The invasive alien plant, prosopis, is a serious threat to of areas in Southerern Africa where it threatens water resources, indigenous vegetation and the livelihood of communities who are dependent on these natural resources.
In the previous issue, Ken Coetzee of Conservation Management Services issued a serious warning that the invasive alien prosopis, which is spreading at an alarming rate, urgently needs action to curb further encroachment.
Prosopis encroachment is already covering 1,8 million hectares of grazing rangelands in South Africa. Based on the present growth rate of eighteen percent per year, the size of the invasion can double every five to eight years. “The rapid spread of prosopis in South Africa is a major catastrophe,” says Ken.
“It is equally clear that the problem is not being effectively addressed as it should be, and that many millions of hectares of grazing rangelands, rivers, wetlands and groundwaters are at risk. “Without human intervention, much of our valuable rangelands will be forever transformed into useless, environmentally damaged prosopis monocultures,” he warns.
Prosopis glandulosa seed pods are highly nutritious and eaten by both game and livestock. (Photo: internet)
Description and adaptation
Prosopis glandulosa var. torreyana or honey mesquite (suidwesdoring) is the primary invasive prosopis species in Southern Africa. It can also hybridise with Prosopis velutina, making accurate identification difficult, but both species, as well as the hybrids, are invasive. Prosopis is a multi-stemmed shrub or small tree that closely resembles an Acacia. It can grow up to ten metres high and forms dense, impenetrable thickets. It has straight paired thorns and the younger branches are reddish-brown in colour.
The tiny yellow flowers are borne on spikes and slightly resemble 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 that are highly favoured by livestock and game. The prosopis tree is a phreatophyte, which means that it can obtain its water from the saturated zone in the soil just above the water table.
Reportedly having the deepest roots of all trees in the world, the roots of the Prosopis can reach up to 50 metres deep in search of groundwater. The roots spread up to 40 metres laterally, which allows it to also make use of moisture effectively in the upper layer of the soil. It can tolerate a wide range of rainfall patterns, ranging from a mean of 100 mm to 1 500 mm per year.
Because of its ecological flexibility, it can tolerate from 100 mm to 500 mm of rain per year, and adapts to a wide range of soil types, including stony substrates, terrace gravels, alluvial dune sand, clayey soils, as well as lime-rich and saline soils. In addition, the prosopis can grow at a rate of up to 30 to 60 cm per year and live up to a hundred years or more. Prosopis glandulosa seed pods are highly nutritious and eaten by both game and livestock. (Photo: internet)
Reasons for concern
There are several serious issues that landowners seem to be unaware of or prefer to ignore. Prosopis is not a benevolent to indigenous plants for various reasons.
Replaces indigenous plants
The leaves, flowers, and purplish new growth of Prosopis glandulosa. Photo: Ken Coetzee.
By making more effective use of available moisture, prosopis replaces locally indigenous vegetation and out-competes trees that are adapted to arid conditions, such as indigenous Acacia species. Dense invasions of prosopis can lower the water table, putting ground water beyond the reach of the natural local vegetation. It is also allelopathic, which means that it changes the soil underneath it to eliminate competition by prohibiting germination of the seeds and establishment of indigenous plants.
Thirsty
Prosopis has been labelled a water waster, especially where dense infestations occur as it uses more water than most indigenous trees. It typically invades overgrazed, eroded areas that have been affected by drought – conditions that are typical of most rangelands in South Africa. Adverse conditions for indigenous plants, such as climate change and accompanied global warming, seem to create even more favourable conditions for the spread of this plant invader at the cost of local plants.
Deep-rooted
Its preference for watercourses results in the widespread damage to natural hydrology. The deep roots of the prosopis can damage borehole pipes, block boreholes, and dry up wells and springs. Dense infestations, along with water carried plant debris, create barriers in watercourses that can divert the normal flow into the surrounding veld alongside drainages, accelerating soil erosion. Thus, prosopis invasions result in the loss of plants like grasses, sedges and reeds that are adapted to wetlands, and which naturally line and protect riverbanks from erosion during flooding.
Cuts off watercourses
Dense prosopis infestations along watercourses sometimes form an impenetrable wall because of the thorns that prevent animals from getting to the watercourse. This can prevent livestock and game from getting to the water and grazing along these watercourses, as well as shade provided by indigenous trees.
Source: Flickr
Costly to control
The cost of clearing prosopis encroachment could prevent many landowners from actively fighting the invasion. Farmers in the Northern Cape spend approximately R20 000 per farm per year to control the encroachment. With the ever-increasing spread of the invasion, and the rising costs of herbicide, transport and labour, this expenditure will probably soon increase substantially, which will further dampen enthusiasm to attempt any effective prosopis control.
Contact details Ken Coetzee at 076 075-5056 or consken@mweb.co.za
References
Bromilow,C. (2018) Problem plants and alien weeds of Southern Africa. 4th Edition. Briza Publications, Pretoria.
Coetzee,K. (2005) Caring for natural rangelands. University of Kwa-Zulu Natal Press, Scottsville.
Henderson, L. (2001) Alien weeds and invasive plants. Plant Protection Research Institute, Handbook No.12, Agricultural Research Council.
Shackleton, R.T., Le Maitre, D.C., van Wilgen, B.W. & Richardson, D.M. (2015) The impact of invasive alien Prosopis species (mesquite) on native plants in different environments in South Africa. S. African J. Botany, Vol.97 (25-31).
Shackleton, R.T., le Maitre, D.C. & Richardson, D.M. (2015) Use of non-timber forest products from invasive alien Prosopis sp. (mesquite) and native trees in South Africa: Implications for Management. Forest Ecosystems, Open Access, Springer Nature.
Strohbach, B.J., Ntesa, C., Kabajani, M.W., Shidolo, E.K. & DÁlton, C.D. (2015) Prosopis encroachment along the Fish River at Gibeon, Namibia. Dinteria, No. 35(53-73).
Van Wilgen, B.W. & Richardson, D.M. (2014) Challenges and tradeoffs in the management of invasive alien trees. Biol. Invasions. 16 (721-734). Springer Nature, Forest Ecosystems, open access on-line.
Zachariades,C. Hoffman, J.H. & Roberts, A.P. (2011) Biological control of mesquite (Prosopis species) (Fabaceae) in South Africa. African entomology,19 (2): 402 -415).
