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Domestication of valuable tree species in agroforestry systems: evolutionary stages from gathering to breeding

K.F. Wiersum
Department of Forestry
Wageningen Agricultural University
P.O. Box 342, 6700 AH Wageningen, Netherlands

Abstract

The domestication of a plant species involves its manipulation and cultivation for specific uses. Through breeding and selection this process can result in rather uniform plant populations with a narrow genetic base. The cultivation of such selected varieties is the last stage in an evolutionary continuum in people-plant interactions. In this evolutionary process a progressively closer interaction between people and plant resources takes place with increasing inputs of human energy per unit of exploited land.

Three dimensions of domestication may be distinguished, i.e., domestication of the biophysical as well as the human environment and the domestication of the plant species. The recognition of these different dimensions is of particular importance with respect to the domestication of tree species, because commercial tree utilization usually starts with the exploitation of trees from the wild in natural forests. The domestication of the forest environment involves, in the first instance, the change from uncontrolled utilization of the wild tree products to their controlled exploitation. The second phase consists of the purposeful cultivation of wild trees in either a resource-enriched natural environment or in indigenous agroforestry systems such as forest gardens. The cultivation of domesticated trees in either agroforestry systems or commercial tree-crop plantations is the last stage of this domestication process.

These three stages of domestication are associated with specific socioeconomic and ecological conditions. During the first stage of domestication of the forest environment the management practices are, in first instance, directed at defining user rights. In the second stage, management practices with biological objectives are also undertaken. These focus at the manipulation of the growing conditions of the trees and of the tree's morphological characteristics. During the third stage of plant domestication the genetic constitution of the tree population is also modified. The recognition of various phases of forest and tree domestication provides for a better understanding of the different options for the production of commercial tree crops in agroforestry systems, under location- and time-specific conditions. It also illustrates that efforts to domesticate commercial forest resources should be focused both on the manipulation of morphological characteristics of a tree (its environment) and on the modification of the tree's genetic characteristics.

Introduction: the nature of plant domestication

The concept of plant domestication is interpreted differently by various scientists (Leakey & Newton 19 94). Some have defined it in a relatively narrow sense as a human-induced change in the genetics of a plant, to adapt it to human agroecosystems; this process ultimately culminates in the plant's inability to survive in natural ecosystems (Harlan 19 75). Others have defined plant domestication in a broader sense, as a process of naturalization of plant species towards specific human-induced growing conditions, during which an increased adoption for specific uses normally takes place. For instance, Prance (1994) describes that extraction of non-timber products from the forests may be considered as the initial phase of domestication of valuable tree species. During the domestication process, wild plants are first brought to some form of management. In a later stage of the process, the wild plants are actively cultivated. Only in its final phase does the process involve the selection and breeding of selected genotypes resulting in rather uniform plant populations with a narrow genetic base.

The concept of plant domestication can thus be interpreted in a restrictive sense as a process of changing of the biological characteristics of a species, or in a more extended sense as a process of change in plant exploitation practices, which brings with it changes in the plant's morphology and genetics, as well as in its growing environment. Some authors (Chase 1989) have even extended the concept of domestication from the conventional biological definition to a more inclusive one, i.e., the process of increasing human-plant interactions. According to this interpretation, domestication not only involves a change in plant characteristics and the biophysical environment, it also involves adaptations in human activities with respect to the use and manipulation of valuable natural resources. The various dimensions involved in the process of domestication can be clarified by relating it not only to plant species but also to landscapes: `A domesticated landscape is one that has been modified by humans from its highly biodiverse state to a state that may still have high biodiversity, but which contains a greater concentration of resources useful to humans' (McKey et al. 19 93).

Such an integrated concept of domestication within the landscape seems particularly apt when we consider agroforestry. Many agroforestry systems develop as a result of the gradual modification of forests, by enriching them with useful crops. Such enrichment not only involves agricultural crops; in many cases it also concerns trees producing non-timber forest products (NTFPs). The first phase of domestication of NTFP trees involves a process of concentration of naturally occurring useful tree resources in natural forests. In subsequent phases, also, new species may be introduced into these forests. At first, this will take the form of transplanting of wildlings or seeding of wild species. But gradually selected varieties will also be introduced. As a result, the natural forests gradually change into a mosaic of managed forests and agroforestry systems (Posey 1985, De Jong 1995, Wiersum submitted). The process of domestication can therefore be considered as an evolutionary process, from gathering to breeding, during which changes at the level of both the landscape system and the plant species occur. Concomitantly, a progressively closer interaction between the tree resources and people takes place. The domestication of NTFP trees in agroforestry systems thus involves a coevolution of human society and nature (McKey et al. 1993, Dove 1994), with the combined processes of natural and cultural selection creating a great diversity of human-influenced agroforest types.

This coevolutionary process consists of various stages along a nature-culture continuum. These stages can be associated with specific socioeconomic and ecological conditions (Wiersum submitted). To understand under what conditions farmers are interested in incorporating commercial NTFP trees in their land-use system, it is necessary to understand the specific characteristics of these different stages. This will allow identification of the kind of domestication practices that offer the most scope under specific socioeconomic conditions and for given agroforestry systems. In this paper an analytical model will be presented, to contribute towards a better understanding about various options for incorporating NTFP trees in agroforestry systems under location- and time-specific conditions. The process of domestication of NTFP trees in agroforestry systems is represented by several phases in tree management with associated ecological and institutional conditions; these phases are arranged sequentially along a gradient of increasing input of human energy per unit of exploited land.

Production of non-timber products in agroforestry systems

The potential of tropical forests to provide a variety of non-timber forest products is at present widely acknowledged (e.g., Beer & McDermott 1989, Falconer 1990, Plotkin & Famolare 1991, Nepstad & Schwartzman 1992, Ros-Tonen et al. 1995). Such products include a variety of food products as well as diverse commercial crops such as gums, resins, rattan. In the past it was often assumed that mainly tribal people who were engaged in traditional methods of forest resource use, such as gathering and swiddening, collected such products for their subsistence. But it is increasingly acknowledged that there exists a long history of commercial trade in non-timber forest products (Dunn 1975), coupled with an active management of these products (e.g., Anderson 1990, Dove 1994). When considering the production of non-timber forest products, it is therefore not appropriate to assume a dichotomy between the gathering of NTFPs from wild trees in a natural forest and the cultivation of domesticated NTFP trees in an agroforestry system. Rather, attention should be focused on the various phases of domestication and on the creative role of human culture in regulating tree resources for human use (Hladik et al. 1993, Wiersum submitted).

Agroforestry is a generic name for a variety of land-use systems in which trees are purposively combined with agricultural crops and/or livestock (table 1). With respect to NTFP trees, a wide range of agroforestry and other forest systems may be distinguished (Olofson 1983, Anderson 1990, Gomez-Pompa & Kaus 1990, Wiersum submitted):

· Native forests in which NTFPs are protected: Specific areas or specific tree species in native forests that are favoured and protected because of their value for providing useful materials.

· Resource-enriched native forests: Native forests, either old growth or fallow vegetation, whose composition has been altered by selective protection and incidental or purposeful propagule dispersion of food and/or commercial species.

· Reconstructed natural forests: (Semi-) cultivated forest stands with several useful planted trees, together with tolerated or encouraged wild species of lesser value, and non-tree plants (herbs, lianas), composed of mainly wild species.

· Mixed arboriculture: cultivated mixed stands, almost exclusively of planted, and often domesticated, tree species.

· Interstitial trees on croplands: Either naturally regenerated and protected trees, or planted and sometimes domesticated trees scattered over agricultural fields.

· Commercial plantations with associated agroforestry practices: Plantations of domesticated tree crops that are (temporarily) intercropped with food plants or grazed by livestock.

Table 1. Examples of different agroforestry systems incorporating NTFP tree species

Main types Examples
Gathering of non-timber products in natural forests
Individually claimed trees tree marking, e.g., Southeast Asia (Persoon & Wiersum 1991)
Resource-enriched natural forests
Enriched natural forests enriched rainforest groves in Amazon (Posey & Balée 1989, Anderson 1990) and S.E. Asia (Jong 1995)
Enriched fallows fallows enriched with fruit/tree crops in S.E. Asia (Foresta & Michon 1993, Salafsky 1995) and Amazon (Posey & Balée 1989, Denevan & Padoch 1987)

palm fallows in Amazon, W. Africa and Indonesia (Anderson 1987)

rattan fallow cultivation in Borneo (Weinstock 1983)

Reconstructed (natural) forests
Forest gardens Ifugao woodlots, Philippines (Olofson 1983)

mixed damar gardens, Sumatra (Mary & Michon 1987)

mixed fruit and rubber gardens, Borneo/Sumatra (Salafsky 1995)

Mixed arboriculture
Homegardens pantropical (Landauer & Brazil 1990)
Smallholder plantations pre-Hispanic cacao plantations Mexico (Gomez-Pompa & Kaus 1990)

mixed damar/coffee gardens, Sumatra (Mary & Michon 1987)

mixed rubber gardens, Indonesia (Dove 1994, Salafsky 1995)

Interstitial trees on croplands
Fruit tree cultivation farmed parklands, semi-arid West Africa (Pullan 1974, Raison 1988)

interstitial trees on croplands in Asia (Olofson 1983)

fruit trees on bunds along rice-fields, S.E. Asia (Grandstaff et al. 1986)

Commercial plantations with associated agroforestry practices
  intercropping in commercial tree-crop plantations (Nair 1983, Wessel 1992)

livestock grazing in commercial tree-crop plantations (Payne 1985)

This categorization is a first approximation of the varied and often complex agroforestry systems incorporating NTFP trees. The different categories are not discrete: gradual transformations from one category to another may take place. Many of these systems are indigenous in nature, and they have gradually evolved in response to changing conditions. Such changes may involve a variety of ecological, socioeconomic, cultural and political factors. The four most important changes (Gilmour 1990, Shepherd 1992, Arnold & Dewees 1995) that may affect agroforestry systems are-

· changed ecological conditions, such as resource depletion or land degradation

· changed technological conditions caused by the introduction of new agricultural and forestry techniques

· changed economic conditions, such as development of new markets and increased commercialization, changed demands for forest products and changed opportunities for off-farm employment

· changed sociopolitical conditions, e.g., population growth and migration, increased interaction with other ethnic groups, changed tenure conditions including gradual privatization or nationalization of forest lands, development of state organizations for forest management and rural development

These changes increase pressure on forest and tree resources. In several cases, this has resulted in deforestation and forest degradation. But in other cases, farmers have reacted by modifying their management strategies, e.g., by intensifying the cultivation of valuable tree species (Dove 1994, Arnold & Dewees 1995). For instance, management for fruit production may evolve from collecting wild fruits in the forest, through their cultivation in enriched fallows and home gardens, to fruit production in orchards (Verheij 1991).

Such changes in indigenous agroforestry systems demonstrate that many local communities have been actively engaged in domesticating the landscape as well as valuable tree species. Efforts to stimulate the further adoption of domesticated NTFP trees in agroforestry systems will be most successful if they build upon and strengthen such indigenous processes of domestication. This is possible only when a good understanding exists about the various dimensions of domestication in agroforestry systems, as well as the specific characteristics of the various phases during this process. To obtain such understanding, it is useful to consider the various dimensions of tree management in more detail.

Management of trees in agroforestry systems

Often, the concept of forest or agroforest management is used with respect to the conscious manipulation of the environment to promote the maintenance or productivity of forest resources (e.g., Anderson 1990). Such a definition is not complete, however; it limits management towards the implementation of certain technical activities. It does not incorporate the notion that management includes not only the execution of activities but also the process of making and effecting decisions about the use and conservation of agroforestry systems, as well as organizing the management activities (Wiersum submitted). Agroforestry management can therefore better be conceptualized as referring to the total set of technical and social arrangements involved in the protection and maintenance of agroforestry resources for specific purposes, and the harvesting and distribution of the products.

Technical management practices

To date, most studies on domestication of NTFP trees in agroforestry systems have concentrated on the scope for either improved regeneration practices or selection and breeding of specific NTFP species (Leakey & Newton 1994). This illustrates that many researchers interpret the process of NTFP domestication as involving basically a change in biological properties of NTFP trees, rather than as a process of change in human-plant interactions. When domestication is considered in this more inclusive perspective, it becomes clear that management practices in agroforestry systems do involve more than purposeful regeneration of selected tree species. Management practices also include the stimulation of the production of the required products and the protection of valuable tree species against damaging agents (Gomez-Pompa & Kaus 1990, Campbell et al. 1993, Wiersum submitted).

When considering how and to what extent domestication relates to these various management practices, it is important to recognize that it is not correct to assume that planting trees is the first stage in agroforestry management. Although this is true when bare land has to be replanted, it is not the case when agroforestry systems are developed in existing natural forests. In the latter case, the first phase of management consists of protecting valuable tree species through controlled harvesting techniques. Such controlled utilization of forest products involves not only biologically oriented practices but also the definition of user rights (Gilmour 1990). If ample forest resources are present, there is no interest in forest protection or tree planting practices, and management practices may then be confined to defining use rights. But if forest resources are perceived as becoming scarcer, management may be intensified by also implementing biologically oriented practices that limit overexploitation, e.g., limiting the harvested amounts or harvesting rotationally. This management may further be intensified by the execution of practices aimed at increasing the presence or productivity of the desired species. Measures may also be developed to favour the useful species indirectly by manipulating the biophysical environment and removing less desirable competitors.

Thus, management practices consist of a group of deliberate activities for the conservation and possible enhancement of useful forest resources and the controlled utilization of those resources. These practices may be directed at influencing either the forest resource directly or its biophysical and social environment. Much variation in the kind and intensity of management practices may occur between various agroforestry systems, depending on the kind of resources being considered and on the kind of management system. As indicated earlier, gradual transformations from one system to another may take place. During this progression, valuable tree species are gradually segregated from the natural forest and cultivated in increasingly specialized agroecosystems. Concomitantly, the vegetation structure becomes increasingly systematized, with randomly spaced trees of random age giving way to even spacing of even-aged trees. Furthermore, the propagation methods change from using seeds to using clonal material, and the location of regeneration units changes gradually from a forest environment to open-field conditions.

Organization of forest management

Agroforestry management involves not only carrying out resource management practices but also the process of making decisions about: (1) the objectives of management, (2) the kind of activities to be carried out by various persons, and (3) the distribution of products. In addition, management also requires the existence of a control system that ensures that the proposed activities are carried out as planned. In most studies on tree domestication no attention is given to such organizational aspects. It seems to be assumed that the production of NTFPs takes place within clearly delineated management units and under a clearly defined management authority. In many cases, however, this is not necessarily the case. In many instances agroforestry development is planned in areas that are characterized by a pluriformity with respect to land and tree tenure (Fortmann & Bruce 1988, Fortmann & Nihra 1992). Several agroforestry systems are managed not by private owners but by local communities who act cooperatively in managing agroforestry systems as a common resource. Such agroforestry systems are subject to individual use, but not to private ownership. The utilization of these resources is governed by a set of regulations on independent user rights of members of a specific user group (Messerschmidt 1993). For such common property forest management regimes to function properly, there should exist a local forest management institution with the following characteristics (Gibbs & Bromley 1989):

· A structure for group members to make decisions on the required resource management practices

· Group control over the behaviour of the group members, which ensures that the planned management practices are carried out

· Control over the distribution of collected forest products

· Ability to exclude outsiders

The common property management regimes are mostly characterized by regulations defining user rights and measures for controlling overexploitation. Often no measures for stimulating biological production are included (Gilmour 1990). But when the management is further intensified by investing labour to increase productivity, the tenure regime often shifts from common property rights to private ownership rights (Shepherd 1992).

Evolutionary stages in the process of tree domestication in agroforestry

According to the basic principles of agroforestry management, the process of domestication of valuable tree species in agroforestry systems may be characterized as a general evolution from extraction of products from the natural forests under common property regimes to the cultivation of domesticated tree crops on private lands. This evolutionary process thus involves both technical and ecological changes as well as institutional changes. Although examples of different stages of this evolutionary process have been described by various authors, to date the systematic comparative analysis of the various stages of domestication has received little attention.

In an attempt to improve understanding about the evolution in forest-people interactions, Wiersum (submitted) has developed a model in which various forest exploitation and management activities are arranged along a gradient of evolutionary stages in people-forest relations. This model was based on a model by Harris (1989) in which various stages in exploitation of agricultural crops were arranged sequentially along a gradient of increasing input of human energy per unit of exploited land. On the basis of the principles of forest management, three major categories of forest management practices may be discerned. These categories can be conceived as representing progressive phases with respect to the input of human energy per unit of exploited forest, i.e.-

· controlled utilization of forest resources

· protection and maintenance of forest stands

· purposeful propagation of valuable forest components through the regeneration of wild or domesticated trees

Consequently, three major thresholds may be postulated between the various phases of forest-people interactions (table 2). The first is between uncontrolled and controlled procurement of wild tree products in the natural forests. As indicated above, the control of forest utilization primarily involves the definition and control of use rights; this requires social transaction costs with respect, for example, to time spent on mobilization, decision making and control. Control measures with a biological objective are developed in the second instance. The second threshold is between controlled procurement of wild products and purposeful regeneration of valuable tree species. And the third threshold is between the cultivation of wild and domesticated tree species; the domesticated trees may be propagated through genotype or phenotype variants.

A gradual transformation of the natural ecosystem into an agroecosystem occurs concomitantly with this increasing input of human energy per unit of exploited forest land. In addition, the human intervention in the reproductive biology of tree species intensifies (table 2). This process of progressively closer interaction between people and forest resources is associated with various socioeconomic trends. In the first place, the socioeconomic conditions relating to forest utilization change: increasing sedentarization, increasing population density, and a gradual shift from a subsistence economy to commercialization. In the second place, the complexity of the indigenous rules and regulations change, with common property rights gradually becoming changed into private land and tree tenure rights. Thus, the forests and trees are incorporated in an increasingly more complex cultural environment (table 3).

This model should not be regarded as an explanatory model indicating unidirectional and deterministic trends, in which the various phases represent preordained steps on a ladder of increasingly `advanced' stages of general societal development (cf. Harris 1989). In effect, in many areas different agroforestry types may coexist, with each type occupying a specific landscape (e.g., Posey 1985) or tenurial niche (Fortmann & Nihra 1992). As agroforestry systems are mostly a component of an integrated farming system, the evolutionary trends are anything but straightforward. Depending on their role in the local farming systems the management practices may either be intensified or deintensified in response to agricultural intensification (Belsky 1993) or to more general changes in socioeconomic conditions (Balée 1992). Nonetheless, the model may assist in clarifying the various stages in the process of the domestication of valuable tree species in agroforestry systems. It illustrates the various phases in the process of domestication of forest resources and identifies the general relations between tree management practices and ecological and institutional conditions. This information can assist in assessing what kind of activities might be considered when stimulating further domestication of NTFP trees in various categories of agroforestry systems (table 1).

Table 2. Major categories of indigenous forest/tree utilization and management practices

Threshold Utilization system Plant-exploiting

practices

Ecological effects
  uncontrolled procurement of wild tree products casual gathering/collecting incidental dispersal of propagules, no transformation of natural vegetation composition and structure
Controlled utilization controlled procurement of wild tree products more or less systematic gathering, collecting same
    systematic collection with protective tending of valuable tree species reduction of competition, limited transformation of forest structure composition
Purposeful regeneration cultivation of wild trees selective cultivation by transplanting of wildings and/or dispersal of seeds, vegetative propagules in forest environment purposeful dispersal of propagules to new habitats, practical transformation of forest structure, composition
    tree crop cultivation (possibly in combination with annual crops) land clearance, total or almost total transformation of forest structure, composition
Domestication production of domesticated trees cultivation of domesticated trees in tree-crop plantations propagation of genotype and phenotype variants, land clearance and soil modification inputs of fertilizer and pesticides

Table 3. Institutional arrangements in indigenous forest and tree management and utilization practices

Utilization system Socioeconomic conditions Indigenous institutions with respect to utilization
Uncontrolled procurement of wild tree products segmented societies, low population density, subsistence economy open access
Controlled procurement of wild tree products low population density, incipient social stratification at community level (often coupled with formal state regulations and dual economic system) common property rights, sometimes priority rights to valuable tree species
Systematic collection of wild tree products and protective tending of valuable tree species increased social stratification and incipient commercialization at local level (often coupled with formal state regulations and dual economic system) combined common property rights on forests and private priority rights on claimed trees
Selective cultivation of wild trees increased population density and socioeconomic stratification priority rights to forest plots for tree planters
Tree-crop cultivation medium high population density, increased incorporation in market economy private land and tree rights
Production of domesticated trees in plantations high population density, fully commercialized resource use private land and tree rights

Conclusion

The domestication of a plant species involves manipulating and cultivating plants for specific uses. Domestication of NTFP trees in agroforestry systems is a multifaceted process in which a progressively closer interaction between people and plant resources takes place. This process consists of three dimensions:

· Acculturalization: the incorporation of trees in an increasingly complex cultural environment through the formation of management entities and the formulation of rules and regulations for resource utilization and management.

· Modification of the biophysical environment: the protection and stimulation of production of useful trees in natural forests, the partial clearing of natural forests, followed by the planting of useful wild species; also the manipulation of the biophysical environment to stimulate production of the valuable tree species.

· Modification of a tree's biological characteristics: the manipulation of its morphological and genetic characteristics.

The utilization of tree species usually starts with the exploitation of trees from the natural forests. Gradually, uncontrolled utilization of the wild tree products is changed to controlled exploitation. Subsequently, wild trees may be purposefully cultivated in either a resource-enriched natural environment or in agroforestry systems, such as forest gardens. The cultivation of selected varieties of trees in either mixed tree plantations or commercial tree-crop plantations is the last phase of this domestication process. Each of these phases is characterized by specific conditions with respect to the ownership and management responsibility, as well as ecological conditions.

The recognition of these various stages of the domestication process is important for understanding the scope for domestication of commercial NTFP tree species in agroforestry systems. It allows better understanding of the various options for the production of commercial tree crops under different ecological and socioeconomic conditions. It demonstrates that the willingness of farmers to adopt commercial cultivation of NTFP trees in agroforestry systems depends not only on access to markets but also on prevailing management conditions. It illustrates that efforts to domesticate NTFP trees in agroforestry systems should focus on both technical and institutional aspects of agroforestry management as well as their interrelations. It also illustrates that efforts to domesticate tree resources in a biological sense should focus not only on modification of a tree's genetic constitution but also on options for modifyinf a tree's morphological characteristics and its biophysical environment.

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Plate 16. A complex agroforest based on rubber and cinnamon in Sumatra in close proximity to paddy rice (photo: R.R.B. Leakey)

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Plate 17. The traditional parklands agroforestry system of the Sahel after the millet harvest. The principal tree species here in Mali are Vitellaria paradoxa (shea nut or karité) and Parkia biglobosa (néré). (photo: R.R.B. Leakey)

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