The Prosopis debate

A briefing paper for local, national and international bodies

responsible for natural resource management in arid and

semi-arid regions experiencing invasions of weedy Prosopis

The Prosopis debate

Prosopis is an important topic of discussion and policy in

many parts of Africa, south Asia, Australia and the

Americas. Ranchers, farmers and ecologists are alarmed

by the invasion of vast areas of land in only a few

decades. They have put pressure on governments who,

in turn, have told forestry departments to stop planting

and begin eradication programmes. However, many

farmers and artisans, as well as researchers, argue that

the tree is a valuable resource. Eradication of Prosopis

has proven to be extremely difficult or impossible, and

there is a need to consider control through its

exploitation as a resource. Better management of

Prosopis can greatly reduce its invasiveness.

What are Prosopis trees?

Prosopis are valuable multi-purpose trees. Where native,

in the Americas, there is a long history of using all tree

parts and trading in Prosopis wood, food and fodder.

However, when introduced to Africa, Asia and

Australia, the indigenous knowledge rarely followed,

and Prosopis remains under-utilised and unmanaged.

Prosopis are fast growing, nitrogen-fixing, very salt and

drought tolerant trees and shrubs. Most are thorny,

though thornless types are known. Seeds are spread

widely by animals that eat the pods, and trees develop a

shrubby growth form if cut or browsed.

The weedy invader

Prosopis often colonises disturbed, eroded, over-grazed or

drought-affected land, forming dense, impenetrable

thickets. In pastures, grass cover and stocking density

are reduced, threatening the livelihoods of ranchers or

pastoralists. Invasions into agricultural land, along

irrigation channels and water courses is also a major

problem. The trees are believed to deplete groundwater

reserves and to reduce growth of neighbouring crops.

Several species have become weedy in native ranges, but

it is where Prosopis has been introduced that the ‘debate’

is strongest, especially in Australia, India, Pakistan,

South Africa, Sudan and much of the Sahel. The

common species worldwide are P. juliflora and P. pallida

in the dry tropics, and mainly P. glandulosa and P.

velutina in the sub-tropics.

Are eradication and/or control

the answer?

For over 50 years, a major eradication programme in the

USA and smaller programmes in Argentina, India,

Pakistan and Sudan have tried to eradicate Prosopis with

a range of herbicides and mechanical removal. Some are

effective for a short time but the Prosopis generally

returns. Millions of dollars have been spent but still no

cost effective solution has been found. Nevertheless,

governments continue to implement new programmes,

now aiming to control rather than eradicate invasions,

using the same techniques. In some countries, biological

control has been effective, for example in South Africa,

where the seed-feeding bruchid beetles Neltumius

arizonensis, Algarobius prosopis and A. bottimeri have been

introduced from North America.

New knowledge applied to the problem

An international team lead by HDRA and funded by

DFID, began a project in 1998 to gather the global

knowledge on Prosopis. An important conclusion is that

eradication is not a simple solution and there are many

management and control techniques that can convert

weedy stands into productive, profitable and sustainable

agroforestry systems.

Prosopis (mesquite, algarrobo):

invasive weed or valuable forest resource?

Prosopis pallida, Peru. Tree products include wood for timber,

posts, poles, chips, charcoal, firewood; pods for fodder, flour,

syrup, honey, resin gums, fibres, tannins and medicines.

HDRA –  the organic organisation

Management by exploitation required

Exploiting Prosopis

With the production of fuelwood, sweet pods and straight trunks for timber, exploitation of Prosopis can be a profitable use of

otherwise unproductive lands. Markets are developing around the world but work is still required to promote Prosopis as a valuable

product of the desert. Integrated development is needed, from basic stand management to product processing and marketing.

Based on cost/benefit analysis, national and state governments should strike a balance between the containment of Prosopis

through current eradication and control programmes, and the development of profitable agroforestry land use systems through

improved management.

(1) Application of improved

management techniques

Stand conversion and improvement:

● Weedy stands are thinned to 100-400 trees per hectare, in stages. Broad strips are

cleared and cut stumps are removed manually or mechanically, or are treated by

stripping the bark or chemically by applying used motor oil or a triclopyr/diesel

mixture directly to the stump. Animals can re-enter immediately as these

chemicals have little mammalian toxicity. Selected trees in the remaining rows are

pruned to single stems at final spacings of 5x5m to 10x10m. The cost of the

operation should be at least covered by the sale of charcoal, wood chips and/or

small timber.

Above:

Prosopis

before

pruning

● Pruning appears to be the single most important technique in improving tree and

understorey yields; weedy shrubs are turned into valuable, productive trees by

removal of side branches. Regularly pruned trees are found to have smaller root

systems, use soil water more efficiently and compete less with neighbouring crops

and grasses.

Right: tree

after pruning

● Stands can be improved by introducing thornless or high yielding varieties by

grafting or interplanting.

Preventing re-invasion:

● Prosopis trees have many competitive ecological advantages over other plants but

the seedlings are sensitive, rarely establishing under mature trees or in tall grass.

Re-invasion can be minimised by maintaining a high-pruned tree canopy and

improved understorey management, including reduced stocking rates.

● Destroying seed or limiting its spread reduces re-invasion. Biological control

including the introduction of seed-feeding beetles has been effective. Also effective

are the collection of pods for stall feeding or processing, or a change of livestock, as

cattle spread seed widely, whereas sheep kill most seed eaten and pigs kill them all.

(2) Development and application of

processing technologies

Promote collaboration between industry

and research and development

organisations:

● Improve pod and timber processing

efficiency.

● Adapt wood and pod processing

technologies for small-scale use.

● Develop high-technology extraction

of high-value bio-products.

Contact: HDRA, Ryton Organic Gardens, Coventry CV8 3LG, UK (research@hdra.org.uk).

This policy brief was written by N Pasiecznik, based on the project ‘Prosopis juliflora and related arboreal species: a monograph,

database and extension manual’. It is an output from a research project funded by the United Kingdom Department for

International Development (DFID) for the benefit of developing countries. The views expressed are not necessarily those of DFID.

(R7295) Forestry Research Programme. HDRA, 2002