Ecology and Drought Management Ecology of Milletia pinnata in the arid regions of Texas By John M Jeapes Introduction Milletia pinnata is a species of tree in the pea family, Fabaceae, native in tropical and temperate Asia including parts of India, China, Japan, Malaysia, Australia and Pacific islands. It is often known by the synonym Pongamia pinnata as it was moved to the genus Millettia only recently. Common names include Indian Beech, Honge and Pongam among others. Naturally distributed in tropical and temperate Asia, from India to Japan to Thailand to Malaysia to north and north-eastern Australia to some Pacific islands; It has been propagated and distributed further around the world in humid and subtropical environments from sea-level to 1200m, although in the Himalayan foothills it is not found above 600m. Withstanding temperatures slightly below 0 °C (32 °F) and up to about 50 °C (120 °F) and annual rainfall of 500–2,500 mm (20–100 in), the tree grows wild on sandy and rocky soils, including oolitic limestone, and will grow in most soil types, even with its roots in salt water. The tree is well suited to intense heat and sunlight and its dense network of lateral roots and its thick, long taproot make it drought-tolerant. The dense shade it provides slows the evaporation of surface water and its root nodules promote nitrogen fixation, a symbiotic process by which gaseous nitrogen (N2) from the air is converted into ammonium (NH4+, a form of nitrogen available to the plant). Specifically M. pinnata is an outbreeding diploid legume tree, with a haploid chromosome number of 11. Root nodules are of the determinate type (as those on soybean and common bean) formed by the causative bacterium Bradyrhizobium. The oil has a high content of triglycerides, and its disagreeable taste and odor are due to bitter flavonoid constituents including karanjin and karanjachromene. It grows well 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 Milletia pinnata 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. See Saxena and Venketeshwarlu, 1991). Plant characteristics A legume tree, it grows to about 15–25 meters (15–80 ft) in height it has a large canopy which spreads equally wide. It may be deciduous for short periods. The leaves are a soft, shiny burgundy in early summer and mature to a glossy, deep green as the season progresses. Flowering starts in general after 3–4 years. Cropping of pods and single almond sized seeds can occur by 4–6 years. Small clusters of white, purple, and pink flowers blossom on their branches throughout the year, maturing into brown seed pods. The 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. Ecological attributes Milletia pinnata has become naturalised in many of the arid and semi-arid parts of the world. 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. A legume tree, it grows to about 15–25 meters (15–80 ft) in height it has a large canopy which spreads equally wide. It may be deciduous for short periods. The leaves are a soft, shiny burgundy in early summer and mature to a glossy, deep green as the season progresses. Flowering starts in general after 3–4 years. Cropping of pods and single almond sized seeds can occur by 4–6 years. Small clusters of white, purple, and pink flowers blossom on their branches throughout the year, maturing into brown seed pods. Milletia pinnata 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 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 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 once the tree is large enough. This species is however, quite susceptible to frost. Young trees lose their leaves in frosty weather, but the leaves regrow strongly in spring. Major growth takes place after the rainy season, and Milletia pinnata 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 Milletia pinnata 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. Uses The Honge Tree is well-adapted to arid zones and has many traditional uses. It is often used for landscaping purposes as a windbreak or for shade due to the large canopy and showy fragrant flowers. The flowers are used by gardeners as compost for plants requiring rich nutrients. The bark can be used to make twine or rope and it also yields a black gum that has historically been used to treat wounds caused by poisonous fish. The wood is said to be beautifully grained but splits easily when sawn thus relegating it to firewood, posts, and tool handles. Although all parts of the plant are toxic and will induce nausea and vomiting if eaten, the fruits and sprouts, along with the seeds, are used in many traditional remedies. Juices from the plant, as well as the oil, are antiseptic and resistant to pests. In addition M. pinnata has the rare property of producing seeds of 25–40% lipid content of which nearly half is oleic acid. Oil made from the seeds, known as honge oil, is an important asset of this tree and has been used as lamp oil, in soap making, and as a lubricant for thousands of years. Long used as shade tree, M. pinnata is heavily self-seeding and can spread lateral roots up to 9m over its lifetime. If not managed carefully it can quickly become a weed leading some, including Miami-Dade County, to label the tree as a invasive species. However this dense network of lateral roots makes this tree ideal for controlling soil erosion and binding sand dunes. Research efforts The seed oil has been found to be useful in diesel generators and, along with Jatropha, it is being explored in hundreds of projects throughout India and the third world as feedstock for biodiesel. It is especially attractive because it grows naturally through much of arid India, having very deep roots to reach water, and is one of the few crops well-suited to commercialization by India's large population of rural poor. Several unelectrified villages have recently used honge oil, simple processing techniques, and diesel generators to create their own grid systems to run water pumps and electric lighting. In 2003 the Himalayan Institute of Yoga Science and Philosophy as part of its Biofuel Rural Development Initiative started a campaign of education and public awareness to rural farmers about M. pinnata in two Indian states. One of the Himalayan Institute's partners developed a consistently high yield scion that reduced the time it takes to mature from 10 years to as little as three. To help the farmers in the transition from traditional crops to M. pinnata the Indian government has contributed over $30 million in low-interest loans and donated 4.5 million kg (5,000 short tons) of rice to sustain impoverished drought-stricken farmers until the trees begin to produce income. Since the project began in 2003 over 20 million trees have been planted and 45,000 farmers are now involved. In 2006 the Himalayan Institute began looking at locations in Africa to transplant M. pinnata into. Initially they began in Uganda but due to the lack of infrastructure and growing desertification the project has been growing very slowly. They have also begun a project in the Kumbo region of Cameroon where conditions are better. There has been some suggestions that M. pinnata could be grown all the way across the continent as a way to prevent the encroachment of the Sahara. Pacific Renewable Energy trial plantation in Caboolture, Queensland. The University of Queensland node of the Australian Research Council Center for Excellence in Legume Research, under the directorship of Professor Peter Gresshoff, in conjunction with Pacific Renewable Energy are currently working on M. pinnata for commercial use for the production of biofuel. Projects are currently focused on understanding aspects of M. pinnata including root biology, nodulation, nitrogen fixation, domestication genes, grafting, salinity tolerance, and the genetics of the oil production pathways. Emphasis is given to analyzing carbon sequestration (in relation to carbon credits) and nitrogen gain. Research has also been put into using the material leftover from the oil extraction as a feed supplement for cattle, sheep and poultry as this by-product contains up to 30% protein. Other studies have shown some potential for bio-cidal activity against V. cholerae and E. coli, as well an anti-inflammatory, anti-nociceptive (reduction in sensitivity to painful stimuli) and antipyretic (reduction in fever) properties. There is also research indicating that M. pinnata can be used as a natural insecticide. 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 septum, coated with a sweet, thin, dry pulp, whereas the whole septa body is enclosed in a dry, yellow pulp containing 20-30% sucrose. 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 M. pinnata stands. Seed extraction What makes Millettia pinnata a superior, more sustainable biofuel species? 1 Carbon Sequestration. Millettia grows into a large tree with a 10-metre taproot, creating a huge carbon sink 2 Resilience. Millettia is resistant to a wide range of adverse climatic conditions: drought, light frost, water logging, moisture stress and salinity 3 Tolerance. Millettia does not require prime arable land otherwise used for food production 4 Millettia is tolerant of extremely poor soil types and does not require prime arable land otherwise used for food production. 5 Water and nutrients. Millettia is a nitrogen fixing leguminous tree that can source water and nutrients deep into in the subsoil 6 Carbon credits. Millettia’s carbon-fixing qualities qualify the tree for carbon credits 7 Millettia easily surpasses plantation oil yield of other oil crops such as oil palm and jatropha with high oil content per seed (45-50%). 8 Long-term yield. The oil yield in a Millettia plantation continues to increase for 15 years 9 Long life-span. Millettia has a lifespan of 100 years with a productive oil seed lifespan of 60 years 10 Millettia plantations can be managed by smaller/unskilled workforce due to lower crop maintenance and ability to utilise mechanical pruning and harvesting equipment. 11 Millettia is a legume therefore minimising irrigation and expensive fertilizer requirements. 12 Huge yield potential. At maturity, Millettia trees regularly produce 800 – 1,000kgs of seed, per tree, every year 13 Flexibility. Millettia thrives in temperatures from zero degrees Celsius, right up to 50 degrees Celsius 14 Environmental friendly. Intensive Millettia crops sequestrate more than 50 tonnes of CO2 per hectare, per year. 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 o C 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.
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