Prosopis -- Management by Exploitation, Not Eradication, Required to Control Weedy Invasions
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Henry Doubleday Research Association,
Coventry CV8 3LG, UK
1. Cadoret, K., N.M. Pasiecznik and P.J.C. Harris. 2000. The Genus Prosopis: A Reference Database (Version 1.0): CD-ROM. HDRA, Coventry, UK. ISBN: 0 905343 28 X.
2. Konda Reddy, C.V. 1978. Prosopis juliflora, the precocious child of the plant world. Indian Forester 104: 14-17.
3. Pasiecznik, N.M. 1998. Prosopis and provenance research in Cape Verde. pp 73-76 in: J.C. Tewari, N.M. Pasiecznik, L.N. Harsh and P.J.C. Harris (eds.), Prosopis Species in the Arid and Semi-Arid Zones of India. The Prosopis Society of India and the Henry Doubleday Research Association, Coventry, U.K.
4. Pasiecznik, N. (1999), Prosopis : pest or providence, weed or wonder tree? ETFRN News 28 : 12-14
5. Pasiecznik, N.M., P. Felker, P.J.C. Harris, L.N. Harsh, G. Cruz, J.C. Tewari, K. Cadoret and L.J. Maldonado. 2001. The Prosopis juliflora - Prosopis pallida Complex: A Monograph. HDRA, Coventry, UK. 170p. ISBN: 0 905343 30 1.
6. Peattie, D.C. 1953. Natural History of Western Trees. Riverside Press, Cambridge, Boston, USA.
7. Tewari, J.C., P.J.C. Harris, L.N. Harsh, K. Cadoret and N.M. Pasiecznik. 2000. Managing Prosopis juliflora (Vilayati Babul): A Technical Manual. CAZRI, Jodhpur, India and HDRA, Coventry, UK. 94p. ISBN: 0 905343 27 1.
The Prosopis 'debate' is now a hot topic throughout many parts of Africa, south Asia and Australia, where it has been introduced as a fuel and fodder species, but has now spread out of control. As only a 'poor man's tree', it received only limited attention from the world's research and development community, and no concerted, global effort, until an international team lead by HDRA, and funded by DFID (UK), began a project in 1998 to gather the global knowledge on Prosopis. One of the conclusions is that eradication is clearly not the answer, and another that it appears that the deserts can be made profitable by adopting simple management, turning weedy stands into productive agroforestry systems.
"So mesquite is something more than a tree, it is almost an elemental force, comparable to fire - too valuable to extinguish completely and too dangerous to trust unwatched" (Peattie, 1953).
Some of the most widespread tree species in the world's deserts, Prosopis(mesquite) are proclaimed both as a saviour and disaster. Foresters, environmentalists and researchers all voice their opinions, rarely coinciding, and send a confused message to extension workers and farmers. Like Eucalyptus, which raise concerns about water use, Prosopis are seen as a weed and worse. Several species have been introduced from the Americas and are now common world-wide, mainly the sub-tropical P. glandulosa and P. velutina and the truly tropical P. juliflora and P. pallida.
These are fast growing, nitrogen-fixing, very salt and drought tolerant trees that coppice well. The wood is hard and durable and an excellent fuel and timber. Flowers provide bee forage, and sweet, nutritious pods are relished by livestock and made into human food. Foliage is rarely browsed but leaf litter improves soil quality. However, large, stout thorns are positively disliked, and after cutting or browsing, trees produce many basal stems leading to a shrubby, multi-stemmed form. Seeds pass through animals' stomachs undigested, the process aiding germination, and are spread widely by livestock and water. Tap roots can reach deep water tables and extensive lateral roots spread well beyond the crown.
The weedy invader
Invading Prosopis tends to form dense, impenetrable thickets. In pastures, it reduces grass cover and stocking density, threatening ranchers' livelihoods, even forcing the migration of traditional 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 the growth of neighbouring crops. Prosopis pollen is said to be a major cause of allergic reactions, the thorns to be poisonous, and the trees to harbour nematodes.
Although the trees have many competitive ecological advantages over other plants, the seedlings are sensitive. They often colonise disturbed, eroded, over-grazed or drought-ridden land associated with unsustainable agronomic practices, such as following the introduction of cattle ranching in the Americas. Millions of hectares of rangeland have been invaded in this century, and the process is still occurring in South Africa, Australia and coastal Asia, where Prosopis species have been introduced. However, the ability to establish easily is an advantage for firewood collectors, who can find Prosopis on even the poorest of sites.
Prosopis the survivor
Prosopis are phreatophytes with deep tap roots to keep trees green during droughts by accessing the water table, and lateral roots to draw on surface water during the rains. Leaf adaptations reduce water loss, as expected in desert plants. Pot studies do not reflect actual water use in the field, and re-appearance of streams after land clearance has been explained by increased soil permeability following stump removal.
Prosopis are not voracious water users. Research on allelopathic effects shows decreased seed germination and seedling growth, with negative effects apparently due to shade and root competition. However, there are many conflicting reports of plants being lusher and growing quicker under Prosopis canopies. Increased nematode populations near Prosopis are unconfirmed. Deaths from thorn pricks have been explained by secondary infection, although stout thorns certainly penetrate most shoes and are likely to cause injury. Where Prosopis are the most common trees, the pollen has been recorded as a major allergen.
The tree of the poor
Views for and against Prosopis come from different quarters. "The popularity of P. juliflorais income related, those that can afford bottled gas for cooking and do not have to raise livestock quickly forget its value as a fuel and fodder tree. Comments concerning its monoculture, lack of aesthetic value and unconfirmed beliefs on the lowering of water tables come only from the more affluent. Rural farmers are invariably aware of its importance" (Pasiecznik, 1998).Prosopis is generally the scourge of ranchers and pastoralists, but a boon to the rural poor. In contrast to negative views of Prosopis as a weed, in India, where Prosopis provides up to 70 % of the firewood needs of rural populations in dry regions, only its value is noted.
"The rebellious sands are subdued and the inhospitable soils are colonised. The dreary scene of dry districts is changed to that of green belts. The bleak tree-less landscape is painted with splashes of brown, green and yellow. The monotony is broken for the traveller and the sheep and goats munch and crunch happily on the proteinous pods. The rural folk, whose lands were getting buried under drifting sands are grateful to the Forester and Prosopis, and the poor folk who had no fuel to burn in their hearths now have Prosopis . They collect the fuel in their leisure and sell in towns for a decent price" (Konda Reddy, 1978).
Eradication the answer?
For over fifty years, ranchers in south-western USA and Argentina tried every possible technique to eradicate or control Prosopis . The end result? Millions of dollars spent and still no cost effective programme found. In Sudan, the eradication programme even trains children to uproot seedlings. In South Africa and Australia, amongst others, eradication or control programmes exist, and new methods of biological control using seed-eating beetles are being attempted. However, it seems that once it has arrived, Prosopis is there to stay, so why not learn how to live in harmony with this new neighbour?
New management required
Some change in land-use systems appears necessary. Cattle spread seed widely, for example, whereas sheep kill most seed ingested and pigs kill them all. A reduction in stocking rates can encourage good grass cover, which prevents seedling establishment. But what to do with dense stands? They must be thinned, which is not a desirable job, to 100-200 stems per hectare. Stumps have to be removed or treated. Remaining trees must be pruned to single stems. Seedlings do not establish under tree canopies, so such a cover will prevent further establishment. Pruned crowns reduce root competition and grass growth will improve. With the production of fuelwood, sweet pods and straight trunks for timber, this can only be a profitable use of otherwise unproductive lands.
Markets are developing around the world, as consumers become aware of the high quality of Prosopis timber, fuel, pod flour, animal feed, honey and gums. The species are over-exploited in their native range Americas, where Prosopis is well-liked and well-used for furniture, food, feed and a source of raw materials for industry. However, where introduced, such local knowledge has not followed, and Prosopis remains a neglected, unrespected weed.
We now have the answers
To counter this, a project was funded by the Department for International Development (DFID) of the United Kingdom, aiming to collate global knowledge on this, one of the most common but under-utilised trees in the world’s dry lands. HDRA co-ordinated an international team, which gathered over 6500 pieces of literature from around the globe, now available on a searchable CD-ROM database (Cadoret et al, 2000). The definitive scientific monograph on Prosopis juliflora and Prosopis pallida, the two most common tropical species (Pasiecznik et al, 2001) will be available from August. This includes detailed information on the origin, biology, resource characters, uses and very importantly the management of these species. A technical manual (Tewari et al, 2000), aimed specifically at the problems in India, is also available.
Copies of the above are available free of charge to individuals and organisations in countries eligible for UK development aid, and at cost price to others. For more information, please contact K Cadoret, HDRA, Coventry CV8 3LG, UK ([email protected]).
A training programme, 'Management and Utilisation of Prosopis juliflora' has also been successfully organised in three states of India in 2001, to excellent reviews, and will be expanded. If you would like more information on training aspects, or wish to request that a training course comes to your district, please contact the author, Nick Pasiecznik ([email protected]).
Desert lands around the world presently attract little inward investment due to very low and variable agricultural production. Many were once fertile, but due to poor management and over-exploitation, they are now degraded, desertified or covered with weeds, and almost worthless. Agroforestry systems based on drought tolerant, nitrogen fixing trees have for decades been seen as a possible solution, leading to sustainable increases in productivity, reclaimation of soils and rural rehabilitation.
However, researchers appeared to overlook the fact that without ready markets for tree products, who would plant, tend and harvest these trees? Also, firewood and animal fodder are hardly the things a goldrush is made of, but specialist niche markets for fine timber, foods and industrial raw materials could provide the impetus needed. Prosopis and Acacia species are well-suited to producing these goods, well-suited to agroforestry systems and well-suited to desert regions.
The humid tropics and temperate zones are reaching their limits of sustainable productivity, if they haven't passed them already; demand on forest land for conversion to agriculture continues unabated; and all this while a third of the world is under-utilised desert. The world's manufacturers are crying out for sources of timber, foods and raw materials, the world's financial institutions are crying out for guaranteed 'ethical' investments, and the people of the world's deserts, are crying out for support for local develoment.
Co-ordinated efforts at national, regional and international level are required now to develop the deserts, not by planting a few trees here and there, but by using markets and investment. Deserts can be made profitable, in fact must be made profitable, if the pressure is to be let off from the rest of the world, and Prosopis is one of the trees already there that will help to do this.
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(This paper is adapted from Pasiecznik, N. (1999), Prosopis : pest or providence, weed or wonder tree? ETFRN News 28 : 12-14.)