I - Overview

Editorial introduction - J.C. Tewari^[1], N.M. Pasiecznik^[2], L.N. Harsh^[3] and P.J.C. Harris^[4]

Prosopis

The genus Prosopis includes species ranging from thorny shrubs to tall attractive trees, found predominantly in arid and semi-arid regions. They occur naturally, or have been introduced, throughout the world and have attracted attention because of their ability to survive in extremely arid, saline and inhospitable locations, sometimes being the only trees able to do so.

Many Prosopis species grow quickly and produce excellent firewood and charcoal, abundant seed pods rich in protein and sugars that are edible by humans and domestic animals, and leaves that can be harvested and stored for dry season fodder. Prosopis trees are an excellent bee forage for honey production and, being legumes, can add nitrogen to the soil.

Traditional value

Prosopis trees have been valued and exploited locally for centuries in India, the Middle East, West Africa and North and South America. In India for example, Prosopis cineraria is a vital component of a traditional agroforestry system and grown together with field crops to provide dry season fodder and fuel. In the Middle East the same species is sometimes the most important tree forming woodlands in deserts such as the Wahiba Sands in Oman.

Worldwide introduction

Given their local value, it is not surprising that Prosopis species have been distributed around the world, to the extent that the most common species, P.juliflora, native to Central and South America is now found throughout the semi-arid and arid tropics. Although now considered a weed in some parts of India and southern Africa where it has invaded rangeland, P.juliflora has nevertheless proved highly adaptable and productive and is one of the main sources of fuel for the rural and urban poor in northern India. P.juliflora has also proved invaluable in a massive reafforestation programme in the Republic of Cape Verde since that country’s independence in 1975.

The transfer of Prosopis species from their native range to new areas, mostly from the Americas to the rest of the world, has been somewhat haphazard. Only a very small proportion of potentially useful species have been introduced into forestry programmes as exotics. Introduction has generally been for one purpose only with little regard for other useful properties. Species may have been introduced as ornamentals ignoring the fact that in their native range they are highly prized for fuel or form the basis of local livestock production systems. Also, when Prosopis species has been introduced to a new area the seed may well have been collected from a very restricted source, not necessarily the best suited for the new location. Only very rarely has a wide range of species, let alone individual provenances, been considered and tested at the introduction stage.

Domestication

Despite their local use, Prosopis trees have not received the same selection and improvement as the major industrial forestry species. Similarly, there have been relatively few attempts to develop or improve agricultural and forestry systems employing Prosopis, or to industrialise processes for exploiting Prosopis products. Among the examples where innovative developments have taken place are a major sheep rearing enterprise in the Atacama Desert of Chile using P.tamarugo, small-scale commercial processing of animal feed from P.juliflora pods in Brazil, and erosion control in Cape Verde with P.juliflora coppiced for fuel. Other promising developments include novel techniques for processing pods and seeds of Prosopis for human consumption in drinks and snack foods in the USA and South America. Improvements in the exploitation of Prosopis for quality timber products has led to renewed interest in this is the USA. The recent refinement of techniques for vegetative propagation of Prosopis has stimulated the commercial exploitation of P.alba clones for ornamental and amenity planting in the USA.

The future for Prosopis

The highest priorities for future work with Prosopis include an urgent need to collect and conserve valuable germplasm and to continue field testing to select and improve the best material for specific sites and uses. However there is also a great need for the research results obtained so far to be integrated into agricultural and forestry development programmes and into the practical and commercial development of Prosopis and its products.

We are convinced that there is considerable scope for innovative exploitation of Prosopis to improve rural livelihoods. To help achieve this, the different experiences with Prosopis, covering its full geographical and ecological range and the variety of its economic uses need to be recorded, evaluated and disseminated, to develop the true potential of diversified output from Prosopis forests and plantations in the dry zones of the world (Harris et al., 1994). Several major conferences have been held which have covered various aspects of work with Prosopis (Habit, 1985; Habit and Saavedra, 1988; Dutton, 1993; Felker, 1997). The proceedings reported in this volume represent the current state of Prosopis research and development in India as a contribution to the international body of knowledge on this important subject.

References

Dutton, R.W. 1993. Prosopis species; Aspects of Their Value, Research and Development, Proceedings of the Prosopis Symposium, University of Durham, 27-31 July 1992, CORD, Durham.

Felker, P. 1997. Prosopis: Semi-arid fuelwood and forage tree; building consensus for the disenfranchised. Washington DC 13-15 March 1996.

Habit, M.A. 1985. The current state of knowledge on Prosopis tamarugo. Papers presented at the International Round Table on Prosopis tamarugo Phil., Arica, Chile, 11-15 June 1984.

Habit, M.A. and Saavedra, J.C. 1988. The current state of knowledge on Prosopis juliflora. II International Conference on Prosopis, Recife, Brazil, 25-29 August 1986.

Harris, P.J.C., Bradbury, M., Pasiecznik, N.M. & Arya, S. 1994. Prosopis: an under-exploited forestry resource. Science and Technology Now 5: 14-17.

Prosopis in the arid regions of India: some important aspects of research and development - L.N. Harsh and J.C. Tewari

Central Arid Zone Research Institute, Jodhpur 342003, India.

Introduction

The genus Prosopis contains 44 species in five sections, spread throughout southern Asia, Africa and America, but predominantly in the latter continent, from western North America to Patagonia. The centre of polymorphism is in central-western Argentina. Prosopis species are generally thorny and frequently phreatophytic. Plants exhibit an enormous variety in growth habits and in their defences, which may be represented by thorns, internodal prickles or be entirely absent.

In the Indian sub-continent, only one species, P. cineraria is endemic, all others having been introduced. P. juliflora which now occurs throughout the arid and semi-arid tropics of the Indian sub-continent, was first introduced last century. In India, the species now grows predominantly in Rajasthan, Haryana, the plains of Uttar Pradesh, Gujarat, North Karnataka, Tamilnadu, Madhya Pradesh, Delhi, Maharastra and also in parts of Punjab. Depending on climatic and edaphic conditions, the density of species varies from 5 to 100 trees/ha.

The growing of arable crops in association with P. cineraria is an age-old tradition in arid parts of India in general, and particularly in the Thar desert. The vegetation of the Thar desert region is classified as dry tropical thorn forest (Champion and Seth, 1968) and in a broader successional gradient, P. cineraria is designated as the climatic climax species, and is the predominant constituent of the vegetation complex of arid parts of India. In its natural range, the species grows very slowly, especially during early growth.

Realising the slow growing nature of P. cineraria, another Prosopis species, P. juliflora was first introduced into India in 1857 from Latin America. The species reflected its potential in terms of adaptability and growth in these new environmental conditions immediately after introduction. The arid and semi-arid parts of the country were the most preferred habitats of P. juliflora, but over time this species has spread into other parts of the Indian sub-continent. P. juliflora is now well naturalised in the arid and semi-arid tracts of the region.

Introduction and distribution of Prosopis in India

Although the earliest recorded introduction of P. juliflora was in 1857, the first systematic plantations were not carried out until 1876 in the Kaddapa area of Andhra Pradesh. It was introduced into parts of Gujarat in 1882 and into Sindh (Pakistan) in 1877. The ruler of the princely state of Marwar introduced P. juliflora into Rajasthan in 1913. Fascinated by its excellent growth rates in the region, the then ruler of the state declared it a “Royal Plant” in 1940 and exhorted the public to protect it. Since then, the species was gradually introduced in many parts of the country including Haryana, Punjab, Uttar Pradesh, Madhya Pradesh, Maharastra and Tamilnadu. Due to its wide ecological amplitude and rapid colonising ability, the species has spread rapidly over a large part of the arid and semi-arid tract, with the exception of frost prone areas.

At present, P. juliflora is thought to inhabit more than 500,000 ha, although plant densities vary with different areas and habitats. It grows in all type of soils, from sandy to saline-alkaline soils. It has proved to be a most versatile species for afforesting shifting sand dunes, coastal dunes, river beds, saline-alkaline lands, eroded hill slopes, mine-spoiled areas and other wastelands. (Muthana and Arora, 1983). Initially, plantations were established mainly for the purpose of conservation, but P. juliflora has now become the main source of fuel in rural areas and also to a large extent in urban areas, fulfilling more than 70% of the firewood requirements of rural people in the tropical arid and semi-arid regions of the country. The seed pod is used as a livestock feed, and the wood is also used in charcoal making and occasionally as a timber.

Introduction of other species of Prosopis only began after 1960. In 1962, P. chilensis and P. tamarugo were introduced into Rajasthan and P. alba and P. pubescens were introduced at CAZRI, Jodhpur in 1966. These were simple introductory trials to assess the adaptability of these species to the arid environmental conditions of India. A further trial involving six other exotic provenances of P. juliflora was established in 1973, and in 1985, under a FAO/IBPGR programme, P. pallida was also introduced at CAZRI, Jodhpur. P. alba, P. chilensis, P. glandulosa, P. hassleri, P. nigra, and P. pallida were introduced into the saline-sodic soils of Uttar Pradesh in 1980 and into the saline-sodic soils at Karnal, Haryana in 1985.

In 1991, under a joint Indo-US research project (PL-480), 232 accessions of several Prosopis species (P. alba, P. chilensis, P. flexuosa, P. nigra, a Prosopis hybrid and Peruvian Prosopis sp.) were introduced at CAZRI, Jodhpur. Convinced by the excellent performance of these Prosopis species in various research stations and their multiple utility, the forest departments of a number of states of India have now started establishing large scale plantation programmes of these species. The following discussion is essentially related to P. juliflora, which still dominates the arid zone, although some important aspects of genetic improvement of other introduced Prosopis species are included.

Habitat and growth

The ecological amplitude of P. juliflora is very wide, excluding frost-susceptible areas, and it can be grown in any soil type. It can tolerate very severe drought and flourishes even in very low and variable rainfall. It can be grown on the poorest of soils, with low organic matter and nutrient status. In waterlogged and highly saline areas where nothing else can be grown, this species thrives well and produces large quantities of biomass. In the Little Rann and Great Rann of Kutch, Gujarat, it is the predominant species, with plant densities exceeding 2000 individual/ha, depending on the rainfall pattern and available soil moisture. It is an aggressive coloniser in areas where soil moisture availability is adequate.

Two forms of P. juliflora are observed in its naturalised habitat. The first is the large ‘tree’ form, with individuals often more than 8 m in height and having a clear single bole up to 3-4 m. Some trees of more than 15 m in height were observed near Jodhpur and in the Allen forest of Kanpur. The second is the small ‘bush’ form, multi-stemmed at ground level, although occasionally the multi-stemmed nature may be due to coppice regrowth. Though initially bushy by habit, with forking taking place at lower heights, if it is pruned from the base leaving one stem, it can be grown straight. It was also observed that as the tree becomes older, it becomes more thornless, thus it appearing that the presence of thorns in the juvenile stage is a protective mechanism.

Establishing the plantation

Reproduction and multiplication

In nature, P. juliflora reproduces through self-seeding, with trees producing large quantities of seeds enveloped in indehiscent pods. Pod production varies from tree to tree, from only a few kilograms to 50-70 kg. The pod shape, weight, length and width also vary between species of Prosopis and also between provenances of same species (Sharma et al., 1993). Similarly, the seed to pulp ratio also varies considerably. Valdivia (1972) reported that the selection of the most vigorous, healthiest and most productive trees for seed, resulted in highly productive plant types. Carvalho (1963) reported that seeds from well formed, vigorous, thornless and highly productive adult trees should be collected for the improved growth of the progeny. Muthana and Arora (1983) and Mutha et al. (1994) also reported that larger and heavier seeds always gave better quality seedlings. However, plantations raised with seed of selected trees exhibited variability over time in growth and subsequent development.

To raise homogeneous plantation stands, multiplication and establishment of cuttings is the only appropriate method, because of the high genetic diversity found in seedlings. Felker and Clark (1981) obtained over 70% rooting with stem cuttings of P. alba, P. chilensis, P. glandulosa, P. pallida and P. velutina. At CAZRI, Jodhpur, vegetative propagation from stem cuttings of introduced Prosopis species gave 40% rooting with P. alba, 11% with P. sp. ‘Peru’ and 6% with P. chilensis in open nursery conditions. In the controlled conditions of a mist chamber, over 80% rooting was obtained with similar stem cuttings.

Recently, another method of vegetative propagation, that of grafting, has been standardised. Successful wedge grafting was obtained both at the nursery stage with 4 month old seedlings, and with 12 month old field-planted saplings of P. juliflora. It was observed that July to early August is the most suitable period for the grafting of 4 month old seedlings, when success rates of greater than 80% were achieved. Wedge grafting in the nursery appears a suitable technology for the production of erect, thornless seedlings for supplying to farmers and other users for direct field planting.

Seed and nursery work

The extraction of pure seeds from the pods is very difficult, as each seed is encased in a hard endocarp shell. In nature, the pods are eaten by grazing animals and the seeds pass through the rumen intact, germinating easily on excretion. Natural germination via this process is common, and the rapid and uncontrolled spread of this species can take place. Therefore, for the revegetation of any land with Prosopis species in a planned manner, large scale seed extraction is required.

At VRTI (Vivekanand Research and Training Institute, Mandvi, Gujarat), a thresher was developed for seed extraction, where by 1993, 1097 t of P. juliflora pods had been collected and processed (Kanzaria and Varshney, this volume). In the state of Gujarat, more than 10 such threshers are in operation. Some 10-15% of the seeds are damaged during processing, but then no pretreatment is required for germination as the seed coat is scarified by the threshing process. Chemical extraction is also possible, by soaking pod segments in 0.1 % hydrochloric acid solution for 24 h or a 4% solution of sodium hydroxide for at least 30 min (Silva, 1988).

The seeds of P. juliflora have a hard seed coat and require pretreatment before sowing. Bohra et al. (1994) reported mechanical scarification to be the most effective pretreatment. However, for large quantities of seeds, only sulphuric acid treatment was found to be suitable. Mutha et al. (1994) observed that larger and heavier seeds produced higher germination rates and better quality seedlings as compared to smaller seeds.

The seeds, once removed from their endocarps, are used for raising seedlings in the nursery and for direct sowing in the field, however in arid regions, direct sowing of seeds has failed to give any encouraging results, primarily due to low and erratic rainfall. For improved establishment, the raising of seedlings in a nursery is prerequisite. Mutha et al. (1995) observed that depth of sowing is very important to obtain high germination. Under nursery conditions, sowing at 1 cm depth gave the maximum germination, while seeds sown at 5 cm depth completely failed to germinate.

In the nursery, seedlings are raised in clear polythene bags or tubes (approximately 11 cm x 4 cm, from 300 gauge plastic). More recently, thermocol pots are also being used for raising seedlings. In Uttar Pradesh and some other states, black polythene bags are being used instead of transparent ones to avoid algal growth, which can occasionally adversely effect seedling development.

Planting

Plant spacing depends upon the objectives of the plantation. If the objective is fuelwood production, then the plant density should be high, with spacings of 2 x 2 m or less. For afforestation programmes for multiple utility, the recommended spacing is 3 x 3 m, especially in the case of saline-sodic soils. For sand dune stabilisation, the spacing used is 5 x 5 m. P. juliflora is often used for live fencing and as windbreaks with a recommended spacing of 1 x 1 m. For silvopastoral and other agroforestry systems, the recommended spacing is 10 x 10 m.

Seedlings attain a height of 35-45 cm after four months and are ready for out-planting at selected sites, normally in early July after the first effective rains. At planting, seedlings should be removed from the nursery container, and if the roots are coiled they should be cut or made straight. Pesticides should be applied to the planting hole to prevent termite damage and irrigation should be provided. Weeding should be carried out twice during the monsoon season and once more the following March. Growing saplings should be trained into single-stemmed trees from the very beginning by removing side branches, unless they are grown for the purpose of live fencing.

Production potential

To determine the optimum age for fuelwood harvesting, a study on yield of P. juliflora was carried out at 4 sites; Sardarshahar (268 mm), Jhunjhunu (395 mm), Gadra Road (285 mm), and Bikaner (285 mm). There were wide variations in the fuel yield with respect to age, habitat and the amount of rainfall received. With the increase in rainfall from west to east in western Rajasthan, fuel production showed a corresponding increase (Table 1). Different workers have reported variable biomass production from different regions. Gurumurti et al. (1984) reported 113.25 t/ha when planted at 1.3 x 1.3 m spacing, while Chaturvedi (1985) reported a harvest of only 6.7 t/ha from two year old plantations of P. juliflora on saline-sodic (usar) soils. Patel (1986) reported that high density plantations of P. juliflora (10000 trees/ha) produced 250 t/ha, 5 years after planting. Rao (1986) recorded a yield of 50 t/ha from Andra Pradesh.

Table 1. Average fuel yield per tree in kg (y) and percentage standard error (cv) as affected by different ages at 4 sites, indicating significant differences (Sig. dif.) between sites and ages.

P. juliflora trees normally start bearing pods 3 years after planting. Pod production in the early years is very poor, increasing with increasing age. In a recent survey, great variability in pod production was recorded, from 5 to 40 kg/tree, depending on rainfall and the habitat. Saxena and Venkateshwarlu (1991) reported pod production of 2300 kg/ha from a plantation with a density of 118 trees/ha. The pod production from saline areas of the Banni region of Gujarat varied from 15 to 20 kg/tree.

The wood of P. juliflora is rated as a high quality timber, although the sapwood is very different from the heartwood. The sapwood is yellow in colour and susceptible to wood boring insects, while the heartwood is a rich dark brown colour and very hard. The grain of the wood is close, straight to slightly interlocking, and medium coarse in texture. The wood is very dimensionally stable and extremely hard, fairly easy to work and finishes smoothly. In Andhra Pradesh alone, 250000 m³ of P. juliflora timber is produced annually (Rao, 1986).

Alternative uses

Gum production is approximately 40 g/tree/yr. From the Kutch districts of Gujarat, 300 t of gum is being collected annually and is used mainly as an emulsifying agent in industry. Where P. juliflora grows abundantly, it is the main source of pollen for honey production, with trees flowering twice a year. Approximately 300 t of honey was collected from P. juliflora plantations in the last 5 years in Gujarat, and simultaneously during the process of purification, about 15 t of beeswax was also obtained over the same period. Also, in addition to fuelwood, the GSFDC (Gujarat State Forest Development Corporation) also manufactures high quality charcoal from P. juliflora wood in substantial quantities.

P. juliflora also has a large role in soil conservation and improvement. Plantations on salt-affected soils have ameliorating effects, by favourably affecting the physical properties of the soil (Singh et al., 1993). Yadav and Singh (1970) observed that P. juliflora plantations on alkaline soils lead to a decrease in the salt content and an increase in organic matter and nitrogen content. Singh et al. (1989) reported the effects of P. juliflora plantations on improving soil water capacity and moisture relations of alkaline soils, with the depth of infiltration after 18 months after planting being only 2.29 cm in control conditions, and 5.21 cm in the soils where plantations were raised. In Rajasthan, P. juliflora and Acacia tortilis are the main tree species used for sand dune stabilisation. Plantations of P. juliflora are now being established with the use of industrial effluent mixed with water, and also for the revegetation of mine-spoiled areas (Aggarwal et al., 1994; Sharma et al., 1996).

Genetic improvement

Despite all the advantages of Prosopis species, rural people do not favour the planting of P. juliflora because of the stiff thorns, and the bushy and invasive nature of the tree. Therefore, efforts are being made to improve the species in order to obtain thornless, straight-boled and high yielding plant types (both for fuelwood and pod production). Sharma et al. (1993) selected 44 plus trees of P. juliflora from different locations in Rajasthan, Gujarat and Uttar Pradesh, and studied the variability in seed and pod characters, and performance at the nursery and one year old plantation stages, and great variability in the different characters was reported. Kumar et al. (1993) also conducted studies aimed at improving the P. juliflora growing in saline-sodic soils of Uttar Pradesh. Fifteen provenances were selected from plantations at Fisher forest and Bovine areas of Etawah; Allen Forest, Kanpur; Brindavan block, Mathura; and Kukrail, Lucknow. A very high degree of variability was observed among the different provenances, due to the obligatory out-crossing breeding mechanism of Prosopis species. Six provenances of P. pallida were introduced in 1985 from Peru and their performance were studied, revealing that accession 1124 had the fastest growth rate, attaining a height of 4.8 m after 6 years (Sharma et al., 1993). Several other studies have been conducted, at CAZRI, Rajasthan (Harsh and Tewari, 1992; 1993), including various species, results from one trial containing 7 species is given in Table 2. Results from several other trials conducted at CAZRI, and some others from elsewhere in India are detailed in this volume.

Table 2. Growth performance of different Prosopis species in the second year of planting, ranked in order of mean annual increment (MAI).

Conclusions

Certain scientists with far reaching future vision predict that two plant genera will have a major impact on man’s survival; Prosopis and Acacia. The development of desert regions is an issue that at present concerns humanity as a whole, because currently between 36% and 46% of the earth’s land area is classified as desert. In India alone, more than 300,000 km² is classified as arid, of which 61% is in the Thar desert of western of Rajasthan. P. juliflora is at present providing more than 70% of the total fuelwood needs of the population of tropical arid and semi-arid parts of India. Moreover, a sizeable proportion of the cattle, goat and sheep population of these areas feed primarily on the pods and leaves of P. cineraria and on the pods of P. juliflora. Experimental results have shown that the majority of exotic species of Prosopis are highly adaptable to the environmental conditions of these areas. Thus the introduction and management of various exotic Prosopis species can play a significant role in uplifting rural communities, especially in the arid and semi-arid tract of the country.

References

Bohra, M.D., J.C. Tewari, U. Burman, N.K. Sharma and L.N. Harsh, 1994. Response of pre-treatments of germination on Prosopis juliflora (SW) DC. Journal of Tropical Forestry 10: 305-309.

Carvalho, R.F., 1963. A Algarobeira. Anuario dos Criadoves de São Paulo 10: 195-199.

Champion, H.G. and S.K. Seth, 1968. A Revised Survey of Forest Types of India. Government of India Publication Branch, New Delhi. 404p.

Chaturvedi, A.N., 1985. Fire wood crops. Uttar Pradesh Forest Department Technical Bulletin, Lucknow, India.

Felker, P. and P.R. Clark, 1981. Rooting of mesquite (Prosopis) cuttings. Journal of Range Management 34: 466-468.

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

Harsh, L.N. and J.C. Tewari, 1992. To evaluate Prosopis species for biofuel and pod production for arid, semi-arid and salt affected soils of India. Project No. FG-IN-749 (IN-AU-420). Annual Progress Report. Central Arid Zone Research Institute, Jodhpur, India. 29p.

Harsh, L.N. and J.C. Tewari, 1993. To evaluate Prosopis species for biofuel and pod production for arid, semi-arid and salt affected soils of India. Project No. FG-IN-749 (IN-AU-420). Annual Progress Report. Central Arid Zone Research Institute, Jodhpur, India. 42p.

Mutha, Neeta, U. Burman, J.C. Tewari and L.N. Harsh, 1994. Effect of seed weight on germination and seedling quality of Prosopis juliflora (SW) DC. Annals of the Arid Zone 33: 252-254.

Mutha, Neeta, U. Burman, L.N. Harsh and J.C. Tewari, 1995. Effect of sowing depth on germination and seedling quality of Prosopis juliflora. Journal of Tree Science 14: 41-43.

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. Role of Prosopis in wasteland development. In: Patel, V.J. (ed.), The Role of Prosopis in Wasteland Development. Jivrajbhai Patel Agroforestry Center, Surendrabag, Gujarat, India.

Rao, E.V.G., 1986. Prosopis in Andhra Pradesh. In: Patel, V.J. (ed.), The Role of Prosopis in Wasteland Development. Jivrajbhai Patel Agroforestry Center, Surendrabag, Gujarat, India.

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

Sharma, N.K., L.N. Harsh, J.C. Tewari and M.D. Bohra, 1993. Variability and changes in genetic parameters for plant height in Prosopis pallida (Humboldt and Barplant ex. Willdenow HBK). Annals of the Arid Zone 32: 273-275.

Silva, S., 1988. Prosopis juliflora (SW) DC. in Brazil. In: Habit, M.A. and Saavedra, J.C. (eds.), The Current State of Knowledge on Prosopis juliflora. FAO, Rome. pp29-55.

Singh, Gurbachan, I.P. Abrol, and S.S. Cheema, 1989. Effect of gypsum application on mesquite and soil properties in an abandoned sodic soil. Forest Ecology and Management 29: 1-14.

Singh, Gurbachan, N.T. Singh and O.S. Otomar, 1993. Agroforestry in salt affected soils. Bulletin No. 17, Central Soil Salinity Research Institute, Karnal, India. 65p.

Valdivia, S.V., 1972. El algarrobo. Una especie forestal prometedora para los tropicos aridos. Boletin de Divulgacion No. 32. Peru Ministry of Agriculture, Lima. 4p.

Yadav, J.S.P. and K. Singh, 1970. Tolerance of certain forest species to varying degrees of salinity and alkali. Indian Forester 96: 587-599.

Review of applied aspects of Prosopis - P. Felker

Center for Semi-Arid Forest Resources, Texas A&M University, Kingsville, Texas 78363, U.S.A.

Introduction

Taken together, Prosopis juliflora and P. cineraria are a very complimentary and powerful pair of species in providing fuelwood, forage and increased fertility to the arid zones of India. Despite the widespread occurrence and utilisation of these species, there has been little genetic improvement of P. cineraria and virtually none of P. juliflora. This paper will review some of the techniques and development opportunities for Prosopis in India.

Pod production

The possibilities for genetic improvement in pod quality and pod production are excellent due to the enormous variation in these factors. For a 5 year old plantation, Felker et al. (1984) found that mean pod production per accession ranged from 7.1 kg/tree to 0.0 kg/tree for 25 accessions. Even more striking was that the accession with the greatest mean pod production had a range of 3.2 to 12.2 kg/tree.

Perhaps even more noteworthy than the variation in pod production per tree was the variation in protein and sugar contents. Odoul et al. (1986) examined the chemical composition of 30 Prosopis accessions grown in 3 environments. He found that the pod sugar composition and pod protein content varied from 11% to 40% and from 9.2% to 18% respectively. Accessions with the high protein and sugar contents were similar at the 3 sites. While heritabilities could not be calculated due to the fact that the male parents were not known, intraclass correlations were 0.40 for pod protein content and 0.30 for pod sugar content over the 3 sites. These values indicated considerable genetic control of these pod characters.

There have been a number of studies that have examined the use of Prosopis pods in animal rations (Felker, 1984). Prosopis pods were a major food source for native Americans in prehistoric times. This author finds that Prosopis flour prepared from the pod mesocarp imparts a pleasant aroma and a much liked flavour to baked pastries, when normal flour is substituted with one third Prosopis flour. Saunders et al. (1986) described a 1000 kg/hr dry milling facility suitable for processing Prosopis pods into a high fibre fraction, a high protein faction, a high sugar fraction and a galactomannam gum fraction. Saunders et al. (1986) also described the suitability of the use of Prosopis flour into chapatis, crackers, flakes, leavened breads and tortilla chips.

Relative value of Prosopis wood products

In designing development projects, it is important to know the relative value of the various uses of Prosopis products. The lowest monetary value is that of the direct energy value of Prosopis wood. Since there are 17 million BTU per dry ton of Prosopis wood, the value of a ton of wood at US$1/million BTU (1 million BTU = 1000 cubic feet of natural gas) is only $17/ton. In contrast, Prosopis is much more valuable when converted into dimensional lumber. This is because Prosopis wood has exceptional dimensional stability with regard to changing moisture conditions (Tortorelli, 1956; Welden, 1986), and that it has similar hardness and density to oak (Quercus spp.) and other exotic tropical hardwoods. When compared with current wholesale prices for cherry (Prunus avium), walnut (Juglans spp.) and oak lumber at US$1-3 per board foot, the value of Prosopis lumber ranges from US$604-1812/dry ton, or US$423-1269/m³.

This clearly points out the fact that economic development strategies should be geared to production of lumber whenever possible. Since Prosopis requires a great deal of pruning to obtain straight poles suitable for lumber, there is a great opportunity for the use of prunings for fuelwood, to be obtained while producing clear boles for high value lumber in long rotations. As pointed out later, we have measured annual growth rates of 1.25-1.50 cm/yr in basal diameter which translates into a 20-30 year rotation for a 40 cm diameter saw log (Cornejo-Oviedo et al., 1991a). We have estimated that these 40 cm basal diameter trees contain about 0.17 m³ of lumber worth a minimum of US$70/tree (Felker et al., 1988).

While Prosopis lumber has exceptional properties and has also attractive colour and figure, it has not been widely used in fine furniture due to the short length and small diameter of the logs. This can be overcome by the fact that there is an increasing tendency to directly produce blanks for furniture parts rather than the standard 8 feet long by 12 inch wide (2.4 x 0.3 m) boards (Araman et al., 1982). A recent survey of hardwood lumber sizes used in the U.S.A. cabinetry industry found that 90% of the actual pieces were less than 60 cm long and less than 15 cm wide (Araman et al., 1982). These authors suggested that standard sized blanks could be produced that are much smaller than normal lumber lengths and widths. These smaller widths and lengths could accommodate smaller sized hardwood logs. This is an approach that would be very feasible with Prosopis.

However, the routine unavailability of 2.4 m long Prosopis logs precluded its grading and thus marketability through official National Hardwood Lumber Association (NHLA) grading channels. Thus Los Amigos dei Mesquite, the organisation of producers and users of Prosopis in the U.S.A., proposed a modified set of grading rules to the NHLA that used shorter and narrower boards, which for the highest grade only needed to be 4 feet (1.2 m) long and 6 inches (0.15 m) wide and have a minimum clear cutting surface free of cracks and defects of 4 inches (0.1 m) by 24 inches (0.6 m). This grading system was presented to the NHLA at their 1993 annual convention in Dallas and was voted on by the membership. Since the NHLA Chief Inspector recommended its approval, it is hoped the general membership will ratify these grading rules. At the NHLA convention, major hardwood buyers from Europe, Japan, Canada, South America and the U.S.A. were present and were all favourably impressed with Prosopis lumber. Thus it appears as if the international climate is ripe for an excellent demand to high value Prosopis lumber.

Technology in use in north-eastern U.S.A. and Canada for small diameter, short length logs needs to be examined for use in India. This technology is adapted to high volume production from 15-30 cm diameter logs that are less than 1.2 m in length. For example, a 3 man crew has been reported to saw small walnut logs at about 1.5 m³/hr with a ‘bolter saw’, and at a value of over US$400/m³ this would be very significant. Small, 2 to 3 person sawmills are commercially available for US$3000-15000, of the bandsaw and circular type. While the circular sawmills take more kerf than the bandsaws, the circular saws are more rugged and can take more abuse with the hard wood of Prosopis.

The 2 types of sawmills that seem most adapted to high lumber production from small Prosopis logs are the bolter mill and the Scragg mill. The bolter mill typically has one 75 cm diameter blade with a 40 hp motor. The logs are placed on a sliding metal table that passes by the saw. The logs are not fastened to the table with ‘dogs’ and are merely pushed through the saw. The Scragg mill has 2 circular blades, about 75 cm in diameter on the same shaft. For small logs, the spacing between the blades is about 10 to 15 cm. In earlier models, the logs fall into a ‘V’ through which has the effect of centring them. They are then pushed between the blades by a clip on a chain. This mill cuts a slab off 2 sides of the log at one time to make a 2 sided cant 10-15 cm thick. A Scragg mill at Texas A&M was set to cut 2 logs per minute. Once a 2 sided cant is produced, it can be laid on one side on standard tables and processed rapidly with standard resaws, planers etc.

Due to the greater hardness and density of Prosopis, cutting tooth angles, horsepower requirements, feed rates and tooth composition must be optimised for Prosopis. Mr. S. Lunstrum, sawmill specialist at the Forest Products Laboratory of the U.S. Forest Service in Madison, Wisconsin has a computer program to optimise sawmill systems. Mr. Lunstrum has been most helpful to the Texas A&M University sawmill project. It would certainly be useful if a centre of excellence for Prosopis sawmill technology were established in India to support this industry.

Genetic improvement of Prosopis

Due to self-incompatibility in Prosopis (Simpson, 1977), the male pollen must come from a different source other than the female parent (mother tree). As a result, there is great genetic variability in Prosopis. Hunziker et al. (1986) has measured the genetic variability and described numerous natural hybrids in Prosopis. Synthetic hybrids have also been created in reciprocal crosses between P. alba, P. velutina, P. juliflora and P. glandulosa var. torreyana. In addition to obtaining useful hybrids through hybridisation, there is the opportunity to do so by merely choosing outstanding trees and strains from existing genetic material of genetically broad-based field trials. Since Prosopis is outcrossed, individual trees do not breed true to type and there is a strong need for asexual propagation.

A review of asexual propagation strategies for Prosopis has recently appeared (Felker, 1992). Early work: (1) examined use of a DMSO based rooting formulation (Felker and Clark, 1981); (2) identified optimum temperature, light and photoperiod requirements (Klass et al., 1984); (3) optimal hormone drenches (Klass et al., 1986); (4) fertilisation regimes for stock plants (DeSouza and Felker, 1986) and (5) development of a solar powered mist system for use in remote sites without electrical power (Wojtusik et al., 1994). When optimum environmental conditions of stockplants and cuttings are achieved, almost no hormones are required for rooting. However, when less than optimal environmental conditions are obtained, A DMSO based formulation with 6000 mg/l IBA and 9000 mg/l NAA appears useful in increasing rooting percentages. High biomass producing clones have been obtained from both P. alba (Felker et al., 1983a; Felker et al., 1989) and thornless Peruvian Prosopis (Wojtusik et al., 1993).

More recently, interspecific graft incompatibility between North American, South American and tropical species has been demonstrated (Wojtusik and Felker, 1993). This will allow genetic improvement of existing weedy, thorny Prosopis without having to produce nurseries, transport seedlings and water to the site or transplant and protect the small seedlings. This will also permit utilisation of root stocks specially adapted to edaphic conditions or perhaps to stimulate early flowering or more resistance to cold weather. In the greenhouse, some Prosopis have been demonstrated to grow in salinities equivalent to full sea water (Rhodes and Felker, 1987).

Biomass estimation

Whether one is developing stand management techniques or estimating biomass in plantations, regression equations relating biomass of volume of standing timber to diameter, height or canopy diameter are required. Felker et al. (1983b) reported a combined regression based on the harvest of over 1300 trees with ranges in diameter from 0.2 cm to 15.0 cm. Felker et al. (1989) reported on regression for larger trees in a 3 year old plantation. El Fadl et al. (1989) used a log loader to develop regression equations for 10 entire mature trees. The basal diameter and weights ranged from 20 cm to 55 cm and from 218 kg to 2540 kg. In addition, the small and large diameter and the length of all the individual stem segments were measured. Using the diameter equation developed by Rogers (1984) between log dimensions and lumber recovery, regressions were developed to relate total volume per tree and total lumber per tree to basal diameter.

Management of existing stands

While genetic improvement has great potential for Prosopis, the immense existing stands of Prosopis dictate that better management of the existing stands be conducted. Undeniably dense stands (>10000/ha) of small diameter trees (<3 cm) are undesirable, whereas large trees (>40 cm diameter) on wide spacings (>7 m) are desirable, in providing pods, shade, firewood and enhanced soil fertility. Johnson and Mayeux (1990) found nodules on nearly all Prosopis in native field settings in Texas. Generally, the dense stands of small diameter trees are of recent colonisation (within 10 years) and if left to mature would eventually thin out due to self-mortality. The question then becomes, what are the combinations of stem diameter and spacing which are sustainable? Felker et al. (1988) measured 27 natural plots ranging in stem density from 18800 stems/ha to 6 stems/ha and observed a negative straight line logarithmic regression equation. This equation predicted that 42 cm diameter trees could be obtained on 9.5 m spacings. Obviously this relation would be sensitive to rainfall and presence of groundwater. The greater the moisture availability, the closer it would be possible to obtain the same large sized trees.

A thinning experiment with dense stands of small trees (Cornejo-Oviedo et al., 1991a) found that stagnated dense stands grew at a rate of only 0.25 cm/yr in basal diameter, whereas stands that were thinned to 10 m spacing and intercropped, grew at a rate of 1.25 cm basal diameter per year. This intercropping also prevented the invasion of dense stands of small trees. Another experiment was conducted with an overstocked stand of large trees (average basal diameter of 16.4 cm) that had a dense shrubby understory, to determine the principle factors determining tree growth (Cornejo-Oviedo et al., 1991b). One treatment consisted of understory removal, one of thinning multi-stemmed trees to single stemmed trees, one of fertilisation with 100 kg P/ha to stimulate nitrogen fixation, and combinations of the treatments above. Despite the fact that the available phosphorus in the soil (Olsens) was only 2 ppm, the phosphorus application had no influence on growth. The only treatments which had a major influence on growth were those that included removal of the dense understory. When these stands were thinned to convert multi-stemmed trees to single stemmed trees, 22 m³/ha of small diameter firewood was obtained.

The self-thinning line that predicts the spacing required for large trees also suggests that if large trees are eventually obtained, they will provide sufficient intraspecific competition to prevent dense stands of small trees from becoming established. Thus rather than rely on herbicides or mechanical eradication even including bulldozers, it appears as if the most sustainable technique for avoiding dense, weedy stands of small trees is to create large trees on wide spacings that will dominate the site. In this thinning process, it may well be necessary to use selective herbicide treatments to kill individual trees. One of the most effective treatments is a basal application of 175 ml of a trichlopyr plus picloram mixture (Grazon P+D) in 8 l of diesel, that is applied 15 cm above ground level to the stump after cutting.

Thus stand management techniques can be very effectively applied to Prosopis to create large, straight, single-boled trees on wide spacings that are useful for lumber, to provide thinnings for fuelwood, to enhance tree growth, and to provide intraspecific competition to prevent the encroachment of dense stands of small trees.

References

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