P. K. R. Nair

Carbon sequestration: An underexploited environmental benefit of agroforestry systems

Agroforestry has importance as a carbon sequestration strategy because of carbon storage potential in its multiple plant species and soil as well as its applicability in agricultural lands and in reforestation. The potential seems to be substantial; but it has not been even adequately recognized, let alone exploited. Proper design and management of agroforestry practices can make them effective carbon sinks. As in other land-use systems, the extent of C sequestered will depend on the amounts of C in standing biomass, recalcitrant C remaining in the soil, and C sequestered in wood products. Average carbon storage by agroforestry practices has been estimated as 9, 21, 50, and 63 Mg C ha−1 in semiarid, subhumid, humid, and temperate regions. For smallholder agroforestry systems in the tropics, potential C sequestration rates range from 1.5 to 3.5 Mg C ha−1 yr−1. Agroforestry can also have an indirect effect on C sequestration when it helps decrease pressure on natural forests, which are the largest sink of terrestrial C. Another indirect avenue of C sequestration is through the use of agroforestry technologies for soil conservation, which could enhance C storage in trees and soils. Agroforestry systems with perennial crops may be important carbon sinks, while intensively managed agroforestry systems with annual crops are more similar to conventional agriculture. In order to exploit this vastly unrealized potential of C sequestration through agroforestry in both subsistence and commercial enterprises in the tropics and the temperate region, innovative policies, based on rigorous research results, have to be put in place.

The enigma of tropical homegardens

Tropical homegardens, one of the oldest forms of managed land-use systems, are considered to be an epitome of sustainability. Although these multispecies production systems have fascinated many and provided sustenance to millions, they have received relatively little scientific attention. The objective of this review is to summarize the current state of knowledge on homegardens with a view to using it as a basis for improving the homegardens as well as similar agroforestry systems. Description and inventory of local systems dominated the ‘research’ efforts on homegardens during the past 25 or more years. The main attributes that have been identified as contributing to the sustainability of these systems are biophysical advantages such as efficient nutrient cycling offered by multispecies composition, conservation of bio-cultural diversity, product diversification as well as nonmarket values of products and services, and social and cultural values including the opportunity for gender equality in managing the systems. With increasing emphasis on industrial models of agricultural development, fragmentation of land holdings due to demographic pressures, and, to some extent, the neglect – or, lack of appreciation – of traditional values, questions have been raised about the future of homegardens, but such concerns seem to be unfounded. Quite to the contrary, it is increasingly being recognized that understanding the scientific principles of these multispecies systems will have much to offer in the development of sustainable agroecosystems. Research on economic valuation of the tangible as well as intangible products and services, principles and mechanisms of resource sharing in mixed plant communities, and realistic valuation and appreciation of hitherto unrecognised benefits such as carbon sequestration will provide a sound basis for formulating appropriate policies for better realization and exploitation of the benefits of homegardens.

An Evaluation of the Structure and Function of Tropical Homegardens

Abstract Homegardens represent land use systems involving deliberate management of multipurpose trees and shrubs in intimate association with annual and perennial agricultural crops and, invariably, livestock, within the compounds of individual houses, the whole crop-tree-animal unit being intensively managed by family labour. Known by different names in various places, these agroforestry systems are common in all ecological regions of the tropics and subtropics, especially in humid lowlands with high population density. An analysis of the structural and functional aspects of ten selected homegarden systems from different ecological and geographical regions shows that the average size of the homegarden units is less than 0·5 ha; yet they are composed of a large number of woody and herbaceous species, carefully structured to form 3–5 vertical canopy strata, with each component having a specific place, as well as function. Food production is the primary function of most homegardens, the vast majority of them being subsistence production systems. While there is a remarkable similarity among the different homegardens with respect to the type and nature of the herbaceous crops, the nature of woody perennials varies, depending on environmental and ecological factors. In general, most woody components produce fruits or other types of food in addition to other outputs such as fuelwood, timber, etc. These various food products provide a substantial proportion of nutritive and energy requirements of the local diet. Moreover, the species diversity and varying production cycles of the different components ensure continuous production throughout the year from the homegarden unit. Little or no research has been done to improve homegarden systems. Structural complexity, species diversity, multiple output nature, tremendous variability from farm to farm, etc., are some of the main characteristics that make the homegardens extremely difficult models to work with according to the currently available research procedures.

Biophysical interactions in tropical agroforestry systems

The rate and extent to which biophysical resources are captured and utilized by the components of an agroforestry system are determined by the nature and intensity of interactions between the components. The net effect of these interactions is often determined by the influence of the tree component on the other component(s) and/or on the overall system, and is expressed in terms of such quantifiable responses as soil fertility changes, microclimate modification, resource (water, nutrients, and light) availability and utilization, pest and disease incidence, and allelopathy. The paper reviews such manifestations of biophysical interactions in major simultaneous (e.g., hedgerow intercropping and trees on croplands) and sequential (e.g., planted tree fallows) agroforestry systems.In hedgerow intercropping (HI), the hedge/crop interactions are dominated by soil fertility improvement and competition for growth resources. Higher crop yields in HI than in sole cropping are noted mostly in inherently fertile soils in humid and subhumid tropics, and are caused by large fertility improvement relative to the effects of competition. But, yield increases are rare in semiarid tropics and infertile acid soils because fertility improvement does not offset the large competitive effect of hedgerows with crops for water and/or nutrients. Whereas improved soil fertility and microclimate positively influence crop yields underneath the canopies of scattered trees in semiarid climates, intense shading caused by large, evergreen trees negatively affects the yields. Trees in boundary plantings compete with crops for above- and belowground resources, with belowground competition of trees often extending beyond their crown areas. The major biophysical interactions in improved planted fallows are improvement of soil nitrogen status and reduction of weeds in the fallow phase, and increased crop yields in the subsequent cropping phase. In such systems, the negative effects of competition and micro-climate modification are avoided in the absence of direct tree–crop interactions.Future research on biophysical interactions should concentrate on (1) exploiting the diversity that exists within and between species of trees, (2) determining interactions between systems at different spatial (farm and landscape) and temporal scales, (3) improving understanding of belowground interactions, (4) assessing the environmental implications of agroforestry, particularly in the humid tropics, and (5) devising management schedules for agroforestry components in order to maximize benefits.

Classification of agroforestry systems

Classification of agroforestry (AF) systems is necessary in order to provide a framework for evaluating systems and developing action plans for their improvement. The AF Systems Inventory (AFSI) being undertaken by ICRAF provides the background information for an approach to classification.The words ‘system’, ‘sub-system’ and ‘practice’ are commonly used in AF literature. An AF system refers to a type of AF land-use that extends over a locality to the extent of forming a land utilization type of the locality. Sub-system and practice are lower-order terms in the hierarchy with lesser magnitudes of role, content and complexity. In common parlance, however, these terms are used loosely, and almost synonymously.Several criteria can be used to classify and group AF systems (and practices). The most commonly used ones are the systems structure (composition and arrangement of components), its function, its socio-economic scale and level of management, and its ecological spread. Structurally, the system can be grouped as agrisilviculture (crops — including tree/shrub crops — and trees). silvopastoral (pasture/animals + trees), and agrosilvopastoral (crops + pasture/animals + trees). Other specialized AF systems such as apiculture with trees, aquaculture in mangrove areas, multipurpose tree lots, and so on, can also be specified. Arrangement of components can be in time (temporal) or space (spatial) and several terms are used to denote the various arrangements. Functional basis refers to the main output and role of components, especially the woody ones. These can be productive functions (production of ‘basic needs’ such as food, fodder, fuelwood, other products, etc.) and protective roles (soilconservation, soil fertility improvement, protection offered by windbreaks and shelterbelts, and so on). On an ecological basis, systems can be grouped for any defined agro-ecological zone such as lowland humid tropics, arid and semi-arid tropics, tropical highlands, and so on. The socio-economic scale of production and level of management of the system can be used as the criteria to designate systems as commercial, ‘intermediate’, or subsistence. Each of these criteria has merits and applicability in specific situations, but they have limitations too so that no single classification scheme can be accepted as universally applicable. Classification will depend upon the purpose for which it is intended.Nevertheless since there are only three basic sets of components that are managed by man in all AF Systems, viz. woody perennials, herbaceous plants and animals, a logical first step is to classify AF systems based on their component composition, into agrisilvicultural, silvopastoral and agrosilvopastoral (or any other specialized) systems. Subsequently the systems can be grouped according to any of the purpose-oriented criteria. The resulting system name can thus have any one of the three basic categories as a prefix; for example agrisilvicultural system for soil conservation.Some of the major AF systems and practices of the tropics are grouped according to such a framework. The scheme appears a logical, simple, pragmatic and purpose-oriented approach to classification of AF systems.

Silvopastoral research and adoption in Central America: recent findings and recommendations for future directions

Human population growth in Central America during the last thirty years has lead to encroachment of forests for food production and resulted in the replacement of forests with pastureland. Deforestation and degradation of productive soils have prompted researchers to investigate agroforestry as an alternative approach to land management. Silvopastoral systems in particular have been studied and their capacity to augment nutrient cycling, enhance soil processes, supply forage for livestock, and provide habitat for flora and fauna have been documented. Despite conclusive research findings, Central American producers are reluctant to integrate well-researched silvopastoral systems into their farming systems. Thus, it is imperative that researchers investigate the reasons for scant adoption, and that adoption is given precedence. We suggest that adoption research be made a priority on local, national, and regional silvopastoral research agendas. To explore incidence of low adoption, research should identify specific barriers, assess the technology generation process, examine the risks that silvopastoral adoption presents, and evaluate the potential for the integration of silvopastoral technologies on-farm. In addition, we recommend that the future Central American silvopastoral research agenda include the development of a clear understanding of the process of adoption specific to potential silvopastoral adopters in the region.

Directions in tropical agroforestry research: past, present, and future

Reflections on the past two decades of organized research in tropical agroforestry raise several issues. Research efforts started with an inductive and experiential approach but have subsequently followed a deductive and experimental approach that includes hypothesis testing and the development of predictive capability; agroforestry research is thus being transformed into a rigorous scientific activity. The research agenda, so far, has given high priority to soil fertility and other biophysical interactions, less priority to anthropological and sociological aspects, and little priority to evaluating costs and returns, pests and diseases, and the so-called non-timber forest (tree) products. Moreover, larger-spatial-scale issues, such as carbon sequestration, water quality, and biodiversity conservation, have been neglected because of the emphasis on field- and farm-scale studies.Overall, the high expectations that were raised about the role and potential of agroforestry as a development vehicle have not been fulfilled. In order to overcome this, it is imperative that research be focused on the generation of appropriate, science-based technologies of wide applicability, especially under resource-poor conditions and in smallholder farming systems. Future research agendas should entail a judicious blending of science and technology. Applied research should build upon the findings of basic research to generate technologies for application at the farm, regional and global levels. Such research should place increased focus on previously neglected subjects, for example, the exploitation of indigenous fruit-producing trees, the agronomic components of agroforestry systems, and the global issues mentioned above. Furthermore, an appropriate methodology that embodies economic, social, and environmental costs and benefits needs to be developed to realistically assess the impacts of agroforestry, and an enabling policy environment that will facilitate agroforestry adoption needs to be made available.Agroforestry research of the 21st century should strive to build bridges from the inductive phase of the past, through the deductive phase of the present, to the future phase of harnessing science and generating technologies for the benefit of the land and its present and future users.

Indigenous Agroforestry Systems in Amazonia: From Prehistory to Today

Understanding the historical development of indigenous systems will provide valuable information for the design of ecologically desirable agroforestry production systems. Such studies have been relatively few, especially in Amazonia. The agroforestry systems in Amazonia follow a trail that begins with the arrival of the first hunter-gatherers in prehistoric times, followed by the domestication of plants for agriculture, the development of complex societies rich in material culture, the decimation of these societies by European diseases, warfare, and slavery, the introduction of exotic species, and finally, the present-day scenario of widespread deforestation, in which agroforestry is ascribed a potential role as an alternative land use. Despite the upheavals which occurred in colonial times, greatly reducing the population of native tribes, a review of anthropological and ethnobiological literature from recent decades indicates that a great variety of indigenous agroforestry practices still exist, ranging from deliberate planting of trees in homegardens and fields to the management of volunteer seedlings of both cultivated and wild species. These practices result in various configurations of agroforestry systems, such as homegardens, tree/crop combinations in fields, orchards of mixed fruit trees, and enriched fallows. Together they constitute a stock of knowledge developed over millenia, and represent technologies that evolved along with the domestication of native forest species and their incorporation into food production systems. This knowledge is the basis for the principal agroforestry practice employed by farmers in Amazonia today, the homegarden, and has potential to contribute to the development of other agroforestry systems.

State-of-the-art of agroforestry systems

Abstract Agroforestry, often paraphrased as ‘a new name for an old practice’, represents the coming of age of some age-old land-use systems involving trees, especially in the tropics and subtropics, and the recognition of their sustained yield potentials, conservation benefits, and multiple output possibilities. Examples of low-input agroforestry systems, in which woody perennials are deliberately mixed or retained with crop- and/or animal production units with a view to optimizing the economic and ecological benefits from the resultant interactions, are many in different parts of the world. However, the major types of agroforestry practices that constitute these different systems can be narrowed down to a few. The results of a global inventory of tropical and subtropical agroforestry systems undertaken during 1982–1987 showed that the existence or adoption of an agroforestry system in a given area is determined primarily by the ecological potential of the area, but the socio-economic factors determine the complexity of the system and the degree of intensity of its management. There are some agroforestry approaches that are common to many ecological regions, but the nature of components that constitute the systems in any specific region will vary depending upon site-specific factors. Additionally, there are some systems and approaches that are particularly appropriate in specific ecological conditions. Important among such specific agroforestry approaches in humid and subhumid tropical lowlands are homegardens, plantation crop combinations, multilayer tree gardens and various intercropping systems including alley cropping. Silvopastoral systems, windbreaks and shelterbelts, and multipurpose trees on farm lands, are the major specific approaches for arid and semi-arid lands, whereas, soil conservation hedges, silvopastoral combinations, and plantation crop systems are common in the tropical highlands. Improved tree fallows in shifting cultivation areas, modified taungya, use of underexploited woody perennials, multilayer tree gardens, tree folders and other aspects of silvopastoral management are some agroforestry approaches to multiple use management of tropical forests. The main examples of agroforestry systems in the industrialized and temperate regions are silvopastoral systems of tree + cattle/grass combinations, and intercropping systems involving hardwood and nut trees. Practically very little scientific effort has been made to increase the productivity of most of these indigenous systems, so that their potentials are far from being understood, let alone exploited. The main program directions in agroforestry in this second decade of agroforestry development are internationally-funded development projects, research initiatives, and education and training programs. The current trend seems to be to concentrate on development projects almost at the exclusion of academic programs of research and education. This is an unhealthy trend and it needs to be reversed. Fortunately, there is a growing interest in agroforestry in different academic circles, but it cannot be sustained and exploited unless the efforts are backed up by financial support. International and bilateral funding agencies are urged to provide financial support to academic programs in agroforestry, not only in developing countries, but also in developed countries, which could be of great benefit world-wide.

Agroforestry research for development in India: 25 years of experiences of a national program

India has been in the forefront of agroforestry research ever since organized research in agroforestry started world-wide about 25 years ago. Considering the country’s unique land-use, demographic, political, and sociocultural characteristics as well as its strong record in agricultural and forestry research, India’s experience in agroforestry research is important to agroforestry development, especially in developing nations. Agroforestry has received much attention in India from researchers, policymakers and others for its perceived ability to contribute significantly to economic growth, poverty alleviation and environmental quality, so that today agroforestry is an important part of the ‘evergreen revolution’ movement in the country. Twenty-five years of investments in research have clearly demonstrated the potential of agroforestry in many parts of the country, and some practices have been widely adopted. But the vast potential remains largely underexploited, and many technologies have not been widely adopted. This situation is a result of the interplay of several complex factors. The understanding of the biophysical issues related to productivity, water-resource sharing, soil fertility, and plant interactions in mixed communities is incomplete and insufficient, mainly because research has mostly been observational in nature rather than process oriented. Methods to value and assess the social, cultural and economic benefits of various tangible and nontangible benefits of agroforestry are not available, and the socioeconomic processes involved in the success and failure of agroforestry have not been investigated. On the other hand, the success stories of wasteland reclamation, and poplar-based agroforestry show that the technologies are widely adopted when their scientific principles are understood and socioeconomic benefits are convincing. An examination of the impact of agroforestry technology generation and adoption in different parts of the country highlights the major role of smallholders as agroforestry producers of the future. It is crucial that progressive legal and institutional policies are created to eschew the historical dichotomy between agriculture and forestry and encourage integrated land-use systems. Government policies hold the key to agroforestry adoption.