Overstory #135 - Medicinal and Aromatic Plants in Agroforestry
Medicinal and aromatic plants (MAPs) play an important role in the healthcare of people around the world, especially in developing countries. Until the advent of modern medicine, man depended on plants for treating human and livestock diseases. Human societies throughout the world have accumulated a vast body of indigenous knowledge over centuries on medicinal uses of plants, and for related uses including as poison for fish and hunting, purifying water, and for controlling pests and diseases of crops and livestock. About 80% of the population of most developing countries still use traditional medicines derived from plants for treating human diseases (de Silva 1997). China, Cuba, India, Sri Lanka, Thailand, and a few other countries have endorsed the official use of traditional systems of medicine in their healthcare programs. For example, the Indian systems of medicine "Ayurveda," "Sidha" and "Unani" entirely, and homeopathy to some extent, depend on plant materials or their derivatives for treating human ailments (Prajapati et al. 2003). People in villages and remote areas primarily depend on traditional medicines as the modern system is out of reach and expensive. Many among the educated in Asian and African countries use traditional medicines for reasons of firm belief that they are more effective than modern medicine for certain chronic diseases, they do not have side effects of some of the modern medicines, and/or for economic reasons. Thus, in many societies, traditional and modern systems of medicines are used independently.
About 12.5% of the 422,000 plant species documented worldwide are reported to have medicinal value; the proportion of medicinal plants to the total documented species in different countries varies from 4.4% to 20% (Schippmann et al. 2002). About 25% of drugs in modern pharmacopoeia are derived from plants and many others are synthetic analogues built on prototype compounds isolated from plants. Up to 60% of the drugs prescribed in Eastern Europe consist of unmodified or slightly altered higher plant products (Lancet 1994). These drugs carry important therapeutic properties including contraceptives, steroids and muscle relaxants for anesthesia and abdominal surgery (all made from the wild yam, Dioscorea villosa); quinine and artemisinin against malaria; digitalis derivatives for heart failure; and the anti-cancer drugs vinblastin, etoposide and taxol. These compounds cannot be synthesized cost-effectively, which means that their production requires reliable supplies of plant material (van Seters 1997).
The global importance of MAP materials is evident from a huge volume of trade at national and international levels. During the 1990s, the reported annual international importation of MAPs for pharmaceutical use amounted on average to 350 000 Mg valued at over USD 1 billion. A few countries dominate the international trade with over 80% of the global import and export allotted to 12 countries each. Whereas Japan and Korea are the main consumers of medicinal plants, China and India are the world's leading producing nations. Hong Kong, United States and Germany stand out as important trade centers. It is estimated that the total number of MAPs in international trade is around 2500 species worldwide (Schippmann et al. 2002).
Medicinal trees in traditional agroforestry systems
Many plants in traditional agricultural systems in the tropics have medicinal value. These can be found (either planted or carefully tended natural regenerations) in homegardens, as scattered trees in croplands and grazing lands, and on field bunds (Table 2). Many Acacia species found in Africa such as Acacia nilotica, A. seyal, A. senegal and A. polyacantha, as well as several species in African croplands (e.g. Faidherbia albida, Vitellaria paradoxa, Adansonia digitata, Markhamia lutea and Melia volkensii) have medicinal value (ICRAF 1992). Similarly, "arjun" (Terminalia arjuna) in India, chinaberry (Melia azederach) in Asia and Erythrina species in Latin and Central America combine many uses including medicinal. Holy Basil or "tulsi" (Ocimum sanctum), drumstick (Moringa oleifera) and curry leaf (Murraya koenigii) are backyard plants in many Indian households and they are routinely used for common ailments or in food preparations. A number of plants used as live fences around home compounds such as henna (Lawsonia inermis) in India (Singh et a. 1996), Ipomoea carnea ssp. fistulosa in Bolivia and Asian countries (Frey 1995) and Euphorbia tirucalli around crop fields in Africa (ICRAF 1992) have medicinal values. Although the medicinal value of these plants is "exploited" locally, they are seldom used for commercial purposes (except in the case of commercially exploited neem). In fact, many of these species are valued for poles, fuel wood, fodder, fruit, shade, and/or boundary demarcation and their medicinal value is secondary.
Forests and forest plantations
MAPs growing in forests require (or tolerate) partial shade, moist soils high in organic matter, high relative humidity, and mild temperatures. Cultivation of such MAPs can be taken up in thinned forests and cleared forest patches, and as intercrops in new forest plantations (Table 3). In China, cultivation of medicinal plants has been an age-old practice under the name of "silvo-medicinal" systems. In northeast China, ginseng (Panax ginseng) and other medicinal plants are grown in pine (Pinus spp.) and spruce (Picea spp.) forests; in central China, many medicinal plants are planted with Paulownia tomentosa and in southern China medicinal herbs are often planted in bamboo (Bambusa spp.) and Chinese fir (Cunninghamia lanceolata) forests (Zou and Sanford 1990). In Yunnan province, China, traditional "Dai and Jinuo" agroforestry systems involve the medicinal crop Amomum villosum in the forest areas cleared of undergrowth (Saint-Pierre 1991). The forest is thinned to give 30% to 40% shade and seedlings or cuttings are planted, which produce an average dried fruit yield of 375 kg ha-1 per annum (Zhou 1993). Gupta (1986) listed a number of indigenous understory herbs and shrubs that can be produced as part of forest farming or in new forest plantations to improve economic returns from forests in India (Table 3). Indigenous people living in the Himalayan forest margins in Uttaranchal, India, are known to have conserved and cultivated several medicinal species for centuries (Kumar et al. 2002).
A farmers' cooperative in the northern lowlands of Costa Rica has successfully demonstrated cultivation of the medicinal herb "raicilla" (Cephaelis ipecacuanha) in natural forests for export to the Netherlands and Germany (Hager and Otterstedt 1996). American ginseng (Panax quinquefolium), a medicinal herb exported to China from the United States and Canada is grown as an understorey in red maple (Acer rubrum) forests (Nadeau et al. 1999) or deciduous hardwoods such as black walnut (Juglans nigra) and sugar maple (Acer saccharum), instead of growing under artificial shade with considerable expense (Hill and Buck 2000). Indeed, cultivation of ginseng and several other medicinal plants in the forests is a common and growing form of forest-farming practice of agroforestry in North America (Table 3). Light demanding understory species (e.g. Echinacea sp.) may be intercropped initially to provide early returns from plantations and after canopy closure, shade-tolerant species such as ginseng and goldenseal can be intercropped (Teel and Buck 2002). Studies in New Zealand have indicated that the American ginseng can be successfully grown under Pinus radiata with best growth under a tree stand of 130 stems ha-1 (Follett 1997). In addition to providing shade, the trees may also benefit the understory component from hydraulically lifted water. Fungal diseases are a major concern in forest farming but the application of fungicide can be detrimental to the forests' health, therefore proper spacing and mixed cropping is recommended (Cech 2002). Mechanical cultivation may not be feasible under forested conditions so labor availability needs to be considered as a constraining factor (Hill and Buck 2000).
As in the taungya system, newly established forest plantations can be intercropped with MAPs similar to food crops until the trees cover the ground. The participation of the local people with right to share benefits of the plantations, especially ownership to crops, has helped governments to establish and protect large-scale tree plantations without conflict with the local people in many Asian countries (Nair 1993). The same approach can be employed for the cultivation of MAPs in the new plantations. In the rehabilitation of degraded forestlands, participatory planning and implementation with local communities and economic benefits from an early stage onwards will ensure commitment of the people (Rao et al. 1999).
Homegardens are complex agroforestry systems involving many plant species characterized by different morphology, stature, biological function and utility, practiced mostly in the humid and subhumid tropics (Kumar and Nair 2004). Food, fruit and timber species may dominate the homegardens and occupy the middle and upper strata, but medicinal plants, spices and vegetables occupy the lower stratum. Three categories of medicinal plants could be noted in homegardens: species used exclusively for medicine, horticultural or timber species with complementary medicinal value, and "weedy" medicinal species. While the first two categories are deliberately planted the latter group is part of spontaneous growth. The species composition, plant density and level of management vary considerably depending on the soil, climate and market opportunity and cultural background of the people. Homegardens of individual holdings generally cover small parcels of land and are established around homesteads. Although these systems in the past were mostly seen to meet the home needs of small-scale farmers in the forest margins, increased urbanization, transport and market opportunities in recent times are helping to produce cash value crops. Multistrata systems involve fewer species (three to 10 species) than in homegardens in definite planting arrangement and can be designed for home consumption as well as commercial production.
Medicinal plants are an invariable component of homegardens, whether they are in the Peruvian Amazon (Lamont et al. 1999), on the slopes of the Mt. Kilimanjaro in Tanzania (O'Kting'ati et al. 1984), or in the humid and semiarid Cuba (Wezel and Bender 2003). The species composition differs depending on cultural background, distance from markets and influence of tourism. Medicinal plants accounted for about 27% of total plant species in the homegardens in Amazon (Padoc and de Jong 1991), 56% in northern Catalonia (Iberian Peninsula) (Agelet et al. 2000) and 45% in the floodplains of the river Jamuna in Bangladesh (Yoshino and Ando 1999). In the Soqotra Island, Yemen, endemic medicinal plants such as Aloe perryi, Jatropha unicostata and Commiphora ornifolia are cultivated in homegardens (Ceccolini 2002). On St. Croix and St. Thomas, U.S. Virgin Islands, the medicinal trees neem, moringa, and noni (Morinda citrifolia) have become popular in homegardens (Palada and Williams 2000).
Riparian buffer zones
An agroforestry system that has received considerable attention in North America is riparian buffers zones (Schultz et al. 2004). Riparian buffer zones can improve water quality and protect streams and rivers from degradation by nutrient loading and chemical pollutants from agriculture and urban areas, from erosion by attenuating peak flows and provide habitat for wildlife. NTFP production can help defray the cost of buffer zone installation and maintenance. Slippery elm (Ulmus rubra), harvested for its aromatic and medicinal inner bark, is commonly found in riparian areas in North America (Teel and Buck 2002). Riparian buffer zones are an ideal location for the production of this species, which suffers from commercial over-exploitation and the Dutch elm disease.
Intercropping of maps
Two types of intercropping systems can be distinguished involving MAPs: (1) medicinal plants as upperstory trees and (2) MAPs as intercrops in other tree crops.
Medicinal plants as overstory trees
Coffee (Coffea arabica), cacao (Theobroma cacao) and tea (Camellia sinensis) are traditionally grown under shade offered by multipurpose trees that produce timber, fruit, flowers, nuts, palms etc. Medicinal tree species that grow tall and develop open crown at the top can also be used for this purpose, for example yongchak (Parkia roxburghii) in India, the protein rich seeds of which are used to treat stomach disorders (Balasubramanian 1986) and Ginkgo biloba in China, the nuts of which are used in Chinese medicine and fetch high value (Shen 1998). In Ivory Coast, 19 of the 41 tree species planted as shade trees in coffee and cacao provide pharmaceutical products for traditional medicine (Herzog 1994). New plantations of coffee, tea, and cacao offer scope for cultivation of forest medicinal trees that are under demand. However, research needs to identify the medicinal trees that can be grown in association with these plantation crops and develop management practices for them. Tall and perennial medicinal trees that need to be planted at wider spacing such as Prunus africana, Eucalyptus globulus (for oil), sandalwood (Santalum album), ashok (Saraca indica), bael (Aegle marmelos), custard apple (Anona squamosa), amla (Emblica officinalis), drumstick or moringa (Moringa oleifera) and soapnut tree (Sapindus mukorossi) can be intercropped with annual crops in the early years until the tree canopy covers the ground. Some of the medicinal trees may allow intercropping for many years or on a permanent basis depending on the spacing and nature of the trees. The intercrops give some income to farmers during the period when the main trees have not started production.
Medicinal plants as intercrops
Many tropical MAPs are well adapted to partial shading, moist soil, high relative humidity and mild temperatures (Vyas and Nein 1999), allowing them to be intercropped with timber and fuel wood plantations, fruit trees and plantation crops. Some well known medicinal plants that have been successfully intercropped with fuel wood trees (e.g. Acacia auriculiformis, Albizia lebbeck, Eucalyptus tereticornis, Gmelina arborea, and Leucaeana leucocephala) in India, include safed musli (Chlorophytum borivilianum), rauvolfia (Rauvolfia serpentina), turmeric (Curcuma longa), wild turmeric (C. aromatica), Curculigo orchioides, and ginger (Zingiber officinale) (Chadhar and Sharma 1996; Mishra and Pandey 1998; Prajapati et al. 2003). Only 10 out of 64 herbaceous medicinal plants tried in intercropping with two-year old poplar (Populus deltoides) spaced 5 m apart gave poor performance (Kumar and Gupta 1991), indicating that many medicinal plants can be grown in agroforestry systems. The trees may benefit from the inputs and management given to the intercrops. Short stature and short cycle MAPs and culinary herbs are particularly suited for short-term intercropping during the juvenile phase of trees. Wherever markets are established, MAPs are remunerative alternative intercrops to the traditionally grown annual crops (Maheswari et al. 1985; Zou and Sanford 1990). The number of years MAPs can be intercropped with a given tree species depends on the size and intensity of its canopy shade, tree spacing and management, especially pruning of branches and nature of the MAPs. Shade-tolerant and rhizomatic MAPs can be grown on a longer-term basis in widely spaced plantations.
Intercropping of medicinal plants in coconut (Cocos nucifera) and arecanut (Areca catechu) stands is an age-old practice in India and other parts of south- and southeast Asia. These palms allow 30% to 50% of incident light to the underneath, which is ideal for some MAPs, including cardamom (Elettaria cardamomum). Kacholam or galang (Kaempferia galanga) - a medicinal herb - is traditionally intercropped in mature coconut gardens in Kerala, India. Kacholam intercropped in a 30 year-old coconut plantation produced 6.1 Mg ha-1 of rhizomes compared with 4.8 Mg ha-1 as a sole crop (Maheswarappa et al. 1998). Twelve-year old coconut trees did not adversely affect the growth and yields of a number of medicinal species grown as intercrops compared to the yields in the open (Nair et al. 1989). In Karnataka and Kerala states, India, arecanut palm is commonly intercropped with ginger, turmeric, black pepper (Piper nigrum) and cardamom (Korikanthimath and Hegde 1994). Some of these intercrops may cause small reduction in arecanut yields but the combined returns from both the components are greater than from arecanut alone. Another plantation crop intercropped with MAPs is rubber (Hevea brasiliensis), for example with Dioscorea floribunda in the state of Assam in India (Singh et al. 1998) and with Amomum villosum in Yunnan province of China (Zhou 1993). In Sikkim, India, large cardamom (Amomum subulatum) is grown under 30 different shade tree species (Patiram et al. 1996). In Fujian Province, China, Cunninghamia lanceolata - an important timber tree - is intercropped with a variety of cereals, cash and medicinal and oil-producing crops (Chandler 1994). Many of the medicinal herbs commonly grown in thinned forests can also be grown intercropped with trees (Zhou 1993).
In the Caribbean islands, there has been increased interest on alternative crops that have better economic potential than traditional crops. For example, in the U.S. Virgin Islands, a number of farmers are now opting for specialty crops such as the West Indian hot peppers (Capsicum chinense), thyme (Thymus vulgaris) and chives (Allium schoenosprasum) instead of vegetables (Crossman et al. 1999). The prospects of growing indigenous MAPs such as "japana" (Eupatorium triplinerve), worrywine (Stachytarpheta jamaicensis), inflammation bush (Verbersina alata) and lemongrass (Cymbopogon citratus) in association with the medicinal trees noni (Morinda citrifolia) and moringa have been explored at the University of the Virgin Island, St. Croix, (Palada and Williams 2000). These local herbs are commonly used as bush teas and very popular in the Caribbean. Medicinal plants and herbs in intercropping produced similar yields to those in sole cropping at the first harvest, but they tended to be lower than in sole cropping at subsequent harvests (Palada and Williams 2000).
Traditional systems of medicine in most developing countries depend primarily on the use of plant products either directly or indirectly. Besides serving the healthcare needs of a large number of people, medicinal plants are the exclusive source of some drugs even for modern medical treatment. The use of plant products as nutrition supplements and in the cosmetic and perfume industry has increased the value of medicinal and aromatic plants in recent years. The over dependence on forests, natural woodlands and long-term fallows for extraction of MAPs is threatening the survival of many valuable plant species. It is imperative therefore that such endangered species are cultivated outside their natural habitats to ensure their regular supply for human needs as well as to preserve the genetic diversity. Cultivation is an important strategy for conservation and sustainable maintenance of natural stocks, but few MAPs are actually cultivated. Lack of basic knowledge on biology, ecology, propagation methods and cultural practices for the concerned species is an important constraint.
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About the authors
Meka R. Rao is an agronomist and worked mostly in CGIAR institutes, first at the International Center for Crops Research in Semi-Arid Tropics (ICRISAT) in India, then at the International Institute for Tropical Agriculture (IITA) in Cameroon and lastly as agroforester at the International Centre for Research in Agroforestry (ICRAF, recently designated as World Agroforestry Centre) based at Nairobi in Kenya. At ICRAF he was leading the Ecosystem Rehabilitation program. He also worked as cropping systems specialist in Brazil for two years. Rao's research covered cropping systems, tree-crop-environment interactions, soil fertility replenishment using trees, and pest management in agroforestry. He is author of over 120 research papers and articles published in leading journals. He has returned back to India after his retirement from ICRAF in 2000 and is available at email@example.com.
Manuel C. Palada is a professor of horticultural science at the Agricultural Experiment Station, University of the Virgin Islands on St. Croix, U.S. Virgin Islands. He has wide international experience in tropical agriculture starting with senior research assistant at the International Rice Research Institute (IRRI) in the Philippines, senior agronomist at the International Institute of Tropical Agriculture (IITA), Nigeria and recently a visiting scientist and crop production/ecosystem specialist at the Asian Vegetable Research and Development Center (AVRDC) in Taiwan. He has previous assignments with the Rodale Institute, Pennsylvania, World Bank in Kenya and Somalia, Asian Development Bank (ADB) in the Philippines and USAID in Tanzania, Liberia and Uganda. His major research area is focused on sustainable agriculture, agroforestry systems, medicinal and aromatic plants, microirrigation, vegetable production, culinary herbs and farming systems.
Brian Becker is a research coordinator with the School of Forest Resources and Conservation's Phytoremediation and Short Rotation Woody Crops Program at the University of Florida, Gainesville, Florida. His work with non-timber forest product management spans six years with Nepal's Community Forest Program, the United States' Appalachian Mountains and Southeastern costal plain. Currently finishing his Master's degree specializing in Agroforestry, Brian studied the effects of intercropping medicinal and aromatic plants (MAPs) with multipurpose trees in the U.S. Virgin Islands. His research interests include the biometrics, management, production and marketing of non-timber forest products. He can be reached at: Brian Becker, Building 191 Mowry Rd, PO Box 110831, University of Florida, Gainesville, Florida, 32611-0831, USA; Tel: 01-352-846-3054; Email: firstname.lastname@example.org.
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