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Contents
page 1. INTRODUCTION ................................................................... 3 1.1 Concept and definition of agro forestry ................................ 6 1.2 Classification of agro forestry system ................................... 6
2. Buffer zone agro forestry ............................................................ 8 2.1 Role of Buffer zone agro forestry in natural forest conservation and development .................................................. 10 3. Role of agro forestry in maintaining soil fertility ..................... 13 3.1 Agroforestry as a practical management option for soil fertility ....................................................................................... 14 3. 2 Effects of trees on soils ..................................................... 18 3. 3 Processes by which Trees Maintain or Improve Soil Fertility ................................................................................................... 20 4. Quality of good agro forestry trees ........................................... 22 5. Agro forestry for Control of Soil Erosion................................. 25 5.1 The traditional approach...................................................... 25 6. Agro forestry Practices for Erosion Control ............................. 29 7. Traditional agro forestry practice in Ethiopia ........................... 34 7.1 Historical and social values of the Gedeo Agro forestry system ........................................................................................ 36 8. Agro forestry for ecosystem services and environmental benefits ....................................................................................................... 40 8.1 Agroforestry for improved air and water quality ................ 41 8.2 Agro forestry for biodiversity conservation ....................... 43 8.3 Agro forestry for carbon sequestration................................ 44 9. Summary ................................................................................... 46 10. References ............................................................................... 48
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The Role of Agro forestry in soil and water conservation By Zinabu Wolde Graduate student of Soil Science, Hawassa University, P. O. Box 05, Hawassa, Ethiopia Email:
[email protected]
Abstract The main purpose of this review paper was to gather Role of Agro forestry in soil and water conservation and IDUPHUV¶ DJURecological knowledge, about trees, their interactions with coffee and other components of the shaded tree systems with farming practices. A systematic approach mainly using repeated semistructured interviews was used for acquiring knowledge about the coffee farm components, their interactions and the environmental services realized from the system. Other methods used were; focus group discussions, ranking exercises, diagram sketching, visual aids and informal talks. Major findings were agroforestry play remarkable role in soil and water conservation. Planting trees on agricultural and nonagricultural lands was a usual practice mainly for southern Ethiopia. For it was inevitable because the area was hilly; the majority of farmers operated on a small scale and would hardly afford intensive management of their crop with integration of suitable trees for the crop they grow. Farmers had detailed knowledge about trees and ecosystems services from the agroforestry system. Ecosystem services provision largely GHSHQGHG RQ WKH V\VWHP¶V FRPSRQHQWV )DUPHUV¶ UDWLRQDOH IRU selecting shade trees was mainly based on the tree attributes and their preference differed between locations. Key words, Agroforestry, Buffer zone, farmers Knowledge.
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1. INTRODUCTION Soil degradation has raised some serious debate, and it is an important issue in the modern era (Gardner 1996). Some believe that erosion and soil degradation have disastrous effects on agricultural productivity (Scherr & Yadav 1996). Others argue that productivity loss due to erosion and soil degradation is hardly 5% (Crosson 1995). Freshwater is now scarce in many regions of the world (Postel et al. 1996). It constitutes only 2.5% of the total volume of water on Earth, and only about one-third of this water (0.77% of the total) is held in aquifers, soil, lakes, swamp, rivers, plant life and the atmosphere (Colenbrander 1978). The number of countries with scarcity of freshwater for human consumption (including agricultural and industrial use) was 20 in 1990, and will increase to 30-35 by 2025 (Engelman & LeRoy 1993). Between 1990 and 2025, the number of people affected by water scarcity will increase from 130 million to about one billion. Soil and water degradation are also related to overall environmental quality, of which water pollution and the `greenhouse effect' are two major concerns of global significance. The atmospheric concentration of CO2 has increased by 30%, from 280 ppm in 1850 to 360 ppm in 1995 (IPCC 1995). The increase has occurred due to two principal anthropogenic activities, namely
ϯ
land use and fossil fuel burning. The global release of soil organic carbon (SOC) from agricultural activities has been estimated at 800 Tg C yr-1 (T=tera=1012) (Schlesinger 1990). Soil biological degradation decreases in SOC and biomass carbon contents is an important factor leading to C emission from soil to the atmosphere, and is closely linked to soil quality. Therefore, to resolve these problem sustainable land use option was given due emphasis like that of agro forestry. Agro-forestry is a collective name for land use systems in which woody perennials (trees, shrubs, etc.) are grown in association with herbaceous plants (agricultural crops, pastures, etc.) or livestock in a spatial arrangement, a rotation or both. It has both productive and VHUYLFH IXQFWLRQV $PRQJ WKH SURGXFWLYH IXQFWLRQV WKH WKUHH µ)V¶ (fuel wood, fodder and fruit) are the most important besides construction wood, gums, resins, medicines, fibers and a host of other economic base and greater food security. The service functions include shade, reduction in wind speed, control of erosion and maintenance and improvement of soil fertility. Agroforestry is a medium and a combination of agricultural and forestry technologies to create integrated, diverse and productive land use systems (Garrett et al. 2000). A broader field is that of soil and water conservation, since reduction in water loss through runoff is an integral part of soil conservation. In turn, soil and water conservation form part of the
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wider aim of the conservation of natural resources which covers also the conservation of other resources, including vegetation (forests, pastures) and wildlife. The relations between agro forestry and soil conservation vary with climate, soil type and landforms. To provide a common frame of reference, the terms used are taken from the generalized classification level of the ICRAF Environmental Data Base (Young, 1985a, 1989b). Ecologically, agro forestry helps rehabilitate and preserve the environment through soil and water conservation in sloping lands. Tree roots hold the soil together thus minimizing erosion and eventually the occurrence of floods during rainy season. Tree canopies also help to conserve the soil from the erosive impact of raindrops. It does not only intercept large amount of rainfall but also large amount of incoming radiation depending on the percent of canopy coverage, leaf structure and crown stratification. The leaf litter and humus built up under the tree stands control flow of water and allow them to percolate into the soil. With these the paper expected to review the role of agroforestry in maintaining soil fertility and reducing loss of water, to describe different types of agroforestry system and to review the role of agro forestry in natural resource management in particular and protection of environment in general.
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1.1 Concept and definition of agro forestry Agro forestry is a new name for a set of old practices (Nair, 1993). Growing trees on farm land is an ancient art for millina. Farmers have nurtured trees on their farm, pasture lands and around their homes. Therefore, neither the concept nor the practice of agro forestry is new (Sen et al. 2004). As a scientific discipline the origin of agro forestry are fairly recent (Wojtkowski, 1998). Agro forestry is a symbiosis of tree growing, crop production and live stock rising where each component is beneficial to each other. It may be a traditional and/or introduced (Bandyopadhyay, 2001). These definitions imply that in an agro forestry system: 1) There are two or more species of plants/ animals at least one of which is woody perennial; 2) There should be biological and economical interaction with in the components; 3) The cycle of an agro forestry system is always more than one year (Mesele, 2002).Therefore agro forestry can be expressed in both its system and practices which could be explained in the section below. 1.2 Classification of agro forestry system 7KHZRUG³V\VWHPV´DQG³SUDFWLFHV´DUHRIWHQXVHGV\QRQ\PRXVO\ in agro forestry literature (Nair, 1993). However, some distinction can be made between these two concepts. An agro forestry system consists of one or more agro forestry practices that are practiced
ϲ
extensively in a given locality or area; the system is usually described according to its biological composition and arrangement, level of technical management or socio- economic features. An agro forestry practice, on the other hand, denotes a specific land management operation on a farm or other
Figure 3.2. Classification of agro forestry systems based on the type of components. Source: Nair (1985a). Management unit, and consists of arrangements of agro forestry components in space and/ or time (Gholz, 1987). All agro forestry
ϳ
systems consists of at least two of the three major groups of agro forestry components; trees (including shrubs), agricultural crops, and pasture/livestock, trees being present in all agro forestry system. Occasionally there may be other components also, such as fish, honey bees, etc. Depending on the nature and type of components involved, agro forestry system can be classified as agrisilvicultural (tree + crops), silvopastural (tree + pasture and /or livestock) and agro silvopastural (all three types of components) (Gholz, 1987).
2. Buffer zone agro forestry 2.1 Concepts of Buffer zone agro forestry The concept of buffer zone management has recently emerged as a relatively new integrated development approach to nature conservation. Buffer zones are seen as an important tool in conserving areas of ecological importance like natural forest at the same time addressing the development issues of the people in the areas surrounding it (Ebregt & Greve, 2000). Lynagh (2002) noted that buffer zone is an area of land peripheral to a protected area, designated with the intention of benefiting the local community, while simultaneously providing an extra layer of protection to a conservation area. Various authors and experts have given their definition of buffer zones from different angles and perspectives (Sayer, 1991; Ebregt &Greve, 2000; Lynagh, 2002).
ϴ
From the Conservation point of view buffer zone is defined as a zone peripheral to a national park or equivalent reserve, where restrictions are placed upon resource use or special development measures are undertaken to enhance the conservation value of the area. From the Conservation and Communities point of view buffer zone is defined as area, often peripheral to a protected area like protected forest, in which activities are implemented or the area managed with the aim of enhancing the positive and reducing the negative impacts of conservation on neighboring communities and of neighboring communities on conservation (Sayer, 1991; Ebregt & Greve, 2000). The buffer zone system, perhaps first conceptualized by UNESCO (1984), consists of a series of concentric areas around protected core; usually, this core area has been designated as a national park, wilderness area, or forest reserve, and its biological diversity is maintained through care full management. Surrounding this core area is a primary buffer zone in which research, training, education and tourism are the main activities. This primary buffer zone is encircled by secondary or transitional buffer zones, in which sustainable use of resources by the local communities is permitted. It is in these transitional zones that great possibilities exist for agro forestry innovation (Nair, 1993). Buffer zones may provide a variety of benefits, depending on the type of buffer zone, natural conditions, investments made and
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other factors. These benefits can be categorized as biological, social, economic, institutional or policy-related benefits (Ebregt & Greve, 2000). 2.1 Role of Buffer zone agro forestry in natural forest conservation and development Human demand for agricultural products leads to land degradation, deforestation, habitat destruction, and loss of biodiversity. Effective management of land for both agricultural and conservation purposes remains central to sustainable development efforts (Duffy et al., 2001). Harvesting of fuel wood and timber has profound effects on the biodiversity of the forest ecosystem, often leading to the change in species composition and vegetation structure. The uncontrolled grazing by domestic livestock is another aspect of removal of biomass from natural ecosystems, which has direct impact on the regeneration process of forest by removing the young saplings and soil loss due to trampling (Shekhar, 2001). Implementation of eco-GHYHORSPHQW SDFNDJHV WKURXJK SHRSOH¶V participation for afforestation of the degraded lands surrounding the human settlements would help to provide employment to local people and also meet their fodder and fuel demand and thus bring down their dependency on the biomass resources of the forest. Alternate income-generating activity such as sale of medicinal herbs, could be a promising venture. Besides providing income to
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the villagers the proposed initiative would minimize the illegal collection of medicinal herbs from the reserve giving enough time to rejuvenate forest flora (Shekhar, 2001). Many different sustainable management systems have been put forward as viable solutions that will take the pressure off tropical forests. One of the more useful systems is complex buffer zone agro forestry. Complex buffer zone agro forestry provides a land-management system that increases production and ecological stability, as well as supporting sustainable development. Complex agro forestry encourages the sustainable development of degraded lands by maintaining human activity, while conserving natural resources. Through its combined economic, environmental and social functions, complex buffer zone agro forestry can make a significant contribution to sustainable development. It is its emphasis on management, economic and environmental qualities that distinguishes complex agro forestry from simple agro forestry systems, such as alley cropping, intercropping or hedgerow systems (Retnowati, 2003; Oke & Odebiyi, 2007). The cocoa agro forests of West and Central Africa are a very good example of complex multistrata agro forestry systems which look like and function as natural forest ecosystems, but are integrated into agricultural management systems in which tree species produce agro forestry tree products including high quality timber (Oke, & Odebiyi. 2007). It was observed that agro forestry increased the
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production of tree components on farmland and minimized the dependency on natural forest for fuel wood and timber that imposed positive impact on natural forest conservation (Essa, 2004). Forestry is gaining from the newly promoted strategies of integrated land use in various ways: less pressure on forest resources and thus less destruction of forest vegetation. Additional lands for wood production outside the forest estate, cooperation instead of confrontation with other target groups and an expanded multiple-use concept increasing the value of marginal forest lands. Agriculture and livestock management are improved by various environmental benefits of the forest component and by the availability of forest products within agro forestry systems (Vonmaydell, 1985). Buffer zone agro forestry has the potential to provide a sustainable supply of tree products which were formerely harvested from the forest as well as improving the sustainability of local agriculture. Apart from reducing the demand for forest tree products by replacing forest resource with equivalent on farm resource, agro forestry can sustain and improve crop yields as well as providing range of additional resources and services. Agro forestry also contributes to natural forest conservation and development program. It has identified as a strategy for restoring degraded DUHDV LQFUHDVLQJ SHRSOH¶V DFFHVV WR YDOXHG IRUHVW SURGXFWV DQG conserving existing forest resources (Gezon & Freed 2008). Agro
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forestry practices provide a variety of ways in which agriculture can be intensified, tree cover enhanced, and biodiversity extended outside protected areas. So those, agro forestry have proven to be one of the most suitable means of combining biodiversity protection with farming (Garrity & Banaynal, 1995).
3. Role of agro forestry in maintaining soil fertility Soil fertility is the capacity of soil to support the growth of plants, on a sustained basis, under given conditions of climate and other relevant properties of land. The inclusion of a sustained basis in this definition refers to the capacity for continuing support for plants. Some initially productive soils have unprotected stores of nutrients and rapidly lose their fertility if transferred from natural vegetation to managed ecosystems. Others, notably nitosols on basic rocks, possess natural recuperative powers, enabling them to restore nutrients from rock weathering. Soil fertility depletion is the fundamental cause of food insecurity and low income of farmers in Africa. The loss of nutrients due to continuous cropping gradually renders the soil less fertile, resulting in poor yields. The magnitude of nutrient losses from agricultural soils is huge with annual average loss of 22 kg N, 2.5 kg P, and 15 kg K for the whole of SA region (Stoorvogel and Smaling 1990). The role of agro forestry in enhancing and maintaining long-term soil productivity and sustainability has been well documented. The
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incorporation of trees and crops that are able to biologically fix nitrogen is fairly common in tropical agro forestry systems. Non N-fixing trees can also enhance soil physical, chemical and biological properties by adding significant amount of above and belowground organic matter and releasing and recycling nutrients in agro forestry systems. A large body of literature, comprised of both original research and synthesis articles, has described the effects of agro forestry on soils in the tropics (e.g. Nair and Latt 1997; Young 1997; Buck et al. 1998; Schroth and Sinclair 2003). 3.1 Agroforestry as a practical management option for soil fertility The more widely applicable option for soil fertility management is agro forestry, as a practical option in farm management, the more necessary it is to appraise its benefits and improve techniques. At an early stage in the modern awareness of agro forestry, it was said to be particularly suited to 'marginal' lands, those with environmental hazards such as drought, erosion or low soil fertility. If this were so, then the extent of its potential application would be substantially reduced, although large areas would still remain. Evidence from the ICRAF agro forestry systems inventory shows that this is not the case, agro forestry systems are found in humid regions, on gently sloping land and on some of the most fertile soils, as well as in more difficult environments. For example, the Chagga home gardens system is found on relatively
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rich soils, whilst systems of intercropping and grazing under coconuts occur mainly on level, alluvial land, in both cases under plentiful rainfall (Nair, 1984-88, 1987b). Current agro forestry research is found in fertile areas as well as marginal, for example on the Lilongwe Plain of Central Malawi, the richest agricultural area in the country. The reason for the early presumption was that land-use problems were generally most serious in marginal lands, and these were where help from agro forestry was first sought. In the early years of the ICRAF Collaborative Programme, steeply sloping environments were over-represented, and they are also common in the systems inventory. Certainly, there are some sets of environmental and social conditions in which the potential for agro forestry is particularly high: densely populated, steeply sloping lands are one such, frequently having problems of erosion, fertility decline, forest clearance and fuelwood shortage (Young, 1986d, 1989d). A constraint of extent of land was noted to apply to fallowing and green manuring, meaning that these practices required land over and above that needed for productive purposes. In the context of agro forestry, there are two critical questions: If trees are grown with herbaceous plants (crops or pastures), is the output from the herbaceous plants reduced?
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If the answer to the above is yes, then does the output from the trees more than compensate for the loss in production from the herbaceous plants? Expressed in economic terms, the first question becomes, in a given combination of trees with herbaceous plants, are these two components complementary (the presence of one increases output from the other), supplementary (no mutual interactions), or competitive (the presence of one reduces output from the other)?' There are examples, from traditional systems and recent research, of both gains and losses in crop or pasture production as a result of the presence of trees. If it were to be found that under a wide range of environments and designs trees led to a loss of food-crop production, then this would seriously reduce the potential of agro forestry. In some spatial agro forestry practices, such as boundary planting or trees on conservation works, the tree component occupies otherwise unproductive land. In others, notably hedgerow intercropping, there is an inevitable reduction in the area under crops (perceived by laymen as one of the major obstacles to agro forestry). Also, a fall-off in crop yield close to the tree/crop interface is commonly observed. The question then becomes whether an increased yield per unit area under crop, brought about by the erosion-control and fertilityenhancement effects of the trees, more than compensates for the loss of land under crop plus any reduction in yield close to the
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interface. This is illustrated in Figure 1, which compares mono cropping with a spatial-zoned agro forestry system in which trees take up 25% of the land. All cases assume a halving of crop yield over a 2 m interface. In Case 1, the crop yield away from the interface is no higher than in the control; crop production is lower, as is the economic return. In Case 2, the presence of trees raises crop yield by 40% away from the interface; this is not sufficient to compensate for the combined effects for displacement plus interface reduction, and crop production is again lower, but this is slightly more than compensated for in money terms by the revenue from the trees. Case 3 shows an 80% increase in yield per unit area under crop (a realistic possibility as a result of erosion control) leading to a 12.5% increase in crop yield for the area as a whole.
Figure3.4.1. Tree/crop displacement yield, production and value.
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3.2 Effects of trees on soils According to Nair (1984), the association between trees and soil fertility is indicated by the high status of soils under natural forest, they relatively closed nutrient cycles, the soil-restoring power of forest fallow in shifting cultivation, and the success of reclamation forestry. More detailed evidence is provided by comparisons of soil properties beneath and outside tree canopies. Trees maintain or improve soils by processes which: Augment additions of organic matter and nutrients to the soil Reduce losses from the soil, leading to more closed cycling of organic matter and nutrients Improve soil physical conditions Improve soil chemical conditions Affect soil biological processes and conditions. Parkland is random scattering of trees in fields with crops grown under storey. Management of trees in this system requires pruning of branches and the tops to reduce shading. The trees provide valuable products such as fuelwood, charcoal, construction materials and fodder for livestock. In Ethiopia some tree species traditionally managed in this system include Faidherbia albida, Acacia tortilis, Balanites aegyptiaca, and Acacia raddiana. The service functions of trees include improving soil fertility, conserving soil moisture and improving micro-climate resulting in increased crop yields. Experiments conducted in Debre Zeit and
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Alemaya showed that wheat and maize yields increased by over 50% under F. albida canopy (within 1.4 m radius) compared to those further away from the base of the tree (Dechasa Jiru 1989, EARO 2000). Sampling in depth showed that for one species, the topsoil enrichment was apparently at the expense of lower values at 20-40 cm, but for others the positive effect of the tree continued in depth. Root excavation showed unexpectedly shallow systems, so these differences were attributed not to abstraction of elements from deep soil horizons but to the cumulative effect over time of preferential retention of atmospheric nutrient inputs, leading to a richer plant-soil nutrient cycle under the tree (Kcllman, M. 1980.). On a sandy luvisol in the semi-arid zone of northern Senegal, soil organic carbon, total nitrogen and the mineral nitrogen flux showed a progressive decrease from the trunk to the canopy margin under Acacia Senegal, Balanites aegyptiaca and baobab (Adansonia digitata) (Figure 6B). This was considered either to be a primary effect of tree litter, or a secondary effect, reduced evapotranspiration allowing better growth of herbaceous plants (Bernhard-Reversat, 1982). 3.3 Processes by which Trees Maintain or Improve Soil Fertility Processes, which augment additions to the soil:
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9 Maintenance or increase of soil organic matter through carbon fixation in photosynthesis and its transfer via litter and root decay 9 Nitrogen fixation by some leguminous and a few nonleguminous trees 9 Nutrient uptake: the taking up of nutrients released by rock weathering in deeper layers of the soil 9 Atmospheric input: the provision by trees of favourable conditions for input of nutrients by rainfall and dust, including via throughfall and stemflow. 9
Exudation
of
growth-promoting
substances
by
the
rhizosphere. Processes that reduce losses from the soil: 9 protection from erosion and thereby from loss of organic matter and nutrients 9 Nutrient retrieval: trapping and recycling nutrients which would otherwise be lost by leaching including through the action of mycorrhizal systems associated with tree roots and through root exudation. 9 Reduction of the rate of organic matter decomposition by shading. Processes that affect soil physical conditions:
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9 Maintenance or improvement of soil physical properties (structure, porosity, moisture retention capacity and permeability) through a combination of maintenance of organic matter and effects of roots 9 Breaking up of compact or indurated layers by roots 9 Modification of extremes of soil temperature through a combination of shading by canopy and litter cover Processes which affect soil chemical conditions: 9 Reduction of acidity, through addition of bases in tree litter 9 Reduction of salinity or sodicity. Soil biological processes and effects: 9 Production of a range of different qualities of plant litter through supply of a mixture of woody and herbaceous material, including root residues 9 Timing of nutrient release: the potential to control litter decay through selection of tree species and management of pruning and thereby to synchronize nutrient release from litter decay with requirements of plants for nutrient uptake 9 Effects upon soil fauna 9 Transfer of assimilate between root systems.
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Figure3.4.2. Processes by which trees improve soils.
ͶǤ According to Schroth G, Sinclair F (2003) trees may be grown in fields while crops are grown alongside or underneath. The practice of growing trees in this way can be done either by protecting and managing the trees that are already there or by planting new trees. These trees are usually grown to provide a product of commercial or subsistence value, such as food, fuel, building poles, fodder or
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gum. They also provide nutrients and organic matter for the soil and provide shade for crops and livestock.
Figure 3.5 .Planted cordia africana tree in agroforestry practice ( Wondogenet woreda)
Further evidence for the effects of trees on soils comes from comparing soil properties under the canopy of individual trees with those in the surrounds without a tree cover. For Acacia albida, cases of 50-100% increases in organic matter and nitrogen under the canopy are known, together with increased water-holding capacity (Felker, 1978). In semi-arid climates it is common to find higher soil organic matter and nutrient content under tree canopies
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than in adjacent open land. Maize and sorghum in pot samples from soils under trees in northern Nigeria grew 2 to 3 times faster than in soil with no trees; the order of fertility was Azadirachta indica > Prosopis juliflora = Eucalyptus camaldulensis > no trees (Verinumbe, 1987). Parkland is random scattering of trees in fields with crops grown understorey. Management of trees in this system requires pruning of branches and the tops to reduce shading. The trees provide valuable products such as fuelwood, charcoal, construction materials and fodder for livestock. In Ethiopia some tree species traditionally managed in this system include Faidherbia albida, Acacia tortilis, Balanites aegyptiaca, and Acacia raddiana. The service functions of trees include improving soil fertility, conserving soil moisture and improving micro-climate resulting in increased crop yields. Experiments conducted in Debre Zeit and Alemaya showed that wheat and maize yields increased by over 50% under F. albida canopy (within 1.4 m radius) compared to those further away from the base of the tree (Dechasa Jiru 1989, EARO 2000). There are different quality needed for agro forestry trees, those are: 9 The best trees to grow together with crops are those with deep roots so they do not compete with crops for water and nutrients.
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9 They should allow light through their leaves to allow crops to grow. 9 They should survive regular pruning and cutting back. 9 They should add nutrients to the soil. 9 Their leaves should provide either animal fodder or soil mulch. 9 They should have uses that help the farm family.
5. Agro forestry for Control of Soil Erosion
5.1 The traditional approach In many parts of Africa, farmers traditionally practice agro forestry. Trees are planted in agricultural or silvopastoral systems to provide shade, windbreak, medicines, or to meet household energy needs. Traditional agro-forestry system takes the form of trees scattered on crop fields, woodlots, homestead tree planting and multi-storey home garden (Eyasu Elias 2002). The earlier or traditional approach, as practiced by soilconservation or land-husbandry departments, is set out in standard texts and handbooks. Most textbooks were directed at US conditions, but that of Hudson (1981) is a clear summary, with a focus on the tropics, which has stood the test of time. The following is a summary of features of the traditional approach. Whilst it may be selective, to point out the contrast with recent trends discussed below, it is not intended as a parody, Features are:
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1. Most attention was given to erosion of croplands, much less to that of grazing lands. 2. Attention was focused on rates of soil loss, as tons per hectare/tons per acre; as a consequence: 9 Research was directed mainly at measuring rates of soil loss; 9 Conservation measures were directed at reducing the rate of soil loss; in the USA, the aim was to design conservation measures which supposedly brought the rate below specified level, called 'tolerable erosion', although not many countries followed this practice of setting a target figure. 9 Attempts to assess the consequences of erosion for productivity, and hence economic analysis, were directed at the effects of reduction in soil depth. 3. The requirements of arable cropping with respect to soil cover were taken as fixed and unalterable; hence conservation works were directed at reducing runoff or breaking the force of downhill flow. This will be referred to as the barrier approach to conservation. 4. Land-capability classification was widely employed as a basis for land-use planning. The approach originated in the USA (Klingebiel and Montgomery, 1961) and was adapted for many tropical countries, for example in Africa, first by Zimbabwe (Conex, 1960) and subsequently Malawi (Shaxson et al., 1977) and
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Zambia (Zambia, Department of Agriculture, 1977). In this approach only land below a certain angle (depending on rainfall and soil type) is classified as suitable for arable use, primarily on grounds of erosion hazard. All steeper land should be used for grazing, forestry or recreation and conservation. 5. Extension was conducted on the basis that soil conservation should come first, as a necessary prerequisite for other agricultural improvements. As a result, conservation projects or campaigns were sometimes conducted in isolation, not linked to increases in productivity. 6. Extension work in soil conservation was often conducted on the basis of a prohibitive policy, either by refusing to allow cultivation of land deemed to have a high erosion hazard, or by compulsory, legally enforced requirements for the construction of conservation works. Some successes were achieved through implementation of this approach, in different country (like, Zimbabwe). Frequently, however, problems arose in applying it to the typical situation in less-developed countries, that of small farms, high land pressure and low capital resources both of farmers and government. Among these problems were: 9 It was often found impracticable to reduce erosion to the supposedly desirable limits.
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9 The costs, or labour requirements, of the physical works necessary to control runoff by such means as bunds and terraces were commonly found to be excessive. Where such works were constructed by mechanical means (with foreign aid), these were not always maintained (e.g. Mwakalagho, 1986; Heusch, 1986; Reij et al., 1986). 9 The results of land-capability classification could not be applied. Through land pressure, moderate and steep slopes were already under cultivation, and it was economically, socially and politically unacceptable to require that these should be abandoned. A way had to be found to make such cultivation environmentally acceptable. 9
Using conventional methods of economic analysis, in particular with time-discounting of benefits, coupled with an approach based on loss of soil depth, it was often hard to justify conservation in economic terms.
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Table 2: Average runoff and soil loss under different land uses (Young, 1989)
Treatment
Run-off (%)
Soil loss (t/ha.)
Chrysopogon fulvus
12.7
8.65
Maize alone
27.5
28.27
Maize + Leucaena
21.4
17.83
Maize + Eucalyptus
20.8
13.51
Leucaena + grass
17.6
10.15
Leucaena alone
2.4
1.74
Eucalyptus + grass
6.3
3.52
Eucalyptus alone
2.1
1.20
Cultivated fallow
38.2
56.58
6. Agro forestry Practices for Erosion Control Many countries, however, have begun to adopt agro forestry practices in erosion control, on a trial, demonstration or extension basis. In some cases these attempts are not based on controlled experimental data, whilst in others there may be unpublished local station records. In many small-scale demonstrations, there is no monitoring of erosion rates. However, observations on the apparent
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success of these developments, even if only qualitative, give an indication of the range of practices available. There is a distinction between supplementary and direct use of trees and shrubs in erosion control. In supplementary use, the trees and shrubs are not the primary means of checking runoff and erosion, but fulfil the functions of stabilizing conservation structures and making productive use of the land with these occupy. This applies mainly to the practice here called 'trees on erosion-control structures'. In direct use, the trees, shrubs or hedgerows are in themselves a major method of reducing erosion. This applies particularly to the practices of plantation crop combinations, multistory tree gardens, hedgerow intercropping, windbreaks and shelterbelts, and reclamation forestry with multiple use (Aina, ., 1979). Shifting cultivation In the large literature on shifting cultivation there are many reports of the rapid increase in erosion rates after the first or second year of cultivation on steep slopes in the humid tropics (e.g. Kellman, 1969; Toky and Ramakrishnan, 1981). As is the case for soil fertility maintenance, erosion rates are acceptable under this system only when a short period of cultivation is followed by a long forest fallow. Where population pressure forces a substantial increase in the ratio of cropping to fallow, severe soil degradation commonly results.
ϯϬ
The forms of shifting cultivation found on savannas in the sub humid tropics are mostly practiced on gentle slopes. Whilst there are severe problems of fertility, erosion is not commonly observed or reported as a contributory factor. Function of trees and shrubs in erosion control could be direct and supplementary: Direct use: 7RLQFUHDVHVRLOFRYHUE\OLWWHUDQGSUXQLQJ 7RSURYLGHSDUWO\SHUPHDEOHKHGJHURZEDUULHUV 7ROHDGWRWKHSURJUHVVLYHdevelopment of terraces, through soil accumulation upslope of hedgerows 7RLQFUHDVHVRLOUHVLVWDQFHWRHURVLRQE\PDLQWHQDQFHRI organic matter. Supplementary use: 7RVWDELOL]HHDUWKVWUXFWXUHVE\URRWV\VWHPV 7RPDNHSURGXFWLYHXVHRIWKHODQGRFFXpied by conservation works. Improved tree fallow Improved tree fallow is intended to simulate the effects of shifting cultivation but with the tree fallow consisting of planted species, selected for their soil-enrichment capacity or useful products. It has been reported on steep slopes in Cebu, the Philippines (Eslava, 1984). It may be expected to interact similarly to shifting cultivation: good erosion control during the fallow but with the danger of substantial erosion, and associated loss of carbon and nutrients, during the period of cropping. The practice would become more acceptable in systems in which a mulch cover was maintained by some means during the cropping period.
ϯϭ
Figure 3.6.2a Examples of agro forestry in erosion control
a. Barrier hedges of double rows of Leucaena with maize developing naturally into terraces, Philippines (after Celcstino, 1985; Pacardo, 1985).
b. Leucaena barrier hedges planted at 90-cm spacing in furrows between rows of maize developing into terracettes, Malawi.
c. Trees on conservation works, Malawi: fruit trees on grass strips and Leucaena on marker ridges (ridges laid out along contours to guide cultivation ridges below).
ϯϮ
d. Alternative arrangements for trees on conservation structures, Cameroon (after Simon. 1983).
e. Alternative positions for trees on fanya juu structures, Kenya. Fanya juu (literally 'throw (earth) upwards') structures are bunds in which the bank is above the ditch, promoting natural terrace formation (after Wenner, 1980; and at 1CRAF Machakos field station).
Figure 3.6.2b Examples of agroforestry in erosion control
ϯϯ
a. Trees on terrace risers, Ethiopia (after a recommendation for trials in von Carlowitz, 1986c).
b. Trees on risers of irrigated terraces, Nepal. c. Hedgerow intercropping with Leucaena laid out on a slope (after a photograph in Kang et at., 1984).
d. Model for land use as an alternative to shifting cultivation, north-east hills region, India (after Borthakur et al., 1979).
e. Plan view of suggested land use on slopes, combining barrier hedges with trees on grass barrier strips, Philippines (after Celestino. 1985).
f. Possible development of reclamation forestry into productive use by selective clearance of contour strips (based on Poulsen. 1984; Young. 1985b).
7. Traditional agro forestry practice in Ethiopia Agro forestry has been an age-old practice in the Ethiopian farming system. Kindu (2001) noted the types of traditional agro forestry practices in Yeku watershed northeastern Ethiopia as trees and shrubs in silvipastoral lands, trees on farmlands, trees along rivers, and trees in homesteads. Growing Acacia albida as a permanent tree crop, on farmlands with cereals, vegetables and coffee underneath or in between, is an indigenous agroforestry
ϯϰ
system in the Harrarghe highlands of Eastern Ethiopia (Poschen, 1986). Home gardens in central, eastern, western and southern Ethiopia are characterized as backyards, front-yards, side-yards and enclosing yards (Zemede & Ayele, 1995). Farmers in WondoGenet, which is located within the Ethiopian Rift Valley, have been planting trees near and around homestead, along external and internal boundaries to a lesser scale as woodlot. Fruit trees, coffee, and Cordia africana in most cases are planted in the home garden together with Ensete ventricosum (Abebe, 2000). There are also numerous types of traditional agro forestry practices in different parts of our country, in southern Ethiopia (Zebene, 2003; Tesfaye, 2005), northern Ethiopia (Kindeya, 2004) and north western Ethiopia (Yeshanew, 1997). Farmers know the reason why they retain different tree species on their farm and also they can Classify tree species that are suitable and unsuitable for agroforestry practice. The research result in Chiapas and Mexico indicated that farmers recorded and classified tree species suitable or unsuitable as shade species based on attributes such as leaf phenology, foliage density, crown shape and the amount and timing of litter decomposition, as well as their overall impact on coffee yield (Pinto et al., 2005).
ϯϱ
7.1 Historical and social values of the Gedeo Agro forestry system According to Tadesse (2002), the Gedeo agro forestry system is a kind of enset food system. Enset is the oldest African food crop,
Figure.3.7.1.Traditional agro forestry in Gedeo, Southern Ethiopia without the reliability provided by the enset crop, an investment in the Gedeo agro forestry system could not have been possible. In a way, enset could be said an elastic crop, easily giving in to demand
ϯϲ
presented to it. There is no lean and harvest season with enset food. Enset is not a mere food crop, it is also a livestock feed and enriches soil and conserves water. In a nutshell, the Gedeo agro forestry system is a result of a long term development of the enset food system. The life of the Gedeo before enset domestication must have been unreliable and in short very difficult. Before domesticating enset/ Enset ventricosum the Gedeo lived on fruits, roots and leaves of wield of which many of the multi-purpose trees are remnants. Ensete is among plants whose roots provided good food. Roots, tubers, rhizomes and leaves provided a ready food. Such plants because of the ubiquity fell in the realm of women, who because of their reproductive and nursing role stayed around the residential areas. Contribution of this agro forestry to water and soil conservation is another noticeable opportunity. The multistrata canopy protects the soil from the direct impact of rain, its dense mixed vegetation and ground cover reduces water run-off and the dense multistrata root system contributes to soil conservation by checking soil erosion and water infiltration. The abundant litter and decaying roots, from mid December to March, increase the organic matter in the soil which can further be enhanced through the recycling of domestic waste and compost production, especially in home gardens. Research result of Temaledegn Feleke(2008) support this view
ϯϳ
(Table 1). The deep layered root system pumps deep-lying nutrients which are brought to the surface by the processes of litter and root decomposition. Soil humidity increases and soil temperature reduces as a result of the protection of soil from direct VXQ OLJKW WKURXJK WKH DJUR IRUHVW¶V FDQRS\ $OO RI WKHVH IDFWRUV contribute to a good soil structure, again to natural forests enhanced biological activity. The above mentioned attribute can be compared with available evidences throughout the tropics. For example, the soil related advantages of inclusion of compatible and desirable woody perennials species on farmland include: increasing organic matter content of soil (e.g. Young 1997; Rao ., 1998), which in turn result in increased activity of microorganisms in the root zone (Khanna, 1998), enhancing efficient nutrient cycling, hence maintaining soil fertility(Nair, 1993; Young, 1997; Temaledegn Feleke, 2008), control of soil erosion(Rocheleau et al., 1998).
ϯϴ
Table.2. Nitrogen (N) and Phosphorus (P) composition of litter from five tree species, coffee and Enset crops (Zebene, 2003)
Nutrient composition Species
Nmgg-1
Pmgg-1
Cordia Africana
19.80
0.78
Enset ventricosum
11.40
1.24
Persea Americana
8.90
0.33
Milletia ferruginea
23.80
0.59
Croton macrostachys
17.80
0.49
Ficus vasta
8.90
0.91
Coffee Arabica
10.80
0.26
In general, the historical premises of Gedeo agro forestry system provide a link between the pre-existing and present Gedeo land use system. It provides insight into the challenges and opportunity under which the Gedeo system developed and is developing. Likewise, social value developed by the Gedeo are useful and social value, show for a better understanding the Gedeo system. These two, historical premises and social values, taken together pave the way for the better understanding and conservation of the Gedeo agro forestry system.
ϯϵ
The Gedeo system provides a unique case, where the relationship between population and land resources has taken a different path. Population pressure has been shown time again to lead to the deterioration of the resource base. The Gedeo agro forestry system has nullified the negative effect of increasing population while providing an increasing volume of coffee. It is postulated that provided with a conducive policy environment, the Gedeo system will continue to absorb more population. As such it presents a unique case that needs to be emulated, particularly in mitigating climate change. ϴ͘ Agro forestry for ecosystem services and environmental
benefits The integration of trees, agricultural crops, and/or animals into an agro forestry system has the potential to enhance soil fertility, reduce erosion, improve water quality, enhance biodiversity, increase aesthetics, and sequester carbon (Garrett and McGraw 2000). It has been well recognized that these services and benefits provided by agro forestry practices occur over a range of spatial and temporal scales (Izac 2003). The multifunctional role of agro ecosystems has also been emphasized by both the Millennium Ecosystem Assessment (2005) and the International Assessment of Agricultural Science and Technology for Development (2008). There is also a great deal of interest in providing financial benefits to landowners and farmers
ϰϬ
for land-use practices that maintain environmental services of value to the wider society (FAO State of Food and Agriculture Report 2007).
Different attempts have been made to quantify
environmental benefits of agro forestry. 8.1 Agroforestry for improved air and water quality Agro forestry practices such as windbreaks and shelterbelts are touted as having numerous benefits. These benefits include effectively protecting buildings and roadways from drifting snow, savings in livestock production²by reducing wind chills, protecting crops, providing wildlife habitat, removing atmospheric carbon dioxide and producing oxygen, reducing wind velocity and thereby limiting wind erosion and particulate matter in the air, reducing noise pollution, and mitigating odor from concentrated livestock operations, among others. In recent years, interest in using shelterbelts as a potential approach to dealing with livestock odor has received considerable attention (Tyndall and Colletti 2007). The majority of odorcausing chemicals and compounds are carried on aerosols (particulates). Vegetative buffers can filter airstreams of particulates by removing dust, gas, and microbial constituents. In their detailed review on this topic with particular reference to swine odor, Tyndall and Colletti (2007) suggested that when planted in strategic designs, shelterbelts could effectively mitigate odor in a socio-economically responsible way.
ϰϭ
Agro forestry practices are also a proven strategy to provide clean water. In conventional agricultural systems, less than half of the applied N and phosphorous fertilizer is taken up by crops. Consequently, excess fertilizer is washed away from agricultural fields via surface runoff or leached into the subsurface water supply, thereby contaminating water sources and decreasing water quality (Cassman 1999). For example, agricultural surface runoff can result in excess sediment, nutrient, and pesticide delivery to receiving water bodies and is a major contributor to eutrophication. Agro forestry systems such as riparian buffers have been proposed as a means to combat non-point source pollution from agricultural fields. Riparian buffers help clean runoff water by reducing the velocity of runoff, thereby promoting infiltration, sediment deposition, and nutrient retention. Buffers also reduce nutrient movement into ground water by taking up the excess nutrients. Several studies have shown that agro forestry vegetative buffers reduce nonpoint source pollution from row crop agriculture (Lee et al. (2003) documented increased nutrient removal efficiency when trees were incorporated into a riparian buffer strip placed on the border of agronomic field plots in Iowa. The authors reported that a switchgrass (Panicum virgatum) and woody stem buffer removed 20% more nutrients compared to a switchgrass only buffer. Trees with deep rooting systems in agro forestry systems can also LPSURYH JURXQG ZDWHU TXDOLW\ E\ VHUYLQJ DV D µµVDIHW\ QHW¶¶
ϰϮ
whereby excess nutrients that have been leached below the rooting zone of agronomic crops are taken up by tree roots. These nutrients are then recycled back into the system through root turnover and litterfall, increasing the nutrient use efficiency of the system (van Noordwijk et al. 1996; Allen et al. 2004). Trees also have a longer growing season than most agronomic crops, which increases nutrient use and use efficiency in an agro forestry system by capturing nutrients before and after the cropping season. 8.2 Agro forestry for biodiversity conservation Ecosystems and species important in sustaining human life and the health of our planet are disappearing at an alarming rate. Consequently, the need for immediate action to design effective strategies to conserve biodiversity is receiving considerable attention worldwide. Scientists and policy makers are becoming increasingly aware of the role agro forestry plays in conserving biological diversity in both tropical and temperate regions of the world. The mechanisms by which agro forestry systems contribute to biodiversity have been examined by (Schroth et al. 2004). In general, agro forestry plays five major roles in conserving biodiversity: (1) agro forestry provides habitat for species that can tolerate a certain level of disturbance; (2) agro forestry helps preserve germplasm of sensitive species; (3) agro forestry helps reduce the rates of conversion of natural habitat by providing a more productive, sustainable alternative to traditional agricultural
ϰϯ
systems that may involve clearing natural habitats; (4) agro forestry provides connectivity by creating corridors between habitat emnants which may support the integrity of these remnants and the conservation of area-sensitive floral and faunal species; and (5) agro forestry helps conserve biological diversity by providing other ecosystem services such as erosion control and water recharge, thereby preventing the degradation and loss of surrounding habitat. Designing and managing an agro forestry system with conservation goals would require working within the overall landscape context and adopting less intensive cultural practices to achieve the maximum benefits. 8.3 Agro forestry for carbon sequestration Carbon sequestration involves the removal and storage of carbon from the atmosphere in carbon sinks (such as oceans, vegetation, or
soils)
through
physical
or
biological
processes.
The
incorporation of trees or shrubs in agro forestry systems can increase the amount of carbon sequestered compared to a monoculture field of crop plants or pasture (Sharrow and Ismail 2004; Kirby and Potvin 2007). In addition to the significant amount of carbon stored in aboveground biomass, agro forestry systems can also store carbon belowground. Carbon sequestered in agro forestry systems could be sold in carbon credit markets where such opportunities exist. The largest amount and most permanent form of carbon may be sequestered by increasing the rotation age
ϰϰ
of trees and/or shrubs and by manufacturing durable products from them upon harvesting. The potential of agro forestry systems to sequester carbon varies depending upon the type of the system, species composition, age of component species, geographic location, environmental factors, and management practices. A large number of studies have appeared in recent years that report carbon sequestration potential of agro forestry systems from the world over, Attempts have also been made to quantify the global carbon sequestration potential of agro forestry systems. For example, Dixon (1995) estimated a total of 585±1,215 million ha of land in Africa, Asia and the Americas under agro forestry and a global potential to sequester 1.1±2.2 Pg of carbon (vegetation and soil) over 50 years. Nair et al. (2009) estimated a land area of 1,023 million ha under agro forestry worldwide. Using the median carbon sequestration potential used by Dixon (94 Mg ha-1), the land area of 1,023 million ha represents a carbon sequestration potential of 1.9 Pg of carbon over 50 years. Considering the large extent of degraded croplands and pasturelands and the potential to improve them using agro forestry, there is enormous potential to sequester additional carbon in such systems.
ϰϱ
ϵ͘ Summary Agro-forestry is a medium and a combination of agricultural and
forestry technologies to create integrated, diverse and productive land use systems. A broader field is that of soil and water conservation, since reduction in water loss through runoff is an integral part of soil conservation. In turn, soil and water conservation form part of the wider aim of the conservation of natural resources, which covers also the conservation of other resources, including vegetation (forests, pastures) and wildlife. The relations between agro forestry and soil conservation vary with climate, soil type and landforms, so to provide a common frame of reference, the terms used are taken from the generalized classification level of the ICRAF Environmental Data Base. Ecologically, agro forestry helps to rehabilitate and preserve the environment through soil and water conservation in sloping lands. Tree roots hold the soil together thus minimizing erosion and eventually the occurrence of floods during rainy season and tree canopies also help to conserve the soil from the erosive impact of raindrops. Generally, today the best way for soil and water conservation is one of the best land use system which support our environment in proper manner not only for these generation but also for the future generation.
ϰϲ
ϭϬ͘ References
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