Agro-forestry, as defined by the World Agro-forestry Centre, is “a dynamic, ecologically based, natural resources management system that, through the integration of trees on farms and in the agricultural landscape, diversifies and sustains production for increased social, economic, and environmental benefits for land users at all levels”. On the other hand, the Association for Temperate Agro-forestry describes it as “an intensive land management system that optimises the benefits from the biological interactions created when trees and/or shrubs are deliberately combined with crops and/or livestock” (IGUTEK, 2011). Agro-forestry offers great potential for carbon sequestration (UNFCCC, 2008).
In addition to mitigation benefits, agro-forestry can also address the need for improved food security and increased energy resources, as well as the need to sustainably manage agricultural landscapes. These functions of agro-forestry are described in the article 'Agro-forestry (adaptation)'.
Terrestrial sequestration is based on the fact that plants take CO2 out of the atmosphere through photosynthesis and store it as organic carbon in above-ground biomass (trees and other plants) and in the soil through root growth and the incorporation of organic matter (figure 1). Thus, the process of carbon loss through land use change can be reversed, at least partially, through improved land use and management practices. In addition to afforestation, changes in agricultural land management, such as the adoption of tillage practices that reduce soil disturbance and incorporate crop residues into the soil, can remove carbon from the atmosphere and store it in the soil as long as those land use and management practices are maintained. Agro-forestry systems will vary by region. However, crops and forests together will elevate the carbon conserving capacity of the agro-ecosystem of a region.
Agro-forestry is one of the important terrestrial carbon sequestration systems. It involves a mixture of trees, agricultural crops, and pastures to exploit the ecological and economic interaction of an agro-ecosystem. Agro-ecosystems play a central role in the global carbon cycle and contain approximately 12% of world terrestrial carbon (Dixon, 1995). Increased C sequestration by agro-forests is an important element of a comprehensive strategy to reduce GHG emissions. According to Richards and Stokes (2004), forest land can fix about 250 million metric tonnes of carbon each year (12% of total CO2 emissions), crop land can sequester about 4-11% of atmospheric C/yr, and grazing land can sequester about 5% of atmospheric C/yr in the USA. The system of planting trees in strategic locations on farms to compensate for the lost carbon due to cutting of trees for agriculture is called agro-forestry. It has the biggest potential for increasing agricultural carbon sequestration in tropical countries (Youkhana and Idol, 2009).
Increasing agroforestry may involve practices that increase emissions of GHGs including shifting cultivation, pasture maintenance by burning, paddy cultivation, N fertilisation, and animal production. On the other hand, several studies have shown that including trees in agricultural landscapes often improves the productivity of systems while providing opportunities to create C sinks (Albrecht and Kandji, 2003). The trees play various functions, including shading crops, erosion control, and nutrient cycling. Shading crops and the rhizosphere by the trees would significantly reduce evapotranspiration (ET) of the cropped area although overall ET of crops plus trees may increase. The soil organic carbon content increases at the rate of 50kg ha-1 yr-1 in the top 10cm depth of an improved forestry plantation of Cassia siamia where the high rate of litter fall under Cassia (5 to 7 Mg ha-1 yr-1) helps to maintain high soil organic carbon content (Lal et al., 1998b).
Bamboo is an especially effective agro-forest sink of CO2 with a C sequestration rate as high as 47% amounting to 12-17 t CO2 per hectare per annum. It also generates 35 per cent more oxygen than other timber species (Aggarwal, 2007). Additionally, bamboo plantations generate income, provide a livelihood for forest dependent people. Degraded lands can be used for plantations of fast growing clones of bamboo species up to an altitude of 1,800m. Bamboo grows much faster than other trees with some species growing up to 150ft in just six weeks, occasionally more than 4ft per day. Bamboo is a pioneering plant that can also grow in over grazed soil using poor agricultural techniques (Aggarwal, 2007).
- Trees act as a buffer against storms to prevent crop destruction.
- Dry land forests apparently manage to sequester carbon by reducing respiration rates and growing rapidly in early spring to take advantage of temperatures most favorable for growth (Rotenberg and Yakir, 2010).
- Trees send their roots considerably deeper than crops, thereby placing organic matter at deeper depths in the soil where tillage won’t accelerate its decomposition and the release of CO2. In some instances trees have extracted water from deeper depths which has become redistributed at shallower depths with positive effects on the growth of understory plants. In other cases of negative effects have also been reported, so the phenomenon remains controversial (Prieto, et al., 2012). While such redistributions could be ecologically important, allowing some species to survive that would otherwise perish, it is less clear that the amounts of water involved would enable significant increases in the yield of agronomic crops.
- Leaf litter generates compost and serves as mulch that reduces runoff from rainfall. It also slows soil water loss from evaporation into the atmosphere.
- Agro-forestry trees also improve land cover in agricultural fields in addition to providing carbon inputs (root biomass, litter and pruning) to the soil. These often reduce soil erosion, which is a crucial process in soil carbon dynamics.
- Carbon sequestration continues beyond harvest if boles, stems, or branches are processed into durable products that do not decompose and release CO2.
- An agro-forestry induced micro-climate improves quality and increases the yield of some crops, although it is difficult to provide an estimation of the yield increase (Ebeling and Yasue, 2008).
- Increasing soil carbon greatly benefits agricultural productivity and sustainability.
- Cost of carbon sequestration through agro-forestry appears to be much lower than through other CO2 mitigating options (Albrecht and Kandji, 2003).
- This technology involves a very slow process of marginal carbon conservation.
- Soil carbon increases only in drier sites and actually decreases in wetter sites of agro-forestry regions (Jackson et al., 2002). As a result, the net carbon balance was marginally positive for the dry sites but negative for the wet sites.
- Under dry environments, the tree-crop competition for water usually results in low crop yields, which makes this technology unattractive for dryland farmers. Under dryland conditions, trees with their effective rooting systems take more water compared to crops with relatively less effective rooting systems, so the crops are more vulnerable to water stress with consequent lower yields (Schroeder, 1995).
- Various species of damaging insects, pests, and diseases have been associated with dead or dying trees. These are a major threat to the development of agro-forestry in the tropics.
- Emissions of other greenhouse gases such as N2O and CH4 in the atmosphere may increase, which reduces overall mitigation potential.
Agro-forestry is practiced to some extent all over the world. It is especially used for crops that benefit from the quality improvements associated with shading. However, the other benefits, including carbon sequestration, are being more recognised, and agro-forestry appears to be growing in popularity.
Light is a limiting factor for crop production in agroforestry system, and most crops yield less when shaded with higher plants. Therefore, unless the several advantages such as quality improvement and carbon sequestration can overcome the yield depression, agro-forestry is not likely to become widespread. In addition, most farmers have the equipment to accommodate only a few similar crops. Adapting to growing both small stature and large stature tree crops presents a greater challenge for them.
Takimoto et al. (2008) experimented with two types of agro-forestry systems (live fence and fodder bank) at the Segou region, Mali. The live fence treatment showed US $96 net present value (NPV), 1.53 benefit cost ratio (BCR) and 25.5% internal rate of return (IRR), while fodder bank project showed $159 NPV, 1.67 BCR and 29.5% IRR.
Promotion of agro-forestry can reduce the amount of carbon emitted to the atmosphere annually by 700,000 million tonnes. This can happen due to controlled grazing, fire management, use of fertilisers, improved cultivars, and re-vegetation of reclaimed lands.
According to Rotenberg and Yakir (2010), agro-forestry in semi-arid regions can sequester as much carbon as forests in temperate regions. Every tonne of carbon added to and stored in plants or soils removes 3.6 tonnes of CO2 from the atmosphere.
According to Lal et al. (1998a), a small agro-forestry enterprise following nutrient recapitalisation had a cost of $87 per tonne of carbon sequestered in East African Highlands. Sudha et al. (2007) carried out a costbenefit analysis of baseline (chili crops – best alternative to agro-forestry and the dominant pre-plantation crop) and agro-forestry (Eucalyptus clones) scenarios in the Khammam district, India. Costs and benefits under both the scenarios can be seen in figure 3.
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Forest management techniques for mitigation (REDD+)
Land management: Restoration of degraded lands
Nutrient management: mycorrhiza
Crop varieties with enhanced carbon sequestration
Bioethanol from sugar and starch based crops