Abstract: Background: This study reviews Pterocarpus angolensis one of the most important hardwood tree species found in the dry woodland savanna of East and Southern Africa. The tree produces one of the best timbers in the region and is also valued for its medicinal purposes. The populations of this species have declined in the wild due over-exploitation for timber, forest fires, drought and disease. The rate at which the tree is harvested is of great concern considering the number of trees in the small diameter classes. Regeneration of Pterocarpus angolensis in the wild is poor and sparsely because of poor survival of seedlings during establishment which has been attributed to damage by forest fires, harsh climatic conditions, browsing by animals, recurrent yearly dieback of seedlings, competition from other plants for resources and delayed seed production. In addition, the tree is vulnerable to mukwa disease which has killed many adult trees in Southern Africa in the past decades. Pterocarpus angolensis was recently introduced in the IUCN Red list as near threatened. Conclusion: Attempts to introduce the tree in commercial plantations failed and therefore more research and conservation efforts are required to find ways of regenerating and protecting this valuable resource from extinction.
INTRODUCTION
Pterocarpus angolensis DC is a member of the family Leguminosae: Fabaceae1. The genus Pterocarpus is composed of about 30 species found throughout the tropics except Australia and Madagascar. Four species including angolensis are found in Africa2. Pterocarpus angolensis is found in the miombo woodlands of East and Southern Africa as well as in the dry evergreen and dry deciduous forests3. Miombo woodland cover between 2.7 and 3.6 million km2 in 11 African countries4 that include, Angola, Botswana, Democratic Republic of Congo, Malawi, Mozambique, Namibia, South Africa, Swaziland, Tanzania, Zambia and Zimbabwe5-11. The tree has also been planted on a small scale in Kenya12 and Mozambique13. It grows in woodland and wooded grasslands14 from 300-1550 m above sea level15. It is an occasional or common tree in all woodland types on Kalahari sands and savanna woodland on alluvial transition soils16. Pterocarpus angolensis is the most prominent tree in the miombo woodland16,17 found growing in association with Brachystegia speciformis, Julbernardia paniculata, Julbernardia globiflora, Parinari curatellifolia, Syzygium guineense, Uapaca species and Isoberlinia angolensis6,10,17-20.
Pterocarpus angolensis grows well in well-drained, medium soil21 and most often in deep sandy22 or light soils with low-moderate fertility12 and pH of 5.5-7. Miombo woodland soils are poor in organic matter, N and P23, but Pterocarpus angolensis possess a vesicular-arbuscular mycorrhizae24 and functional N-fixing root nodules25 that enable it to efficiently utilise available nutrients23. The species is adapted to survive and tolerate severe environmental conditions such as fire and drought23. It grows well in areas with a well-defined wet and dry season and can tolerate rainfall as low12,26 as 500 mm year–1 and as high12 as 1500 mm year–1. Pterocarpus angolensis can withstand temperatures2,15,27,28 as low as 4°C.
DESCRIPTION OF THE TREE
Pterocarpus angolensis is a deciduous tree with a straight stem and leafy open flat or rounded spreading crown16,29. It is a medium to large sized tree growing up to 16 m high, reaching 28 m under favourable conditions3,29. A height of 36 m was recorded from some trees growing in Mozambique5. Stems are occasionally swollen at the base3 and can reach a breast height diameter of 50-60 cm5,12. A stem diameter of up to 122 cm was recorded in trees growing in favourable conditions12,16. Currently, there are no or very few stems in the younger age classes to eventually replace their mature counterparts in the wild. This has been exacerbated by loggers who have resorted to harvesting these immature trees to supply timber to the ever expanding market for this valuable species28. The lack of young trees indicates that the species is under threat of extinction and therefore calls for an urgent need and measures to conserve it. The bark is grey to brown or black, rough and fissured with deep cracks that make rectangular sections5,22,29,30 resembling a crocodile skin16,29. The thick bark is very resistant to fire14 and secretes a blood-red sap that contains colorant and other organic compounds such as tannin (76.7%) and muningin when damaged31.The leaves are drooping, alternate5,12,16,29, growing up to 38 cm long5 and in pairs29,30 of 5-9 with a thin and oval central leaf let up to 7 cm30. Leaflets are elliptic-lanceolate to oval, 2.5-7×2-2.5 cm; upper surface is hairless and surface hairy when young and becomes hairless at maturity29. The leaf base is rounded to slightly cordate, apex accumulate, hairy on both sides when young, with 10-14 pairs of fine lateral veins12. Leaves fall between May and June3,30 to give way to their new counterparts that emerge between September and October14. Flowers are sweet-scented, orange-yellow21,22,29,30 pear shaped and appearing in large branched sprays of 10-20 cm long29,30. They are bisexual12 and appear between August and December, just before the appearance of new leaves3,29,30. Flowers are sweetly-scented and succeeded by a distinctive round bristly pods surrounded by a stiff papery3 wing 26.
Fruits or seed pods are round with a distinctive papery wing and contain one seed30. The pods are borne in hanging clusters and can grow up to 10 cm in diameter21. They appear between January and June3,29,30 and the paper-like wing of the pod enables it and seeds within to be carried or blow away by wind from the mother tree21,25. Wind is the main seed dispersal mechanism that can even spread pods32 for several km. The hard, spiny center of the pod does not split open on its own without external forces such as physical abrasion or fire25. Fire breaks down the woody pod and facilitate germination in the wild33. Trees start flowering and seeding between 15 and 20 years of age7 and continue producing seeds until they die. The wood has a grey or creamy-white sapwood and heartwood which varies from pale golden brown to a darker reddish or purplish brown3,5. It has a density of 640 kg m–3 at 12% moisture content34, durable and resistant to fire, decay, wood rotting fungi, termite attack, terrestrial and marine borers12,35. The wood grain is straight to inter-lock with a medium to course texture12. It is easily worked with hand and machine tools36.
THE USES MUKWA
Pterocarpus angolensis produces one of the best timbers in Southern Africa22,29,33 which is in high demand for its high value8. The wood is widely used for furniture, veneer and carving and as multi purpose timber3,8,12,29,37,38. It is extensively used by artists producing wild animals to supply the curio industry21. It is flexible, resistance to marine borers and light weight which make it suitable for making boats, doors and window frames12. The wood shrink’s very little during drying from fresh condition and this together with its high durability, makes it suitable for boat and canoe building29. It is also used to make dishes, drums and mortars29 and constitutes the largest volume of wood carvings found in curio shops in Southern Africa39. Due to its high value, the timber is only occasionally used as firewood or to make charcoal12,30. The wood produces a rich, resonant sound and can make many different musical instruments29.
The bark, roots, flowers, sap and seeds are used in traditional medicine30 to cure various diseases. The sap is used to stop nose bleeding29, kill ring worms and cure ulcers40. Several studies reported that the sap is used to treat eye cataracts, malaria, blackwater fever, skin inflammation29,31 and urinary schistosomiasis41,42. The bark can be heated in water and mixed with figs to message breasts to stimulate milk production29. A cold infusion from the bark provides a remedy for nettle rash and is also used to relieve stomach disorders, headaches and mouth ulcers29. The bark can be peeled off with its red sap and used to treat diarrhoea, heavy mensuration, nose bleeding, headache, stomach-ache, schistosomiasis, sores and skin problems12. The bark has also been used to poison fish. The bark or roots can be boiled with fresh meat and used as a preliminary accelerator in treating gonorrhoea13. Cleaned roots can be soaked in clean water to produces an eye wash and used in cleaning corneal ulcers29. The powder from crushed seeds can be applied as a wound dressing12 to speed up the healing process.
The heartwood from roots can be pounded to make a brownish red dye that has been used in the cottage industry in Namibia and Zimbabwe to dye palm-leaf fibres used in the basket weaving industry12. Pterocarpus angolensis fixes atmospheric nitrogen12,30 which is beneficial to the soil. It has also been used in soil conservation projects to fix sand dunes12 and as an ornamental tree. Leaves and shoots are browsed by wild animals, especially elephants43. The tree is liked by elephants and evidence show that many trees in Botswana and Zimbabwe have been pushed over to make leaves more accessible44,45. Elephants strip off the bark to access the red sap and this exposes trees to fire damage and mukwa disease33,46-48. Flowers are a source of forage for bees12. Pods are eaten by baboons and monkeys43.
MANAGEMENT OF MUKWA
Regeneration of Pterocarpus angolensis in the wild is poor and sparsely9,49,50 because of poor seedling surviva during establishment10. The low survival rates have been attributed to forest fires, hostile climate, animal browsing8,24,28,51 and recurrent annual dieback of seedlings, competition from other plants for light and delayed seed production9. Boaler25 estimated that only 2% of seeds produced germinate in the wild. Studies conducted in the Copper belt of Zambia recently did not record seeds to a depth of 20 cm and observed only 95 saplings ha–1 in disturbed miombo woodlands52,53. Caro et al.9 recorded few seedlings in areas covered by thick grass and dense tree canopy suggesting that Pterocarpus angolensis requires light to achieve its potential growth rates. Pterocarpus angolensis is a pioneer species in burnt areas2.
Natural regeneration in the wild has shown to be a bottleneck due to failure by young seedlings to survive during establishment10. Seed germination in situ is positively influenced by forest fires25,54 that burn and supress existing vegetation to minimise competition for nutrients, moisture, sunlight and growing space23. Forest fires also remove wings and bristles from pods, crack the seed and improve contact with soil2,24,25,55. Forest fire slows seedling development but accelerate the growth of saplings25. Several attempts to raise seedlings in the nursery38 and establish commercial plantations have not succeeded in many countries1,39,40,49,55. This has been attributed to delayed seed production, failer of pods to open without seed being damaged, low seed germination rates, slow tree growth, competition from dense grass and other trees for sunlight9,25. However, small scale plantings were successful in warm areas of Mozambique, where clear-cutting, fire control and weeding was done in the first 10 years after planting13.
Individual seedlings of P. angolensis remain in a suffrutex stage for a number of years with yearly diebacks during the dry season until the root system has grown deep enough to extract sufficient moisture and nutrient to support a shoot to survive harsh conditions during the dry season2,25,32. This phenomenon makes it difficult to use the species to establish plantations25. High densities of suffrutescent structures have been reported within the population in the wild25,56. The suffrutex stage last for about 14 years25, but many seedlings fail to survive this stage because of drought, forest fires, lack of nutrients and browsing. The suffrutex stage is stimulated by above average rainfall, protection from fire or browsing and release from competition to enter stages of rapid above ground growth25,32. Rapid growth of P. angolensis from seedling to sapling stage is stimulated by availability of light, lack of fires and competition for resources25. Trees growing in suitable sites can start flowering and seeding 15-20 years after reaching the sapling stage7,34.
The species can be propagated by seeds and stem cuttings12. However, stem cuttings have a low survival rate in the nursery57 and poor results were reported in Malawi, Mozambique, South Africa Swaziland and Zimbabwe32. A single tree can produce 100-400 fruits and 1 kg of seeds holds 3400-4000 seeds12,58. Seed collection is difficult because the pods are not easy to open without damaging seeds. The hard, spiny centre of the pod does not split open without physical abrasion or other mechanisms for seeds to germinate25. Pods can be opened using secateurs. Several studies found that germination of untreated seeds was irregular both in the wild and nursery39,57,59 because seeds require moderate levels of fire to stimulate their germination33. Germination rates of 30-70% were recorded 3 weeks to 6 months after sowing32,60. Germination of P. angolensis seeds is not hindered by dormancy caused by water impermeable hard seed coat61,62. In addition to fire, physical, mechanical and chemical scarification treatments may also be used to stimulate seed germination28. Chisha-Kasumu et al.10 used different pre-treatment methods and recorded 72-99% germination in seeds that were nicked which was significantly higher than other treatments.
FACTORS THREATENING MUKWA
The unregulated exploitation of Pterocarpus angolensis started way back in the 1950s or earlier in the Democratic Republic of Congo, Tanzania, Zambia and Mozambique7,63-65. The tree has been harvested either legally or illegally to supply its market which is constantly expanding66. Timber exploitation for either the local market or export7 has significant enhanced decline in populations39. Harvesting big trees has led to a truncated size class profile39. Loggers have resorted to immature small diameter trees due lack of big trees28. Immature trees are felled before they become fertile to produce seeds for the next generation trees, which has probably contributed to the decline in the population density noted all over southern Africa2,32. Land clearing for a arable agriculture, housing, infrastructural development, human population expansion and heavy browsing of small trees also contribute to the decline in the species. Several studies have attributed he decline in the population density to annual forest fires and damage by elephants47,48,67. The World Conservation Monitoring Centre carried out an assessment of P. angolensis status68, after which it was introduced on the IUCN Red List as near threatened69.
A disease that is killing P. angolensis is well documented in Botswana47,48, South Africa23,70,71, Zambia and Zimbabwe23,72. The disease is commonly known as mukwa disease which refers to the common name of the tree in these areas23,73. The characteristics of the disease differ from country to country. In Botswana, Zambia and Zimbabwe the disease is characterized by wilt, dieback, bark discoloration, vascular and phloem streaking and the production of epicormic shoots48,72. In South Africa branch dieback, heart rot and the death of mature trees are characteristics of the disease70,71. Symptoms are most intensive during the rainy season23 and infected trees occur in patches and die 2-3 years after infection74,75. Isolation of Fusarium oxysporum Schltdl from discoloured wood revealed that the fungus cause the disease that is attacking and killing P. angolensis trees23,32,72,74-76. Rainfall records from Zambia and Zimbabwe correlated the disease and death of P. angolensis to several years of drought23. This fungus causes damping off, crown and roots rots as well as wilting diseases77 in many leguminous trees78.
CONCLUSION
Pterocarpus angolensis is a very useful multipurpose tree in east and Southern Africa that is threatened by excessive exploitation for is valuable timber and by land use changes. Over exploitation which is unsustainable endangers natural populations of Pterocarpus angolensis in its habitat. Forest fires, elephant damage and failure of seedlings to survive during establishment are also implicated in low population densities of mukwa. Attempts to use it in commercial plantations were not successful. A large scale collaborative research is required to find ways of improving Pterocarpus angolensis seed germination rates, reducing the suffrutex stage of seedlings and accelerate the growth of saplings if this valuable resource is to be saved from extinction.