J. Beer 1, C.A. Harvey, M. Ibrahim, J.M. Harmand, E. Somarriba and F. Jiménez
This article presents a brief review of the main environmental service functions that are provided by agroforestry systems (AFS):
These service functions complement the products that AFS provide (for commercial and home use; e.g. fuelwood, timber, fruits) but farmers are rarely rewarded for them. More research is needed on the possible trade-offs between the different service functions and the negative effects on the traditional products/uses of AFS when the tree component of agricultural systems is increased; e.g. maximizing carbon capture with high-density tree monocultures will have negative effects on biodiversity conservation and could eliminate the source of food supplements, fibres, medicines, etc. used by rural families. Methods for managing financial incentives, as rewards to farmers who provide these services by adopting/improving AFS, in order to leverage better land use, also have to be developed and tested in different socio-economic frameworks. A major limitation to the promotion of AFS is the dearth of economic analyses that include valuation of these service functions.
The formal study and promotion of agroforestry systems (AFS), a method of land management used since time immemorial throughout the "old" as well as the "new" worlds (see references to ancient Greek and other writers in Robinson, 1985), started at the end of the 1970s (De las Salas, 1979; Steppler and Nair, 1987). Initially the focus was on the description, possible biological and socio-economic advantages as well as disadvantages and the inventory of traditional AFS, mostly in the tropics (Budowski, 1982; Nair, 1989). This was followed by evaluations of productivity of both existing and novel AFS and more recently studies on the interactions between the component species with a view to improving management and profitability (or reduced risk) (Schroth and Sinclair, 2003). At the end of the 1990s, increased international concern about environmental issues led to new treaties (e.g. Kyoto Protocol) and emphasis on the environmental service functions of alternative land uses. It was rapidly recognized that AFS have many advantages over monocultures in terms of the increasing demand for multifunctional agriculture and that AFS provide important environmental services. Other recognized potentials of AFS include aesthetic values (e.g. city parklands and tree savannahs), buffering of protected areas and agroecotourism (e.g. guided tours of indigenous cacao AFS in Costa Rica and Belize).
The payment of incentives to farmers whose land use protects natural resources and hence provides a service to the local, national and global community is a new option which could contribute to the financial viability of farms. The title of this congress "Forests, source of life" offers an opportunity to emphasize and review this important new focus in AFS programmes; i.e. the quantification and valuation of service functions of tree-crop and/or tree-animal production systems. The main service functions of the AFS considered in this paper are soil conservation, conservation of water quality, carbon capture (climate change) and biodiversity conservation.
The concepts of soil amelioration by trees in AFS have been reviewed by Young (1989) and Buresh and Tian (1998) among other authors. Soil improvement in AFS is linked to the growth of nitrogen-fixing trees or deep-rooted trees and shrubs that increase nitrogen availability through biological fixation, recycle plant nutrients from depth (especially in dry zones) and build up soil organic matter (Beer, 1988; Rao et al., 1998).
Formal AFS research (especially in Africa) initially focused on ways of maintaining soil fertility in annual cropping systems by using leguminous shrub species; e.g. in parkland AFS (Charreau and Vidal, 1965), in alley cropping (Kang and Reynolds, 1989) and tree-improved fallows (see below). Less research has been carried out on "barrier" AFS (alley cropping along the contour of slopes), though the use of strips of grass and other annual species to trap sediments and nutrients, slow runoff and increase infiltration has been widely promoted by non-governmental organizations in Central America and Asia. Although many of these AFS studies gave promising results on-station or in researcher managed on-farm trials, for productivity and soil fertility parameters, adoption of alley-cropping systems was disappointing because of: high labour and land requirements; in some cases because of the lack of commercial or home use products from the tree/shrub component; and the long time required to show positive changes (Carter, 1995).
Planted tree fallows are a potential solution to declining soil fertility due to shortened fallow periods in areas where slash-and-burn is still practised (Anderson and Sinclair, 1993; Harmand and Njiti, 1998; Ganry et al., 2001). Nitrogen availability, determined by inorganic soil nitrogen or aerobic nitrogen mineralization at 0 to 20 cm depth, and crop yields can be significantly higher after a rotation of nitrogen-fixing trees than after other tree species or grass fallows (Harmand and Balle, 2001). Relative to herbaceous fallows (leguminous or non-leguminous), greater accumulation of organic material and nutrient storage in biomass, increased root density as well as greater vertical extension of tree roots help maintain nutrient stocks by reducing leaching losses or by taking up nutrients from deep layers. Szott and Palm (1996) reported that, in comparison to leguminous herbaceous fallows, leguminous tree fallows greatly increased the total of phosphorus, potassium, calcium and magnesium stocks in the biomass, litter and exchangeable cations/ available phosphorus (soil; 0-45cm). These authors suggested that fast-growing leguminous trees can accelerate restoration of nitrogen, phosphorus and potassium stocks in the crop layer but may not completely restore calcium and magnesium stocks.
The benefits of perennial crop (e.g. coffee and cacao) shade trees include reduced soil erosion as natural litter fall or pruning residues cover the soil and reduce the impact of raindrops, improve soil structure, increase soil nitrogen content and enhance nutrient retention (Beer et al., 1998; Fassbender et al., 1991).
Although economic analyses of all the above-mentioned systems are available (e.g. Sullivan et al., 1992) they do not take into account all the short- and long-term benefits of including the trees, such as improvements or maintenance of soil fertility, nor the possible impact on profitability of service function incentives.
The potential of AFS to help secure water supplies (quantity and quality) is the least studied service function. The trees in AFS influence water cycling by increasing rain and cloud interception (with possible negative and positive effects), transpiration and retention of water in the soil, reducing runoff and increasing infiltration. For example, Bharati et al. (2002) reported that infiltration in areas cultivated with maize or soya, or under pastures, was five times less than under riparian strips cultivated with a variety of plant and tree species, suggesting that the latter had a much higher potential to prevent surface runoff (containing contaminating substances) reaching water courses. Moreover, trees in AFS can cycle nutrients in a conservative manner preventing their loss through nutrient leaching (Imbach et al., 1989). Hence AFS can reduce ground water contamination by nitrate and other substances that are harmful to the environment and human health. As a result of less runoff and leaching, micro-watersheds with forest cover or AFS that cover a high percentage of the soil surface produce high quality water (Stadtmüller, 1994).
A series of studies in Costa Rica have illustrated some of these interactions. For example, rain interception was 16 and 7.5% in coffee (Coffea arabica) plantations associated with regularly pruned Erythrina poeppigiana (555 trees/ha) or unpruned Cordia alliodora (135 trees/ha), respectively (Jiménez, 1986). Nitrate losses through leaching were higher from unshaded coffee plantations than from those containing shade trees in areas where high coffee yields had been achieved through large additions of nitrogen from chemical fertilizers (Babbar and Zak, 1995) probably because of higher rates of transpiration in the AFS (Avila, 2003). In this country, legislation recognizes the environmental services of AFS as well as those of forested land but once again economic analyses that take into account the medium to long term environmental benefits, are needed to determine the true value of the AFS.
Highly productive AFS, including silvopastoral systems, can play an important role in carbon sequestration in soils and in the woody biomass (above and underground). For example, in Latin America, traditional cattle management involves grass monocultures which degrade about five years after establishment, releasing significant amounts of carbon to the atmosphere. Veldkamp (1994) estimated that the cumulative net release of CO2 from low productivity pastures (Axonopus compressus) varied from 31.5 (Humitropept soil) to 60.5 Mg C/ha (Hapludand) in the first 20 years after forest clearing. Well managed silvopastoral systems can improve overall productivity (Bustamanate et al., 1998; Bolivar et al., 1999), while sequestering carbon (López et al., 1999; Andrade, 1999), a potential additional economic benefit for livestock farmers. Total carbon in silvopastoral systems varied between 68-204 t/ha, with most carbon stored in the soil, while annual carbon increments varied between 1.8 to 5.2 t/ha.
The amount of carbon fixed in silvopastoral systems is affected by the tree/shrub species, density and spatial distribution of trees, and shade tolerance of herbaceous species (Nyberg and Hogberg, 1995; Jackson and Ash, 1998). On the slopes of the Ecuadoran Andes, total soil carbon increased from 7.9% under open Setaria sphacelata pasture to 11.4% beneath the canopies of Inga sp. but no differences were observed under Psidium guajava. Soils under Inga contained an additional 20 Mg C/ha in the upper 15 cm compared to open pasture (Rhoades et al., 1998).
Few studies have been conducted to determine how payments for carbon sequestration will affect farm income and land-use changes on livestock farms (Ruiz, 2002). An ex ante analysis showed that farmers can increase income by more than 10% when 20% of grass monoculture pastures are transformed into silvopastoral systems (e.g. fodder banks and dispersed trees in pastures) and secondary forest. This economic analysis, conducted on dual purpose cattle farms, suggested that gross potential income generated from carbon stored in the trunks of trees was US$253 per year for a 70 ha farm (carbon price US$7/t) (Pomareda, 1999). Incentives for farmers to adopt silvopastoral systems that store more carbon and prevent pasture degradation are being developed and tested in Colombia, Costa Rica and Nicaragua (a Global Environment Facility [GEF] project coordinated by the Tropical Agricultural Research and Higher Education Center [CATIE]) but a lot more work is needed to realize the full potential of this approach.
AFS also can play an important role in the conservation of biodiversity within deforested, fragmented landscapes by providing habitats and resources for plant and animal species, maintaining landscape connectivity (and thereby facilitating movement of animals, seeds and pollen), making the landscape less harsh for forest-dwelling species by reducing the frequency and intensity of fires, potentially decreasing edge effects on remaining forest fragments and providing buffer zones to protected areas (Schroth et al., In press). AFS cannot provide the same niches and habitats as the original forests and should never be promoted as a conservation tool at the expense of natural forest conservation. However they do offer an important complementary tool for conservation and should be considered in landscape-wide conservation efforts that both protect remaining forest fragments and promote the maintenance of on-farm tree cover in areas surrounding the protected areas or connecting them, e.g. in the Central American Biological Corridor.
The degree to which AFS can serve conservation efforts depends on a variety of factors, including the design and origin of the AFS (particularly its floristic and structural diversity), its permanency in the landscape, its location relative to remaining natural habitat and the degree of connectivity within the habitat, as well as its management and use, particularly pollarding, use of herbicides or pesticides, harvesting of timber and non-timber products and incorporation of cattle, goats, etc. In general, the more diverse the AFS, the lower its management intensity and the nearer it is to intact habitat, the greater its ability to conserve native plant and animal species. Certain AFS, which closely mimic natural ecosystems (e.g. home gardens, agroforests as well as rustic coffee and cacao AFS), provide a variety of niches and resources that support a high diversity of plant and animals, though usually less than that of intact forest (Perfecto et al., 1996; Rice and Greenberg, 2000). However, even AFS with low tree densities and low species diversity may help in maintaining biotic connectivity (Harvey et al., In press).
Equally important is the attitude of local people towards biodiversity conservation and the perceived resulting benefits (products, services) and losses (e.g. crop damage or raiding, loss of animals), which in turn cause local people to favour or discourage native plants and animals. When hunting intensity is high, populations of game species within AFS are unlikely to be viable regardless of whether there is appropriate habitat available.
While there is a growing literature on biodiversity within AFS, important questions still remain about the long-term viability of animal and plant populations in AFS and what will happen to these populations if the surrounding landscape is increasingly deforested. Most studies to date have monitored or inventoried biodiversity within landscapes that still retain some forest cover, have focused on a few taxa and have been conducted on small spatial and temporal scales. Multi-taxa, multi-scale and long-term studies are needed before the true value of AFS for conservation is known.
Despite these limitations in our current knowledge, there is already sufficient evidence that AFS offer more hope for conservation of plant and animal species than the monoculture crops they usually replace. This finding has led to exciting new initiatives to use AFS as tools for conservation in already deforested and fragmented landscapes. Many of these initiatives include either the direct payment to farmers for biodiversity conservation (e.g. the GEF project led by CATIE; payment for environmental services for AFS in Costa Rica) or the certification of products stemming from these AFS as biodiversity or ecologically friendly (e.g. bird-friendly coffee [Smithsonian Migratory Bird Center, 1999]).
The service functions provided by AFS, such as soil conservation, carbon capture, water quality and biodiversity conservation are gaining the attention of researchers, planners and politicians, but since these benefits accrue over the medium-long term, are not tangible to farmers and/or the beneficiaries are found beyond the farm boundaries, conservation/adaptation of AFS may be severely limited. The introduction/promotion of trees on farms also has many disadvantages from the farmer´s point of view, not least of which is competition with existing crops and pastures. Mechanisms to reward farmers for all of the products and services they can provide are required to encourage the use of AFS.
Although some results are already available on the environmental services of selected AFS in selected sites, more research is clearly needed on the potential trade-offs between the different services involved and on the valuation and financial mechanisms required to directly benefit farmers who provide these services. Since women and children are often the main beneficiaries of the products from traditional AFS (e.g. food supplements, fibres, medicines), possible negative impacts of intensifying the use of a particular tree species (e.g. for timber production) also need to be evaluated. Complex integrative studies, which focus on the possible changes in all the potential products and service functions when the tree component of agricultural systems is increased, as well as on productivity and profitability of AFS, are going to be needed to achieve optimal land use. Without doubt, conceptual, process and other models will have to be used to achieve this goal. Solid baseline studies, monitoring and evaluation, to validate and demonstrate to different levels of our societies the positive impacts of AFS on the long-term ecological and financial sustainability of these multifunctional agricultural production systems, are also needed.
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1 Apartado 44, CATIE, Turrialba, Costa Rica. [email protected]. Department of Agriculture and Agroforestry, Tropical Agricultural Research and Higher Education Center (CATIE), Turrialba, Costa Rica.