Overstory #205 - What smallholder agroforestry systems are appropriate for carbon storage?
Tropical forests have the largest potential to mitigate climate change amongst the world's forests through conservation of existing carbon pools (e.g. reduced impact logging), expansion of carbon sinks (e.g. reforestation, agroforestry), and substitution of wood products for fossil fuels (Schlamadinger et al., 2007; Brown et al. 1996; Brown et al. 2001). In tropical Asia, it is estimated that forestation, agroforestry, regeneration and avoided deforestation activities have the potential to sequester large amounts of carbon.
Tree-based land-use systems – natural forest, forest plantations and agroforestry systems – sequester CO2 through the carbon (C) stored in their biomass. By promoting land-use systems which have higher C contents than the existing plant community, net gains in C stocks (hence sequestration) can be realized. The most significant increases in C storage can be achieved by moving from lower-biomass land-use systems (e.g. grasslands, agricultural fallows and permanent shrublands) to tree-based systems. As many efforts to achieve increased forest C storage may have negative implications for the rural poor, options that support human livelihoods deserve special attention.
Globally, the greatest potential area for expanding agroforestry practices and other forms of land-use intensification is in areas considered 'degraded' at the margins of the humid tropics, such as many secondary forest fallows, Imperata grasslands, and degraded pastures (Sampson and Scholes 2000). It is estimated that a total of 10.5106 ha can be put into agroforestry yearly with enabling government policies such as those described by Fay et al. (1998) and Tomich et al. (1998). According to the IPCC, the land availability for afforestation options (which include agroforestry among others) depends on the price of carbon and how that competes with existing or other land-use, financial returns, barriers to changing land uses, land tenure patterns and legal status, commodity price support, and other social and policy factors (Nabuurs et al., 2007). In the short-term (2008- 2012), it is estimated that up to 5.3 million ha is available in developing countries for afforestation/reforestation under the Clean Development Mechanism (CDM). In addition, to its role in mitigation, agroforestry systems can help smallholders adapt to climate change (Verchot et al., 2007).
Southeast Asia contains vast areas of degraded and underutilized lands that could be used for C investment. Best estimates indicate that there are 35106 ha of Imperata grasslands in Southeast Asia (Garrity et al. 1997). Originally forests, these lands include pure grasslands, cyclic fallows and shrublands, and are acknowledged to be underutilized. There is clear interest, at both the governmental and smallholder farmer levels, to convert some of these Imperata grasslands and other degraded lands to more productive landuse, including tree-based systems (Roshetko et al. 2007; Roshetko et al. 2002; Tomich et al 1997). The establishment of agroforestry systems on underutilized sites would sequester C and could prevent further deforestation by providing on-farm sources of trees (Sanchez 1994; Schroeder 1994). Agroforestry is one means by which smallholder farmers could benefit from C investment projects (CIFOR 2000; Sampson and Scholes 2000). Smallholder agroforestry systems maintain high tree densities and may contain high C stocks. On a per area basis tree-rich smallholder systems accumulate a significant amount of C, equaling the amount of C stored in some secondary forests over similar time periods (Tomich et al. 1998). Individual types of agroforestry systems differ greatly as do the conditions under which each type is appropriate. The question we address here is: What types of agroforestry systems are appropriate for C storage?
What Types Of Smallholder Agroforestry Systems Are Appropriate For C Storage?
Reforestation and afforestation activities are defined by the UNFCCC as follows (Decision 11/CP7 2001)
" 'Afforestation' is the direct human-induced conversion of land that has not been forested for a period of at least 50 years to forested land through planting, seeding and/or the human-induced promotion of natural seed sources;"
" 'Reforestation' is the direct human-induced conversion of non-forested land to forested land through planting, seeding and/or the human-induced promotion of natural seed sources, on land that was forested but that has been converted to non-forested land. For the first commitment period, reforestation activities will be limited to reforestation occurring on those lands that did not contain forest on 31 December 1989."
Agroforestry is a 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 (ICRAF 2004). Agroforestry systems maybe defined as land-use systems in which woody perennials (trees, shrubs, palms, bamboos) are deliberately used on the same land management unit as agricultural crops (woody or annual), animals or both, in some form of spatial arrangement or temporal sequence (Huxley and van Houten 1997). The period of tree cover may vary from a few to many years, as the period becomes longer the agroforestry system may resemble a forest. Smallholder agroforestry systems refer to small landholdings or parcels managed by individuals or groups of farmers. Traditionally producing multiple goods primarily for home consumption, now most smallholder agroforestry systems are at least partially market-oriented. Depending on local needs or opportunities, systems may focus on tree crops, agricultural crops, livestock or a combination. These various systems also differ greatly in size, species component, tree density, longevity and management intensity. Smallholder agroforestry holds potential for C sequestration as a means of converting low-biomass landuse systems (e.g. grasslands, agricultural fallows and permanent shrublands) to tree-based C-rich systems.
Not all smallholder agroforestry systems hold the same potential. To evaluate various smallholder systems from a C sequestration perspective, we may group them into the following categories: agroforests; tree gardens; plantations; improved fallows; rows or scattered trees; livestock systems; community forests and assisted natural regeneration. These landuse categories suggested by each set of authors differ due to perspective. The key characteristics that differentiate our categories are: tree density, C stocks, and products from the system. A short description of each smallholder agroforestry system category and their characteristics are given below in Table I.
Tree density is important as it relates directly to the systems' ability to store C. Simply put more trees – denser spacing – equals higher C stored per area. Those systems with longer maximum ages have higher potential C stocks. It is worth noting that homegarden systems contain lower C stocks than other 60-year systems because they contain a significant number of low-biomass, but nonetheless economically important, species such as coconut and banana. They may also have lower tree density rates than agroforest and forest systems. There is no fixed density or planting pattern for trees growing scattered on farmlands or in silvopastoral systems. Tree densities in these systems are commonly 50-400 ha-1 (Paterson et al 1996). This is significantly less than agroforests, gardens and plantations, which commonly contain 625-850 trees ha-1, assuming tree-spacing of 34 to 44 m, or more. Data concerning the C stocks of scattered tree and silvopastoral systems is not readily available. However, with tree stocking rates only 8-47% of other systems it can be assumed that these systems contain much low C stocks. Additionally, livestock, the main component of silvopastoral systems, are a significant contributor of methane and nitrous oxide, greenhouse gases that are accounted under IPCC guidelines (Sampson and Scholes 2000). Considering these points we generalize that in most cases scattered tree and silvopastoral systems offer a less attractive C investment option compared to systems with high tree densities. Improved fallows/intercropping and assisted natural regeneration are transient systems commonly used to establish any tree-based landuse system. Both are appropriate methods by which to establish a tree-based smallholder agroforestry system for C sequestration. Intercropping is particularly attractive as the management practices undertaken to assure good agricultural crop yields – cultivation, weed control, fertilization – also enhance tree survival and growth; and the agricultural crop yields will provide the farm family with food and income.
Systems that produce a variety of tree products, both wood and non-wood, are preferred by smallholders as a means of securing tree products for household needs, generating income and limiting risk. The great majority of any tree-based agroforestry system's aboveground C stock is found in the wood of the trees. Most non-wood tree products – fruits, vegetables, spices, oils, resins, etc – can be harvested with negligible impact on the C stock of a system. The data in Table I are from systems that primarily produce non-wood products. Conversely, the harvest of wood products, particularly timber in single-objective plantations, has a negative impact on the system's C stock and raises concerns of 'permanence'. However, a limited amount of timber or other wood products can be harvested from a smallholder agroforestry system and still achieve appreciable C sequestration.
Tree density and tree rotation age are not the only factors that affect an agroforestry system's C stock. The soils of agroforestry systems contain significant quantities of C also. Generally the amount of C stored in a system's soil remains steady, increasing slowly with time. As a portion of the system's total C stock, soil C decreases with time as the tree component grows and dominate the system. Studies in Indonesia show that the portion of C stored in 13-year-old homegardens, 30-year-old agroforests and 120-year-old natural forests were 60%, 60% and 20% respectively (Hairiah 1997; Tomich et al. 1998; Roshetko et al. 2002). Pre-existing soil C levels are an important baseline that will be measured at the beginning, and monitored throughout the duration, of any C sequestration project. Any loss in soil C will have a negative impact on the C sequestered over the life of the project. Cleaning, weeding, burning and relocation of biomass are common management practices that lead to steady loss in soil C when practiced to excess. For example, when these practices are applied in natural forests or grasslands soil C losses of 20-50% can occur within a few years (Sampson and Scholes 2000). Such losses are not easily reversed by converting fallow lands back with tree cover (Detwiler 1986). The soil C levels on such sites are expected to increase for decades or centuries (O'Connell and Sankaran 1997, in Schlamadinger and Karjalainen 2000). Appropriate management practices are required to protect against the loss soil C stocks. It is recommended that cultivation of crops be limited to the first 1-3 years when the tree-based agroforestry system is being established and that management practices control soil erosion and maintain/return biomass to the soil. Model simulations indicate that these soil management practices can maintain, and possibly increase, soil C levels, soil nutrient levels and system sustainability (Wise and Cacho 2002).
In summary, to achieve high stocks of quantifiable sequestered C, smallholders should convert low-biomass landuse systems into agroforestry systems that maintain high tree density, contain species with long maximum age, manage the system for long rotation and manage the soil to avoid a loss of baseline C. It may also be beneficial to limit the number of low-biomass species – such as coconuts and bananas. These considerations must be balanced with livelihood and market objectives of the smallholders' management plan. Carbon is a new and mysterious product for smallholder farmers, even less tangible than other environmental services – watershed protection or biodiversity conservation. Farmers must feel confident that they will benefit from their efforts. Agroforestry systems that provide tangible socioeconomic benefits are less likely to be converted to other landuse system. In most cases, the systems should be multiple species, with the mix determined by household needs and market demand. Management must be flexible to limit risk and enable farmers to adjust to changing market opportunities (Mayers and Vermeulen 2002; Tyynela et al. 2002). It is recommended that farmers receive a carbon payment for tree cultivation to promote transparency and farmers' understanding of the services their agroforestry system provide. However, any income received from C payments should be treated as an additional return for the service. This approach will help protect smallholders from project or market failure. Within the domain of economically viable agroforestry systems, clear opportunity exists for smallholders to select management practices that lead to higher C stocks at the system level. C sequestration projects may not make farmers rich, but they could enhance local livelihoods, assuring that smallholders benefit from C investment. Under conditions of strong and steady market demand smallholder polyculture or monoculture might be justified as segregated landuse sub-systems in a larger landscape mosaic. Questions of economic risk and vulnerability need to be clearly evaluated before smallholders opt for these systems.
A Closing Comment Regarding Redd
Reduced emissions from deforestation and forest degradation (REDD) in developing countries is under consideration as a component of the post-Kyoto regime currently under negotiation. It is not clear yet how smallholder farmers can benefit from REDD should it be included as a carbon mitigation option. This is because unlike the current Clean Development Mechanism (CDM) which is project based (for reforestation and afforesation), REDD will likely be monitored at the national or regional scale and sole address reducing deforestation and forest degradation.
Categories and description of smallholder agroforestry systems and their characteristics from a C storage and CDM prototype perspective*.
AGROFORESTS - multi-storey combinations of various tree crops, often with a predominance of a few species of high economic value, in an extensive system resembling a forest. Tree Density: High. C stock Mg ha-1 (Maximum age of system): 350 (+60 yrs) Products: Multiple products for household use and market sale. Comments: Privately owned or communal land rights. Commonly 1-10 ha. Communal areas maybe up to100 ha. May have developed from natural forests. Provides watershed and biodiversity environmental services.
TREE GARDENS - multistory combinations of various tree and annual crops in a system that is obviously planted and managed. Includes homegardens (HGS) and forest gardens. Tree Density: High. C stock Mg ha-1 (Maximum age of system): Forest 350 (+60 yrs), HGS1 280 (+60yrs), HGS2 240 (+60yrs), Rubber 200 (+30yrs), Coffee 160 (+25yrs) Products: Multiple products for household use and market sale. Comments: Usually privately owned, 0.25-5 ha, could be larger or as small as 0.10 ha. Communal gardens may be up to 100 ha. Provides watershed and agro-biodiversity environmental services. HGS2 includes timber production on a 20-year rotation.
PLANTATIONS - of timber, fruit or other commodity (coffee, rubber, etc) containing one or few species. Tree Density: High. C stock Mg ha-1 (Maximum age of system): Timber 300 (+40yrs), Rubber 190 (+25yrs), Oil Palm 180 (+20yrs), Coffee 100 (+25yrs) Products: A few products primarily for market sale. Comments: Privately owned, 0.25-5 ha. Possibly provides watershed environmental services. These systems are vulnerable to market fluctuations and contain very low biodiversity levels.
SCATTERED TREES ON FARMLANDS - on farms, including border plantings, contour plantings, windbreaks, and irregularly spaced trees. Tree Density: Low to medium. C stock Mg ha-1 (Maximum age of system): Unknown (Low) Products: Varies. Possibly multiple products for household use and market sale. Comments: Privately owned, 0.25-5 ha.
LIVESTOCK (SILVOPASTORAL) SYSTEMS - combining trees at irregular or uniform spacing with livestock production, including hedgerows of fodder trees used for intensive feed production. Tree Density: Low to medium. C stock Mg ha-1 (Maximum age of system): Unknown (Low) Products: Livestock products for home use and market sale Comments: Privately owned or communal land rights. Commonly 0.5-5 ha. Communal areas maybe up to 100 ha.
COMMUNITY FOREST LAND / FOREST PRESERVES same as 1 and 2 above?- areas of natural or secondary forests managed by communities for environmental goals (biodiversity or soil/water conservation). Tree Density: High. C stock Mg ha-1 (Maximum age of system): 350 (+60yrs) Products: Low-intensity extraction of Non-wood products Comments: Communal land rights, 10-1000s ha. There maybe individual rights for sub-units of 0.5-5 ha. Provides watershed and biodiversity environmental services.
IMPROVED FALLOWS / INTERCROPPING - combining annual crops with trees, including taungya or alleycropping systems. Often, a method used to establish a tree dominant system. Tree Density: Low tree density during the development stage. C stock Mg ha-1 (Maximum age of system): Low Products: Annual crops for household use during the development stage Comments: Methods used to establish tree-based landuse systems on either private or communal lands.
ASSISTED NATURAL REGENERATION - stimulating the growth of natural seedlings and saplings, may include some planting. Often, a method used to establish a tree dominant system. Tree Density: Depends on site and stage of development. C stock Mg ha-1 (Maximum age of system): Low Products: Low productivity during the development stage Comments: Methods used to establish tree-based landuse systems on either private or communal lands.
* Some systems definitions adapted from Friday, Drilling and Garrity (1999) and Nair, PKR. (1993). C stocks adapted from Tomich et al (1998), Roshetko et al (2002), and van Noordwijk et al 2002. Information in the table is indicative, not definitive, and intended for comparison between systems.
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About The Authors
Jim Roshetko has worked with agroforestry systems and species in Southeast and South Asia for over 24 years. He also has workled in the Pacific, the Caribbean and the United States. He is currently the Trees and Markets Unit Leader for Southeast Asia, Winrock International and the World Agroforestry Centre (ICRAF). His work focuses on improving smallholder tree farming systems to enhance local livelihoods and converse environmental resources. He can be contacted at: ICRAF/Winrock, PO Box 161, Bogor 16001, Indonesia; Tel: 62 251 625-415; Fax: 62 251 625-416; E-mail: J.Roshetko@cgiar.org.
Rodel D. Lasco has over 25 years of experience in natural resources management and climate change reearch. He is one of the authors of the Intergovernmental Panel on Climate Chnage (IPCC}, the 2007 co-winner of the Nobel Peace Prize. He is the Philippines Coordinator of the World Agroforestry Centre (ICRAF), a center devoted to promoting "trees on farms." He can be reached at firstname.lastname@example.org.
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