Prosopis Invasion

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.