Acacia nilotica (L.) Del. (Mimosaceae)

A. arabica (Lam.) Willd.

A variable and widely used tree from Africa and the Indian subcontinent. Subsp. indica, introduced to northern Australia, has spread into dry grassland, slowly at first, but then with population explosions during wet years.

Species characteristics

Life form, size, lifespan

Thorny small tree growing to a height of 7 m. Lives at least 60 years.

Taxonomy, variation and plasticity

Nine sub-species are recognised. Hybridization between the various ssp. occurs and has been influenced by human's seed dispersal. Within the A. nilotica complex the pods are very variable.

Reproductive biology

The yellow sweetly scented flowers are nectarless and found in round heads. Most flowers are functionally male with a few hermaphrodites and are mainly bee-pollinated. Pods are indehiscent. Seed production is high (up to 175,000 seeds/tree) and yearly provided water is plentiful like along water courses. In Kenya seed set per pod was 10.8 and trees produced between 153 and 34,000 seeds. It forms large seedbanks and in Australia the half-life of seeds is 10-12 months.

Leaf production and fall are affected by rainfall whereas temperature affect flowering and fruiting. In Sudan A. nilotica flowers irregularly but generally between June and September and seed fall takes place from march to May. In Australia trees flower from March to June and green pods are produced within four months but ripe pods fall from November to February. Most of the leaf fall occurs during the dry period when the tree bears green pods.

Resilience and resistance

Species is sensitive to frost but is tolerant of grazing, drought (< 2 years), fire and salinity.

Environmental requirements and successional status

Requires water for seed germination and seedling establishment. Can regenerate under pioneer woody plants in seasonally flooding habitats and subsequently outcompete them.

Products and uses

Extensively used as browse, firewood and timber. The bark and seeds are a source of tannin while bark, roots and flowers are used for medicinal properties. The gum is sometimes used. The wood is suitable for paper production. Planted for forestry or reclamation of degraded land.

Status in native range

Range and abundance

Native to India, Pakistan and much of Africa where it is often dominant.

Climate

Grows in areas where the mean monthly temperature of the coldest month is above 16°C. It tolerates a wide range of rainfall from less than 350 mm to more than 1500 mm or more per year.

Site requirements

A. nilotica subsp. indica is commonly found on soils with a high clay content, but in areas of higher rainfall it may grow on deep sandy loam. Often found growing close to waterways on seasonally flooded river flats. In Sudan, along the Nile, regeneration has been reported as occurring once every ca. 15 years following high floods.

Weediness

With the exception of A. nilotica spp. kraussiana in southern Africa, there does not appear to be much evidence to show that the tree is weedy in its native range.

Pests and diseases

Many wild mammals feed on seed pods and a large number of insect species attack the mature seed.

Status in invaded regions

History of introductions and intensity of invasions

A. nilotica subsp. indica, native to the region spanning from Ethiopia to Burma, was introduced to Iran, Vietnam, the Caribbean and Australia. Introduced to Australia around 1900 as a shade tree along bore drains in Queensland. Found elsewhere in Australia but the distribution is limited by frost. Around 1960 it was found in 7% of the Mitchell grassland areas and by the mid 1980s it covered 6.6 million ha or 25% of the area. Its distribution and population densities are still increasing. Since 1969 introduced to the Baluran National Park (Java, Indonesia) as a firebreak and is spreading vigorously.

Patterns of invasion and time-lag

Dramatic increase in A. nilotica, particularly on town commons, took place in the 1950s as a result of a series of years with above average rainfall. Following this population outbreak the species was declared a noxious weed. Instead or removing the plant as required, many landholders continued to plant them along open drains distributing artesian water. The constant water supply ensured fast grow rates and the production of large seed crops, which were spread by cattle throughout the surrounding countryside forming large seedbanks. During a second period of above average rainfall in the mid-1970s germination of the seedbank occurred and resulted in a 1000 fold increase in A. nilotica populations. The highest densities of A. nilotica are found on properties which had the tree planted along its drains prior to the 1970s and had cattle rather than sheep grazing. In the 1980s a prolonged drought has reduced tree density in some areas by up to 80%. In Indonesia it invaded, with apparently no time-lag, wild grasslands.

Site and climate

Clay soils in dry tropics.

Floristic region and vegetation types

In northern Australia, A. nilotica is spreading into grassland dominated by Astrebla spp.

Pests and diseases

Two insect species, including one Coleoptera introduced as an attempt at biological control, eat the seeds unless they are quickly ingested by mammals soon after ripening. The passage through the digestive track of cattle apparently make seeds nearly immune to insect attack.

Impact on ecosystem

The vast grasslands of northern Australia are changed into woodland. In Indonesia it is altering the physiognomy of the invaded ecosystem and depriving native mammals of grazing grounds.

Impact on humans and related activities

The invasion reduces the livestock carrying capacity of Queensland's Mitchell grasslands. It may have an impact on the number of tourists frequenting the Baluran National Park as most visit the park to view large herds of herbivores.

Control

In Indonesia both mechanical and chemical control have been practised and the former was found to be more economical. In Australia a biological control programme has been initiated.

Ecological differences

Existence of ecological equivalent species and competitive interactions in invaded regions

The invaded ecosystem is grassland and therefore is treeless. This is an example of a lifeform hitherto absent being able to spread.

Differences in status and ecology between invaded and native ranges

A. nilotica appears to suffer from fewer insect pests, particularly seed pests, in Australia than in native range.

Selected references

* Alikodra, H.S. (1987) Tanaman eksotik akasia (Acacia nilotica) dan masalahnya bagi ekosistem savanna di Taman Nasional Baluran. Duta Rimba 13(79-80), 30-34.
** Brown, J.R. & Carter, J. (1998) Spatial and temporal patterns of exotic shrub invasion in an Australian tropical grassland. Landsc. Ecol. 13, 93-107.
** Carter, J.O. (1994) Acacia nilotica: a tree legume out of control. In Gutteridge, R.C. & Shelton, H.M. (Eds) Forage trees as legumes in tropical agriculture, pp. 338-351. CAB International, Wallingford.
# Fagg, C.W. & Greaves, A. (1990) Acacia nilotica 1869-1988. Annotated Bibliography - CAB International F42, 1-81.
** Mackey, A.P. (1997) The biology of Australian weeds. 29. Acacia nilotica spp. indica (Benth.) Brenan. Pl. Prot. Quart. 12, 7-17.
Marohasy, J. (1995) Prospects for the biological control of prickly acacia, Acacia nilotica (L.) Willd. ex Del. (Mimosaceae). Pl. Prot. Quart. 10, 24-31.
* Suharti, M. & Santoso, E. (1989) Analisa biaya pengendalian Acacia arabica di Taman Nasional Baluran Jawa Timur. Bul. Pen. Hutan 505, 1-8.
# Tybirk, K. (1989) Flowering, pollination and seed production of Acacia nilotica. Nord. J. Bot. 9, 375-381.
Pierre Binggeli

May 1997