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.
Thorny small tree growing to a height of 7 m. Lives at least 60 years.
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.
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.
Species is sensitive to frost but is tolerant of grazing, drought (< 2 years), fire and salinity.
Requires water for seed germination and seedling establishment. Can regenerate under pioneer woody plants in seasonally flooding habitats and subsequently outcompete them.
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.
Native to India, Pakistan and much of Africa where it is often dominant.
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.
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.
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.
Many wild mammals feed on seed pods and a large number of insect species attack the mature seed.
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.
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.
Clay soils in dry tropics.
In northern Australia, A. nilotica is spreading into grassland dominated by Astrebla spp.
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.
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.
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.
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.
The invaded ecosystem is grassland and therefore is treeless. This is an example of a lifeform hitherto absent being able to spread.
A. nilotica appears to suffer from fewer insect pests, particularly seed pests, in Australia than in native range.
Pierre Binggeli
May 1997