than 3 m high with trunks 3-10 cm in diameter were cut at about 15 cm above ground and the stumps treated on five different dates with the following herbicides: picloram diluted 2:1 with water, picloram + 2,4-D both undiluted and diluted 2:1 with water and a commercial 2,4-D + 2,4,5-T (now banned) mixture diluted 40:1 in dieseline. Picloram treatments gave the best results and were more effective and cheaper than the 2,4,5-T + 2,4-D mixture. June treatments were least effective but August treatments killed 100% of plants. Prior burning of bushes increased their susceptibility to herbicides. Burning after treatment had no effect. In Ankole and Masaka districts, A. hockii is usually associated with the grass weed Cymbopogon afronardus and it is desirable to clear both species at the same time (Harrington, 1973a).
In contrast, however, Harrington (1973b) also draws attention to the possibility that expensive clearance of bush (and especially of A. hockii) on savannah grazing land in Uganda may not lead to any appreciable increase in grazing value, and discusses the possible reasons for this, including loss of fertility from leaf-fall, insects and birds, loss of shade, and an increased rate of surface soil drying.
Monitoring and Surveillance (incl. Remote Sensing)
Mapping is an important tool in aiding management decisions. Spatial prediction showed the potential distribution of A. hockii in Burkina Faso, highlighting the areas of high and low occurrence, useful for the management and valorization of forest resources under climate change and growing human pressure. Validation showed that the models for A. hockii were stable and adequate, with a high Spearman correlation value of 0.84 (Traoré et al., 2008).
NARO, 2004. Implementation of invasive plant prevention and control programmes in Uganda. Report submitted to the CAB International Africa Regional Centre under the PDF-B phase of the UNEP/GEF Project: Removing Barriers to Invasive Plant Management in Africa. Entebbe, Uganda: National Agricultural Research Organisation.