
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
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