FEATURE
younger branches are a reddish-brown colour . The tiny yellow flowers occur in spikes , looking somewhat like a small yellow bottlebrush . The feathery compound leaves are dark green and each tiny leaflet is 10 to 20 mm long . The fruits are narrow yellowish or purplish woody pods which are highly favoured by both livestock and game .
The Prosopis tree is phreatophytic which means that it can obtain its water needs from the saturated zone in the soil , just above the water table . It is able to survive extreme water stress as its taproot can reach deep groundwater , penetrating more than 50 m deep and its roots can spread up to 40 m laterally enabling very efficient use of both upper soil moisture as well as much deeper groundwater . Prosopis is reputed to have the deepest roots of any tree in the world .
Figure 2 . Prosopis glandulosa trees dominate riverbank vegetation .
invasion can double every 5 to 8 years based on the rate of spread to date and the invasion can increase at a rate of 18 % per year . Do the calculation , the result is frightening !
Prosopis replaces the locally indigenous vegetation , it out-competes other aridadapted trees ( such as Acacias ) by making use of water more effectively . Dense invasions of Prosopis can actually lower the water table , putting groundwater beyond the reach of the natural local vegetation . It is also allelopathic , which means that it “ poisons ” the soil under it and by so doing prevents the germination of seeds and the establishment of indigenous plants that may compete with it .
Prosopis uses more water than most of the indigenous trees , and where it occurs in extensive and dense infestations , has been labelled a water “ waster ”. Prosopis typically invade overgrazed , eroded and drought-affected areas . Unfortunately , these are conditions that are typical of most of the arid rangelands of South Africa . Climate change and global warming will create even more favourable conditions for the spread of this plant invader .
The deep-rooted Prosopis can damage borehole pipes , can block boreholes and it can also dry up wells and springs . Its preference for watercourses results in widespread damage to natural hydrology . Dense infestations together with water-carried plant debris create barriers in watercourses which can divert the normal flow into the surrounding veld alongside drainages resulting in accelerated soil erosion . The Prosopis invasions result in the loss of wetlandadapted plants like grasses , sedges and reeds which naturally line and protect river banks from erosion during flooding .
Dense Prosopis lining watercourses can prevent livestock and game from accessing the water or watercourse grazing and shade . These plants sometimes form an impenetrable “ wall of thorns ” that can prevent the movement of all larger animals through it .
A prime concern with Prosopis is the cost of control . It has been found that farmers in the Northern Cape spend approximately R20 000.00 per farm per year . With the ever-increasing spread of the invasion , and the rising costs of herbicide , transport and labour , this expenditure will probably increase substantially in the near future . This will unfortunately dampen the enthusiasm of landowners to do any effective Prosopis control .
Description and adaptation
Prosopis glandulosa var . torreyana ( honey mesquite ) is the primary invasive Prosopis species in Southern Africa but it can also hybridize with Prosopis velutina making accurate identification somewhat difficult . Both species , and the hybrids , however , are equally invasive .
Prosopis is a multi-stemmed shrub or small tree that closely resembles an Acacia . It can grow up to 10 m high and forms dense impenetrable thickets . It has straight paired thorns and the
Prosopis can thus tolerate a wide range of rainfall patterns ranging from a mean of 100 mm to 1500 mm per annum . This extremely wide ecological flexibility enables Prosopis to adapt to a wide range of soil types including stony substrates , terrace gravels , alluvial dune sand , clay soils , lime-rich soils and saline soils .
Despite the arid environments to which Prosopis is very well adapted , it can live for 100 years or more and can grow at a rate of up to 30 to 60 cm per year , an impressive growth rate by any standard .
How Prosopis invasions affect the environment
Ecosystem services like water supply and grazing potential are unavoidably affected by Prosopis invasions . The invasions also have a negative impact on bird and invertebrate diversity and biology and this is certain to have a knockon effect on pollination services in the affected areas .
Prosopis invasions have been known to cause the death of keystone tree species like Acacia erioloba . The invasions are known to reduce the density , species richness and diversity of indigenous woody plants and also reduce the cover of indigenous herbaceous plants , particularly grasses and herbs as a result of the allelopathy characteristic of Prosopis .
The general loss of the protective ground cover under the Prosopis canopy results in accelerated soil erosion because the now unprotected soil surfaces are more easily carried away by runoff water . Soil erosion is thus typical of Prosopis infestations because of the loss of the natural indigenous vegetation soil cover .
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