Description of a plant well adapted to invade
Prosopis glandulosa var. torreyana (honey mesquite) is the primary invasive Prosopis species in Southern Africa but it can also hybridise with Prosopis velutina making accurate identification somewhat difficult. Both species, and the hybrids, however, are equally invasive.
Prosopis is an extremely successful invader. Originating from an arid region, it is well equipped to survive drought and it flourishes under conditions of rangeland overgrazing and extended drought.
Prosopis is a multi-stemmed shrub or small tree that closely resembles an Acacia. It can grow up to 10m high and forms dense impenetrable thickets It has straight paired thorns and the 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 20mm 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 50m deep and its roots can spread up to 40m 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.
Prosopis can thus tolerate a wide range of rainfall patterns ranging from a mean of 100mm to 1500mm 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.
The Prosopis has been classified as a Class 1 invader species due to its aggressive spread and the threat that it represents to natural rangelands and water security in arid areas.
The Prosopis is native to north eastern Mexico and the southwestern United States of America. It was first introduced into Southern Africa in 1897 in South West Africa (now Namibia) where German settlers planted it for shade and livestock feed. It was found to be very useful but by 1912 it had established itself in the wild and by the 1960’s, dense invasions, that were very difficult to control, were already a recognised problem. The invasion had begun.
In South Africa it was planted for windbreaks and shade in Upington from where it has spread into the Great Karoo and the Kalahari Thornveld. There are approximately 45 species of Prosopis and some species have become a worldwide problem, invading rangelands in the USA, Australia, Namibia, East Africa, India, Hawaii and the Middle East regions. Prosopis juliflora is a rampant alien invasive tree problem in Central and northeast Africa and most particularly in Kenya. Some Prosopis species are native to Africa including the djembe (Prosopis africanus).
Prosopis glandulosa seed pods.
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 60cm per year, an impressive growth rate by any standard.