II - Ecology and Management

Ecology of Prosopis juliflora in the arid regions of India - S.K. Saxena

Central Arid Zone Research Institute, Jodhpur 342003, India

Introduction

Prosopis juliflora was introduced into the Indian sub-continent to Sind province (Pakistan) in 1877 from South America, and later on it was introduced into many dry regions of India (Muthana and Arora, 1983). This species has since become naturalised in the arid and semi-arid parts of the country. With its wide adaptability to arid environments and its drought and disease tolerance, it has virtually exceeded all indigenous species in covering the arid and semi-arid tract. It forms thickets on marginal lands, sandy, rocky and gravely wastelands and alongside roads and railways, and is now one of the major sources of fuelwood to the landless and poor populations. In Tamilnadu, Haryana, Gujarat and Karnataka, farmers raise P. juliflora hedges along field boundaries. Its presence all over the area has reduced the threat to important indigenous tree species like P. cineraria, Acacia nilotica and A. senegal, and today it is one of the most important species for desert reclamation (Saxena and Venketeshwarlu, 1991).

Plant characteristics

P.juliflora (Swartz) D.C. (vernacular names: vilayti, pardesi, babul or mesquite), is a small to moderate sized evergreen tree of 3-4 m height with long drooping branches and spreading crown. This species is largely represented in a shrubby state. There are straight, conical and spinescent stipules of 3-9 mm length. Leaves are bipinnate, leaflets in 13-25 pairs, oblong (3 x 1.7 mm) and dark green. Flowering spikes are axillary, hanging 6-8 cm long, cream to yellow in colour. The pods are highly variable in size and shape, mostly curved or sickle shaped, cream-coloured on ripening and indehiscent. They contain 3-25 ovoid seeds, brown to light chocolate in colour, firmly embedded in the pod.

P. juliflora flowers twice a year, in February-March and August-September, and is a prolific seeder. The pods of autumn flowering mature by May or early June and are dispersed before the onset of the monsoon. The monsoon flowering pods mature from early November to mid December. Summer pods mature over a short period of time, whereas maturity of post-monsoon pods are staggered. In drought years, autumn flowering is very much affected, with trees often failing to flower, but the same plant flowers and fruits subsequently when there is adequate rainfall.

Ecological attributes

P. juliflora is well adapted to warm and dry tropical climates. It grows well in areas receiving 250-600 mm annual rainfall. It is a fast growing tree and has a deep to very deep, well meshed root system. It is capable of growing on inhospitable habitats such as rocky and saline soils, under adverse climatic conditions. Being drought hardy, disease resistant and having foliage non-palatable to livestock, it does not require any special care for rehabilitating marginal lands or wastelands. Large scale plantation work can be undertaken on such habitats without any fencing. This species is however, quite susceptible to frost, and its distribution is very much restricted in areas where there is a regular occurrence of frost during winter months. such as the interdunal zones of Barmer, Jaisalmer, Bikaner and Churu (all in Rajasthan). In areas which experience frost once every 4 to 8 years, the plant suffers dieback to ground level, and later in February-March the plant sprouts back and assumes its original height within a 9-10 month period.

Major growth takes place after the monsoon, and P. juliflora has a great capacity for regeneration and is capable of withstanding drought for long periods of time. In consecutive drought and famine periods, most other plant species fail to withstand the environmental harshness, whereas P. juliflora has been recorded to establish and colonise many habitats during such periods due to its aggressiveness and lack of competition. In low rainfall years or prolonged droughts, growing seedlings survive by folding both the cotyledons and protecting the young leaf. Such hardiness makes it valuable for afforestation and rehabilitation work in arid areas. Studies by Dahl (1982) showed that this species is capable of reducing its transpiration rate drastically and drawing firmly held water from the lower substratum. It is also capable of tolerating water-logging for certain periods.

Distribution

P. juliflora is now found in many states of India; Andhra Pradesh, Karnataka, Maharastra, Orissa, Punjab, Uttar Pradesh, Tamilnadu and West Bengal (Gurumurthi et al., 1984; Patel, 1986), and in almost all districts of Harayana, Gujarat and Rajasthan. In all 11 districts of western Rajasthan, P. juliflora has spread widely, with high plant densities encountered in the districts of Pali, Jalore, Jodhpur and Nagaur. There has been a great spread into east and south Rajasthan in the last 25 years. Initially introduced along roads, it has now spread into many habitats, including rocky terrains and saline lands.

In Gujarat, the forest department has undertaken a massive afforestation programme on the coast of Bhavnagar and Kutch districts with great success. High plant densities are encountered in Kutch, where it has fully infested almost all habitats including saline lands. In the Kutch, excessive plant populations of P.juliflora have suppressed the growth of native species such as Salvadora persica, Acacia senegal and P. cineraria. Many drainage lines are fully covered by P. juliflora, and dense thickets occur in Banni grassland areas which experience high levels of salinity. This extraordinary colonisation of Kutch proves the ability of this species to withstand a high degree of salinity. A span of about 30 years (1962 onward) has converted Kutch into P. juliflora woodland.

In districts of Haryana adjacent to Rajasthan, there are also high plant densities of P. juliflora, especially on wastelands, roadsides and gravelly areas. All the saline/alkaline wastelands and other marginal lands of Mathura and Agra districts of Uttar Pradesh also have high density thickets. The Delhi region contains a similar situation with widespread stands of P. juliflora.

Natural regeneration

P. juliflora is known to invade vacant sites and establish itself quite successfully. The extensive and quick propagation under natural conditions on various habitats can be attributed to the palatability of its pods to livestock. Seeds are chemically scarified while passing through the intestines of an animal and are encapsulated in the droppings. Each small faecal pellet may contain 1-3 seeds, and provides the desired moisture and nutrition to the germinating seedling. Even under drought conditions the pellet holds moisture for longer, which enables the seeding to overcome the adverse conditions and establish itself in the first year. These features, and the acclimatisation of saplings, has made it valuable for wasteland development.

Pod production

There is a high variability in pod size and shape, varying from 8-25 cm in length. Seeds are enclosed in a protective square shaped septum, coated with a sweet, thin, dry pulp, whereas the whole septa body is enclosed in a dry, yellow pulp containing 20-30% sucrose. This makes the pod palatable to livestock, also being low in tannin content (0.74-1.5 %) (Felker, 1982.).

Very little information is available in India about the pod yield per plant or on a per hectare basis. In New Mexico, U.S.A., Garcia (1916) reported 17 kg pods/tree, while Jurriaanse (1973) reported 90-150 kg from 10 year old trees in central Africa. Smith (1953) reported pod yield of 4-20 t/ha from mature P. juliflora stands. Studies conducted in Anand gave production at about 19 kg pods/tree/yr (Anonymous, 1981). P. juliflora pods contain 13% crude protein, 60% nitrogen free extract, 0.15-0.44% phosphorus and 0.30-0.50% calcium on a dry matter basis (Shukla et al., 1984). Ground pods fed to cattle, did not show any adverse effects, but under uncontrolled feeding the pods gave deleterious effects, resulting in the formation of a compact ball of indigestible pods in the rumen, which caused sickness and even the death of cattle.

Seed extraction

Each seed is enclosed in a gummy septum which provides resistance to losses from insect attack. A few methods have been developed for the removal of seeds from this septum (Saxena and Khan, 1975). Pods are collected and spread in the sun until they are dry and brittle. These pods are threshed mechanically into small segments, each generally comprising of one seed encased in its gummy septum. The removal of seeds from the endocarp can then be undertaken by soaking the broken segments in: 0.1 M ammonium chloride solution for 24 h; 95% sulphuric acid for 30 min; 1% solution of sodium hydroxide for 30 min; or in 70-80 ^oC water for 10-15 min and room temperature water for 24 h. The segments should then be well rinsed with water, and rubbing with a coarse cloth ensures 100% seed removal.

Germination and establishment

Untreated seed gave only 25% germination, but seed extracted with sodium hydroxide, sulphuric acid or boiled water were also scarified in the process and gave 77-90 % germination. Rapid water uptake of treated seeds was observed. Sulphuric acid treated seeds showed noticeable swelling and began to germinate within 24 h of sowing (Saxena and Khan, 1975).

Most seeds start to germinate in 1-3 days after sowing. The primary root emerges and elongates rapidly for 2-3 days while the cotyledons expand slowly and turn from pale yellow to light green in colour. The cotyledons unfold, and the primary leaf emerges within 3 days of germination. In the first week after germination there is rapid growth in the root rather than the shoot. Seven day old seedlings have a 1:4 shoot:root ratio (Gupta and Balara, 1972). In nature, seedlings have a great capacity to survive despite weed competition. Once the seedlings grow 5-10 cm above ground level, they have the capacity to survive and endure the hot summer months. The upper portion of the stem develops many dormant buds which can sprout and elongate in a suitable environment. It takes 3 years to initiate flowering and fruiting. P. juliflora generally develops a bushy shape with multiple stems in natural stands. If the apical bud is maintained and side branches pruned, then the plant can develop into an erect, well crowned tree.

In young plantations, termite attack in sandy soils may kill seedlings, and can be controlled with a single application of Aldrex (30% Ec @ 0.01%). During drought conditions the desert rat (gerbil) will eat bark for moisture and food, and can kill plants by completely debarking them at ground level, so periodic rat baiting is desirable in the sandy plains.

Biomass production

Large scale, close-spaced plantations (1.3 x 1.3 m) of P. juliflora were planted in Gandhinagar, Gujarat (700 mm m.a.r.), which produced 113 t/ha of biomass dry matter after 4 years (28 t/ha/yr), and the total biomass (root + shoot) produced was 148 t/ha (Gurumurti et al. 1984). Fuelwood yield studies on P.juliflora plantations conducted by Muthana and Arora (1983) on various habitats and different rainfall zones has shown that 5 year old trees produced 4.3 kg/tree at Bikaner and 1.5 kg/tree at Gadra road (Barmer). At Jhunjhunu, 10 year old trees produced 27.4 kg/tree of fuelwood. Annual fuelwood production increases with age, however rainfall and habitat factors have a significant impact on production.

The ability to coppice well is an important attribute in fuelwood species, to ensure continuous biomass production. Tiwari (1983) reported that P. juliflora withstood annual coppicing from the second year of establishment, thus becoming a reliable renewable source of wood. Established plants coppice very well after cutting and assume good growth within 1 year.

Fixation and inhibition

This genus, like many other leguminous genera, Acacia, Albizia, Pongamia etc., is capable of fixing atmospheric nitrogen. Jarrell et al. (1982) found that the growth of Prosopis in Mexico was enhanced by inoculating with effective Rhizobia, leading to high productivity. Gupta and Balara (1972) found nodule formation after 6 weeks. Growth inhibition studies by Lahiri and Gaur (1969) showed that Prosopis leaves contain an inhibitor which effects both the process of germination and the growth of seedlings of other species. Large quantities of leaf litter below the tree canopy do not allow other seed to germinate.

Salinity/alkalinity tolerance

P. juliflora grows well in areas having high salinity or alkalinity levels, and can tolerate alkalinity as high as pH 9.5 (Singh et al., 1993). It will grow normally in situations where pH is below 9.6 and electrical conductivity (Ec) is below 1.20 ds/m. Similar observations were made in the saline grasslands of Banni (Kutch), where thickets of P.juliflora (>2000 trees/ha) have been recorded, on soils with a surface pH of 8.1-8.4 electrical conductivities of between 0.29-0.87 ds/m. It also can continue to grow in a scattered fashion (10-40 trees/ha) with 8.1-9.3 pH and Ec of 4.8-5.91 ds/m. Even in areas of high salinity, it can be planted by amending the soil with farm yard manure and gypsum.

Utilisation

In western Rajasthan, P. juliflora is a major source of fodder, fuelwood, charcoal, timber and gums, and has established itself as a species of soil conservation and for aesthetic purposes. It also provides utility to wildlife, where in the arid tracts of Rajasthan and Kutch, ripe fruits of this species have become the main source of nutritive food to wildlife. Clusters of bushes or thickets provide hiding places for wildlife from hunters and natural enemies. Ripe pods are consumed by livestock in large quantities, and in drought and especially in famine conditions, the pods serve as the main protein source for milk cattle.

Presently 70% or more of the fuelwood demand of people in the arid and semi-arid tract of Rajasthan, Gujarat and Haryana is met by this species. It has a high calorific value and is preferred over indigenous species. A high quality charcoal is produced, which has virtually replaced Anogeissus pendula (Dhokra) as the main charcoal producing species in the arid zone. The timber is hard, heavy,strong, close grained and does not crack. The heartwood is deep brown, while sapwood is thin and yellow, and exists in the ratio of 1:9. Strong and durable timber is obtained from the heartwood, which serves as a valuable material for agricultural implements and household furniture and utensils. The plant produces gum from February to April which can be used for sizing cloth and paper.

Acknowledgements

The author is thankful to the Director and Head of Division of Resource Survey and Monitoring of the Central Arid Zone Research Institute, Jodhpur for providing the necessary facilities.

References

Anonymous, 1981. Utilization of agricultural by-products and industrial waste materials for evolving economic ration for livestock. AICRP Annual Progress Report, Animal Nutrition Dept., G.A.U. Anand, India.

Dahl, B.E., 1982. Mesquite as a rangeland plant. In: Parker, H.W. (ed.), Mesquite Utilization. Symposium on mesquite utilization. College of Agricultural Sciences, Texas Tech. University. Lubbock, Texas. ppA1-A20.

Felker, P., 1982. Mesquite - an all purpose leguminous arid land tree. Herbarium Abstracts 52: 46.

Garcia, F., 1916. Mesquite beans for pig feeding. New Mexico Agricultural Experimentation Station, 29th Annual Report, New Mexico, U.S.A. pp77-79.

Gupta, R.K. and G.S. Balara, 1972. Comparative studies on the germination, growth and seedling biomass of two promising exotics in the Rajasthan desert Prosopis juliflora (Swartz) DC and Acacia tortilis (Forsk) Hayne. ssp. tortilis. Indian Forester 98: 280-285.

Gurumurti, K., D.P. Raturi and H.C.S. Bhandari, 1984. Biomass production in energy plantations of Prosopis juliflora. Indian Forester 110: 879-894.

Jarrell, W.M., R.A. Virginia, D.H. Kohl, G. Shearer, B.A. Brayan, P.W. Rundel, E.T. Nilson and M.R. Shartifi, 1982. Symbiotic nitrogen fixation by mesquite and its management implication. In: Parker, H.W. (ed.), Mesquite Utilization. Symposium on mesquite utilization. College of Agricultural Sciences, Texas Tech. University, Lubbock, Texas.

Jurriaanse, A., 1973. Are they fodder trees? Forestry Dept., PVT, Pretoria, South Africa.

Lahiri, A.N and Y.D. Gaur, 1969. The nature and role of germination inhibitors present in leaves of Prosopis juliflora. Proceedings of the National Institute of Sciences 5: 60-71.

Muthana, K.D. and G.D. Arora, 1983. Prosopis juliflora (Swartz) D.C. A fast growing tree to bloom the desert. CAZRI monograph No.22. Central Arid Zone Research Institute. Jodhpur, India. 19p.

Patel, V.J., 1986. Wasteland development through agroforestry. Gujarat Energy Development Agency, Vadodra, Gujarat, India.

Saxena, S.K. and W.A. Khan, 1975. A quick method for obtaining clean seeds of Prosopis juliflora. Annals of the Arid Zone 13: 269-272.

Saxena, S.K. and J. Venkateshwarlu, 1991. Mesquite: an ideal tree for desert reclamation and fuel wood production. Indian Farming 41: 15-21.

Singh Gurbachan, N.T. Singh and J.C. Dagar, 1993. Mesquite for development of salt-affected soil. Wastelands News VIII: 7174.

Smith, T.R., 1953. Tree Crops - A Permanent Agriculture. Devin Adair Publishing Company, New York.

Shukla. P.C., P.M. Talpada and M.B. Pande, 1984. Prosopis juliflora pods - a new cattle feed source. Technical Bulletin. Animal Nutrition Dept., G.A.U., Anand, India.

Tiwari, K.M., 1983. Social Forestry for Rural Development. International Book Distributors, Dehra Dun, India.

Ecology of succession of Prosopis juliflora in the ravines of India - S.Chinnimani

Indian Council of Agricultural Research, Krisi Bhawan, Delhi

Introduction

Two Prosopis species occur in the ravines of India, the exotic P. juliflora and the indigenous P. cineraria, both being very important and versatile trees. The ecology of P. juliflora has been studied for the past 40 years (Chinnamani, 1992) and is a pioneer species with a very aggressive colonising habit. The pods and seeds are dispersed by water and wind, but mainly by ruminants, especially goats (Chinnamani and Rathore, 1983). Impenetrable thickets can form in places where there is alkalinity with adequate water. It is constantly being cut as a source of firewood, and it also tolerates severe biotic interference and thrives well even in highly unfavourable situations.

P. juliflora in ravines of some important Indian rivers

P. juliflora has been introduced for ravine reclamation for the last 40 to 60 years in the shallow, medium and deep ravines of India. These ravines are found mainly on the banks of the rivers Yamuna, Chambal, Beas, Mahi, Banas, Sabarmati, Narmada and Tapi and their tributaries, covering an area of 3.7 million ha. P. juliflora has become naturalised in these semi-arid ravinous tracts, regenerating naturally and establishing well at the head, sides and beds of ravines, including precipitous side slopes.

With continuous and rigid protection they tend to allow the natural regeneration of indigenous tree species over time. After 30 to 60 years of rigid protection of stands of P. juliflora, the proportion of this species declined from 80-95% to 15-40%, and is gradually replaced by neem (Azadirachta indica), sissoo (Dalbergia sissoo), babool (Acacia nilotica), khair and others, finally becoming one of the associates of the northern tropical thorn forest, the plant composition of which is given in Table 1. In the ultimate dry deciduous forest, P. juliflora is completely eliminated in the late successional stages, as can be seen in the ravines of Uttar Pradesh, Rajasthan and Madhya Pradesh.

Successional trends

In the northern tropical thorn forest, P. juliflora starts as a pioneer or coloniser on denuded abandoned ravines. The speed in which it proceeds to colonise is amazingly fast. In 5 to 10 years the natural regeneration is complete and one can encounter frondose habit of bushy saplings to small trees. Where it was introduced, its cover expanded in the 2nd, 3rd, 4th, 5th and 10th year to 1-5%, 5-15%, 10-20%, 20-50% and 35-90% of the total area respectively. It is cut for firewood from the 2nd or 3rd year, and this is repeated once every 6 to 18 months thereafter. P. juliflora tolerates this heavy abuse and even appears to thrive on it, and having no competitors this results in gregarious, thick and pure patches of this species which dominates sites under severe biotic and edaphic conditions.

Table 1. Tree species composition of northern tropical thorn forest in the ravines of north India.

Table 2. Successional trends in northern tropical thorn forest under two different managements.

If these areas are protected and side branches are pruned leaving one or two leaders, the bushy saplings assume an erect tree form and the natural progression begins. If side branches are not pruned or cut, natural pruning of side branches occurs, but this is a slow process. From the fifth year after initial establishment, the tree gives way to other indigenous species such as neem, sissoo, babool and khair (Sharma et al. 1981). Thus protection and pruning leads to a erect tree form in P. juliflora with 3 to 5 m clear bole in 30 to 45 years, as can be seen in ravines in the protected private forests in the Chambal area.

In the northern tropical thorn forest, P. juliflora occurs in private and government revenue land. In the 50th to 60th year after initial establishment in such lands, P. juliflora forms only 15-20% of the total tree composition, there being a drastic reduction in the compositional gradient from the 5th to 10th year when P. juliflora accounts for 35-90% of the total. This shows that this species is a pioneer coloniser in ravines and forms better conditions for other species to colonise in the course of time. Existing as one of the associates of the northern tropical thorn forest, P. juliflora is completely eliminated by secondary successors with further time course changes (Table 2.), leading to dry deciduous forest in 100 to 200 years as species of a higher order replace it in the successional gradient.

References

Chinnimani, S., 1992. Silvipasture and natural grasslands of Mahi ravines and ravines of Gujarat. Journal of Range Management & Agroforestry 13: 175-181.

Chinnimani, S. and B.L. Rathore, 1983. Palatable rating of natural feeds of Chambal ravines at Kota. Paper presented in a goat meeting, Agra, Uttar Pradesh, India. May, 1983. (unpublished).

Sharma, A.K., I.P. Pradan, J.P. Nema and K.G. Tejwanti, 1981. 25 years research on soil and water conservation in ravine lands of Gujurat. Monograph No.2, CSWCRTI Research Centre. Vasad, India.

Ecology and management of some important species of Prosopis - J.C. Dagar

Central Soil Salinity Research Institute, Karnal 132001, India

Introduction

Prosopis, commonly called mesquite, is distributed in the arid and semi-arid parts of the tropical and sub-tropical regions. Many of the species are now pantropic in their distribution and play an important role in the rehabilitation of wastelands including salt affected lands. A brief account of important species with special reference to their distribution, morphological description, environmental requirements, management and utilisation is presented here.

Distribution

Most of the species are indigenous to North and South America, but also Africa and Asia. Arboreal species such as P. alba, P. chilensis, P. hassleri and P. nigra are the largest sub-tropical species found in Argentina, P. glandulosa is common from Mexico to Kansas and Louisiana, while P. pallida is native to Peru. Many species have been introduced in dry and coastal regions of India, Pakistan, Brazil, Senegal and Sudan, where they have become naturalised and are widely used as a source of fuelwood.

In the Indian sub-continent, P. juliflora was introduced into Sind in 1877 and later into many other parts including Hageri, Bellary, Punjab, Agra and Delhi. Now it has spread into a variety of habitats, including saline and alkaline lands, eroded hills and ravines, along river beds, and dry and degraded wastelands, often where rainfall is low and variable. P. cineraria is the main native species, found throughout the arid and semi-arid regions, while the shrub P. farcta, also native to Asia, has been reported to occur in parts of Punjab and Gujarat. Although less common, P. glandulosa is reported to be found introduced in northern India (Maheshwari, 1963; CSIR, 1969). Recently, many other species such as P. alba, P. articulata, P. glandulosa, P. laevigata, P. tamarugo and P. velutina from many provenances have been introduced into India particularly in dry regions and wastelands.

Morphology and taxonomy

Prosopis Linn. (family Leguminosae, subfamily Mimosoideae) is a genus of generally fast growing, mostly thorny shrubs and trees, having very wide morphological variations and an ability to withstand wide climatic and edaphic diversity. These hardy and fast growing trees also have a remarkable coppicing ability. They have been seen as excellent plants for afforestation programmes, especially in India, because of their ability to grow in dry and degraded lands, having extensive lateral root systems and long taproots (penetrating up to 24 m), resistance to grazing, low cost of production and good quality firewood. The following is a brief description of the species which are grown in India, introduced, naturalised or indigenous.

P. alba Grisebach is a round crowned tree, (introduced into India from the U.S.A, but of South American origin), 5-15 m tall, sometimes with a trunk as large as 1 m in diameter and a short bole with many branches. The sapwood is yellowish and the heartwood dark brown. It is a thorny tree, but thornless varieties are also found. The leaves are bipinnate, with rachis up to 5 cm long, bearing long leaflets, 1-2 mm wide. The foliage is pinkish on young branches, developing a bluish tinge when mature. Flowers are abundant, greenish-white, on spike-like racemes. The pods are beige to off-white (from where the species name originates), typically 18-22 cm long, 4-5 mm thick and sickle-shaped. In contrast, other Argentinean species have red-tinged to dark purple pods (e.g. P. flexuosa and P. nigra).

P. articulata S. Watson has rachis up to 3 cm long, bearing two pairs of pinnae (7-9 mm long) with 30-50 pairs of leaflets per pinnae (6-8 mm long and 1 mm wide). Foliage and branches have a bluish tinge when young.

P. chilensis (Molina) Stuntz emend. Burkart is a small to medium sized tree up to 20 m in height and a bole up to 1 m in diameter. Trees are fast growing, spiny or sometimes unarmed, semi-evergreen to evergreen, large-crowned with drooping branches. The bark is greyish or dark brown, fissured, exuding gum. Leaves are dark green, bipinnate with 2-4 pairs of pinnae, with 10-48 pairs of leaflets, each leaflet 5-20 mm long. Flowers are small, greenish-yellow, numerous, in dense spikes. Pods are straight, falcate, flat or cylindrical (10-25 cm x 8-15 mm), containing 10-30 seeds being ovoid, flattened, hard, yellowish-brown, shiny and 7 x 3 mm in size (there being 9800-36000 seeds/kg). Germination is 50-100% following scarification in sulphuric acid or hot water. This species has been previously wrongly identified as P. juliflora or one of its varieties, but is easily distinguished by its longer and wider spaced leaflets and red-tinged seed pods.

P. farcta (Solander ex Russell) MacBride (synonym P. stephaniana Kunth ex Spreng.) is native, being a spiny bushy shrub or small tree, with downy, slender whitish branches, and armed with copious slender stramineous prickles. Leaflets similar to P. cineraria but smaller and downier, and with larger flowers. Pods are oblique and oblong, 1.5-3.0 cm long, 8-10 mm thick, black, rugose, obtuse, and contracted abruptly into the stalk, occasionally with galls.

P. glandulosa Torrey (synonym P. juliflora D.C. var. glandulosa Cock) is a shrub or small tree, 3-7 m tall with straggling or pendulous branches, stout spines. The tree has 1-2 pairs of pinnae each with 10-18 pairs of leaflets, rather distant along the rachis, linear and oblong in shape, 6-8 mm long and 1 mm broad. Flowers are yellow, in axillary spikes, with pods 13-20 cm long and 5-10 mm in diameter, linear, straight or falcate. It has also been wrongly identified as P. juliflora, but is distinguished from it by being deciduous, having falcate pods and larger leaflets without a mid rib. Two varieties are recognised, var. torreyana growing primarily in the deserts of the south western U.S.A. and northern Mexico (Hilu et al., 1982), and var. glandulosa found from Mexico north to Kansas and east to Louisiana (Burkart, 1976).

P. laevigata (Humboldt & Bonpland ex Willdenow) M.C.Johnston has been introduced in Karnal, Haryana. Native to Mexico, it has 1-2 pairs of pinnae, each 2-3 cm long and having 12-18 pairs of leaflets 3-5 mm long.

P. juliflora Swartz D.C. is the most common species in India. It is a fast growing, thorny, drought resistant, evergreen, large crowned, deep rooted bush or tree up to 10 m or more in height, with drooping branches. There are usually 4 pinnae with 17-20 pairs of dark green leaflets, obliquely oblong in shape. Flowers yellowy green, while pods are straw coloured when ripe, 12-25 cm long, curved, with brown seeds.

P. pallida (Humboldt & Bonpland ex Willdenow) H.B.K. (synonym P. limensis Bentham), native to Peru, Colombia and Ecuador, is a scraggly tree, thriving well in hot and saline regions. It grows to 8-20 m in height with a trunk of 60-80 cm in diameter, and branches reminiscent of twisted rope. On poor sites it is shrubby, resembling P. juliflora. It has sharp spines and pale-yellow flowers, with greyish green leaves and abundant pods 12-24 cm long, but thornless trees are also found.

P. tamarugo F. Philippi, native to the Atacama desert in Chile, is a long lived tree up to 18 m tall, extremely drought resistant and salt tolerant, with a deciduous, open crown. It has golden yellow flowers on long cylindrical spikes, with curved pods containing about 7 seeds embedded in a brownish pulp. It produces a dense mat of lateral roots and a quick growing tap root.

P. velutina Wooton (synonyms P. chilensis var. velutina Standley, P. juliflora var. velutina Sarg.) grows up to 16 m in height and reaches a bole diameter of 60 cm, with usually 2 pairs of pinnae, 4-5 cm long, having 18-24 pairs of leaflets. Leaves and inflorescence are densely tomentose.

Environmental requirements

Most of the species are drought tolerant and suitable for planting in arid and semi-arid regions in a variety of soil conditions. P. juliflora can be found growing in many environments such as mine spoils, along roadsides, railway tracks, alkali and saline tracts and on rocky terrain. It is predominant in the extremely saline soils in the Rann of Kutch, Gujarat, where most other species, except some true halophytes, fail to survive. It has been found growing in very warm, dry climates and tolerates high sodicity (pH up to 10 or more) and salinity (electrical conductivity 44 dS/m or more). P. chilensis is equally salt tolerant, even tolerating sea-water salinities. It is native to regions experiencing lower winter temperatures and low rainfall with ground water between 3 and 10 m deep. P. alba is native to high plains up to 1500 m elevation and tolerates high sodicity (pH 10). P. pallida is a frost sensitive species, native to the incredibly dry, saline coastlines of Peru and Ecuador, remaining shrubby on poor sites. P. glandulosa grows in a wide range of soils and is moderately salt and frost tolerant. It thrives in very high temperatures (>38^oC) and in areas with low rainfall, but usually in areas with accessible ground water reserves.

P. tamarugo tolerates a wide temperature range (12-36^oC) and is extremely drought resistant and salt-tolerant. It is found in areas with thick, surface salt-encrustations. Many species can grow in soil salinities equivalent to sea water, and Felker et al., 1981) found that all species tested could tolerate salt concentrations of 6,000 mg/l with no reduction in growth. P. velutina, P. pallida and P. tamarugo could grow in salinity levels of 18,000-36,000 mg/l. These species are reported to be capable of fixing nitrogen, thus having potential as nitrogen fixing halophytes. Characteristics of 5 important species are given in Table 1.

Table 1. Characteristics of important species of Prosopis.

Propagation and silviculture

Prosopis seeds are not easily removed from the gummy pulp. The pods can be processed in a grinder after drying at about 50^oC overnight, which will also scarify the seeds. Germination pretreatments include mechanical scarification, soaking in concentrated sulphuric acid for 20 min or a 20% solution for 60 min, or immersed in boiling water and allowed to cool for 24 h. There are 25,000-36,000 seeds/kg. Outstanding plus trees of P. alba have been cloned by vegetatively propagating shoot cuttings requiring the control of light intensity and air temperature (Felker, 1991).

Plant spacing varies, for energy plantations being 1 x 1 m or 2 x 2 m, but for a silvopastoral system 2 x 4 m is recommended, which may increase to 4 x 4 m for arable crops 3 years after planting. Soil with compacted hard pan layers, as in many alkali soils of the Gangetic plains, should have a hole dug with a tractor-mounted auger, 20-25 cm diameter and 1-1.5 m deep and the holed refilled with the original soil with 8 kg manure, 3 kg gypsum 10 g zinc sulphate and some insecticide (Dagar and Singh, 1994). Trees respond well to irrigation, with excellent coppicing ability, and maximum fodder yields are obtained when the trees are pollarded on a three year rotation. The best growth is achieved in areas when the root system can reach groundwater, but in low rainfall areas, especially in fast draining soils, irrigation may be required during establishment. Early pruning to encourage an erect form is recommended.

Biomass production

In India, P. juliflora grows rapidly on all kinds of soil, with mean annual increments of 66 cm in height, 2.51 cm in collar diameter and 1.73 cm in breast height diameter recorded, and a total yield of 50-60 t/ha after 10 years and 75-100 t/ha after 15 years (Vimal and Tyagi, 1986). In semi-arid Gandhinagar in Gujarat, a 4 year old plantation (1.3 x 1.2 m spacing) produced 148.63 t/ha dry biomass (Gurumurti et al., 1984). Studying the response of 3 planting techniques, trench (30 x 30 cm deep), pit (30 x 30 x 30 cm), and auger hole (15 x 90 cm deep), the lopped harvested biomass was 34.3, 36.0 and 42.6 t/ha respectively. In response to the soil amendments of gypsum, gypsum + rice husk, and gypsum + farm yard manure, a total biomass of 34.3, 49.5 and 52.3, t/ha, respectively was obtained, compared with only 3.1 t/ha without any amendments, in highly alkali soils (pH 10.4).

References

Burkart, A., 1976. A monograph of the genus Prosopis (Leguminosae sub fam. Mimosoideae). Journal of the Arnold Arboretum 57:219-249, 450-525.

Council of Scientific and Industrial Research (CSIR), 1969. Wealth of India - Raw materials. Vol 8. CSIR, New Delhi. pp245-249.

Dagar, J.C. and G. Singh, 1994. Mesquite - a promising tree for degraded and problematic lands. Tree World 3: 2-4.

Felker, P., 1991. Prosopis alba and Prosopis chilensis: subtropical semi-arid fuel and fodder trees. NFT Highlights, NFTA, Hawaii. pp91-06.

Felker, P., P.R. Clark, A.E. Laag and P.F. Pratt, 1981. Salinity tolerance of the tree legumes: mesquite (Prosopis glandulosa var. torreyana, P. velutina and P. articulata), algarrobo (P. chilensis), kiawe (P. pallida) and tamarugo (P. tamarugo) grown in sand culture on nitrogen-free media. Plant and Soil 62:311-317.

Gurmurti, K., D.P. Raturi and H.C.S. Bhandari, 1984. Biomass production in energy plantations of Prosopis juliflora. Indian Forester 110: 879-894.

Hilu, K.W., S. Boyd and P. Felker, 1982. Morphological diversity and taxonomy of California mesquites (Prosopis, Leguminosae). Madroño 29: 237-254.

Maheshwari, J.K., 1963. The Flora of Delhi. Council of Scientific and Industrial Research, New Delhi. pp145-146.

Vimal, O.P. and P.D. Tyagi, 1986. Fuelwood from Wastelands. Yatan Publications, New Delhi.