Species Focus: Prosopis cineraria
The genus Prosopis, which consists of over 40 species of leguminous trees and shrubs, has attracted considerable attention for many years. Perhaps best known is Prosopis juliflora, a South and Central American species, which has been widely used for reforestation in arid and semi-arid areas. Less attention has been paid to the Old World species of Prosopis, the most important of which is P. cineraria. The Henry Doubleday Research Association (HDRA) coordinates an international research program to investigate this species for use in arid areas.
P. cineraria, also referred to under its synonym of P. spicigera, is a thorny, nitrogen fixing tree reaching 10m in height and with deep tap roots penetrating 30m or more below the surface. It is indigenous to Arabian Peninsula, Iran, Afghanistan, Pakistan, and India. P. cineraria grows in areas with 75-850mm annual rainfall and dry seasons of 8 months or more. It tolerates high temperatures and is somewhat resistant to frost. P. cineraria is often found at low altitudes of up to 600 m, in desert areas. P. cineraria is capable of growing when irrigated with 50% seawater. This tree also forms open woodlands on black cotton soils and will grow on dry stony alkaline land where the pH may reach 9.8.
P. cineraria is a genuine multipurpose species. It yields excellent firewood and produces high-quality charcoal. The wood is hard and reasonably durable with a variety of small timber uses. The foliage provides nutritious fodder, maximum yields being produced when the trees are pollarded on a three-year rotation. Palatability is good. The pods also provide good fodder, and when young, are a popular vegetable in the human diet in parts of India and Pakistan. In times of famine the bark of the tree is pounded into flour and made into cakes. The bark yields edible gums, is used locally in leather tanning, and is reported to be used to treat scorpion stings and snake bites. The flowers are valuable in honey production.
Value of Prosopis for agroforestry
P. cineraria is considered suitable for intercropping in dry areas. Its very deep tap roots obtain water and nutrients from great depths and would not compete with other crops for moisture. In northwest India and Pakistan it is claimed that growth of grasses and crops such as sorghum are enhanced the closer they are to the trunk. The tree coppices well and casts minimal shade during the growing season. P. cineraria is suitable for use as a hedgerow species when planted at 1m spacing. In sub-humid areas this tree spreads freely by root suckers, forming dense thickets and becomes a persistent weed.
Together with colleagues from Shandong Teachers University in China, HDRA investigating the tree's varying salt- and drought-tolerance. They hope to identify superior populations and even individuals for subsequent propagation and use in field trials. Recent work at Durham, U.K. suggests that early performance of the seedlings during germination may be used as a screening for salt-tolerant individuals.
P. cineraria is usually grown from seedlings, and under protected conditions direct sowing can also be successful. Seedlings are usually raised in nurseries for 2 to 3 months before planting out. A slow initial growth rate means that weed control is required during early growth.
Conventional methods of vegetative propagation with Prosopis species have not proved easy. With a few exceptions, Prosopis cuttings have shown poor rooting, although there are reports of successful air layering of P. cineraria. Alternative methods of vegetative propagation utilizing tissue culture techniques have been reported by several groups. In collaboration with Coventry Polytechnic in England, HDRA scientists are currently developing techniques for rapid clonal propagation of P. cineraria and other woody legumes.
In field trials in India, P. cineraria was found to grow more slowly than either Acacia senegal or A. tortilis, the latter of which is considered a slow grower. In early species elimination trials on the Cape Verde Islands, the growth of P. cineraria compared poorly with other Prosopis. However, the favored species, P. juliflora, has proved too vigorous a competitor for contour planting among field crops. Attempts are being made to introduce P. cineraria for this purpose. Growth rate should not be used as the sole criterion for species selection if agroforestry, rather than tree biomass production, is being considered.
Within its natural range, P. cineraria represents one of the most important indigenous trees with a key economic and environmental role in some of the most hostile arid environments. Successful, large-scale plantings have been made, for example in Abu Dhabi and in Rajasthan, India, where P. cineraria has been planted to stabilize shifting sands, in an area with 300mm of rainfall. In collaboration with the National Agricultural Institute of the Republic of Cape Verde, HDRA has introduced over 100 accessions of promising arid-zone trees including 13 accessions of P. cineraria. HDRA is interested in establishing further trials with p. cineraria to explore its potential geographical and climatic range, and its suitability for agroforestry.
Conservation of genetic diversity
Collaborators at Durham University, in England and Sultan Qaboos University, in Oman are studying natural P. cineraria populations in the Wahiba Sands of Oman. Within this vast desert are large P. cineraria woodlands. Here P. cineraria appears to be a highly variable species with individuals ranging from tall straight trees to low, branching shrubs. Populations exhibit considerable differences in their resistance to salt and drought. P. cineraria is vital to the local inhabitants, who have historically used it in a sustainable way. Recently, however, increased pressure on the woodlands has halted regeneration and threatened the trees' survival in Oman and in other Middle Eastern countries.
Prosopis in the Future
To date, insufficient research has been carried out to determine conclusively whether or not P. cineraria can be productively incorporated into sustainable forestry and agroforestry systems outside the present distribution range. However, its high degree of tolerance to adverse soil and climatic conditions, and the existence of a wide and underexploited genetic diversity within the species, suggest that it may have a role to play in agroforestry or silvopastoral programs where volume of timber production is not the prime concern. While such investigations are undertaken, it is essential to collect and conserve as wide a representation of the genetic diversity of the species as is possible.
For more information contact:
Dr. P.J.C. Harris, Overseas Projects Coordinator
The Henry Doubleday Research Association
Ryton-on-Dunsmore Coventry CV8 3LG U.K.
Fax: 44 0203 639229