Throughout the tropics there are numerous perennial woody species that have provided indigenous peoples with many of their daily needs for millennia. Many of these people have now left the land for urban life, but they still demand traditional food, medicines, and other natural products. These traditionally important woody plants are virtually undomesticated. These neglected "Cinderella" species have great genetic diversity and also play a key role in biological, chemical, and hydro-logical cycles, protecting soils and providing ecological niches. The food-producing species are also important for food security, especially in the dry season, as well as a source of vitamins and minerals critical for the health and nutrition of children and pregnant women.
There are four groups of wild trees, shrubs, and lianes which could be rapidly domesticated for agroforestry and which can be viewed as potentially important sources of income for farmers. These trees produce
1. The traditionally important wild foods, mostly fruits, nuts and leaves for vegetables;
2. The traditionally important fibers;
3. Locally and industrially important pharmaceuticals and other extractives such as gums and resins;
4. Commercially important quality timbers and woods.
The domestication of tree species is a dynamic process which develops from deciding which species to domesticate and proceeds through background socioeconomic studies, the collection of germplasm, genetic selection and improvement to the integration of domesticated species in land-use systems (Leakey and Newton, 1994b; Leakey and Jaenicke, 1995). Domestication is an ongoing process in which genetic and cultivation improvements are continuously refined. In genetic terms, domestication is accelerated and human-induced evolution. Domestication, however, is not only about selection. Simons et al. (1998) contend that it integrates the four key processes of the identification, production, management, and adoption of tree genetic resources.
Strategies for tree domestication will vary depending on the value of the products, the extent of intraspecific variation, and many other factors, but for high-value species such as those producing marketable forest products the vegetative propagation of superior genotypes identified from within the existing wild populations will be appropriate (Leakey and Simons, 1998a). This is the approach generally followed in horticulture. Thus, an individual with superior yield, fruit flavor, stem form, or wood quality can be mass produced by vegetative propagation. In this way it is possible to select those clones likely to develop above-average characteristics in any given trait. By a series of ongoing selections and an ever-increasing intensity of selection, it is also possible to achieve rapid and substantial genetic improvements. In situations where desirable traits are not easily identified, it is possible to multiply a number of copies of each plant and to establish these in field trials and then to observe the development of distinct genetic differences between each clone (Longman and Jenik, 1987).
When identifying and selecting trees for cloning by vegetative propagation, there are two important criteria. It is important, first, to ensure that those that are highly superior for the desired traits are chosen and, second, to ensure that the selected individuals are unrelated and as genetically diverse in other traits as possible. This calls for a risk-aversion strategy (see Leakey, 1991) that allows intensive selection (e.g., 1 out of 100 to 10,000 trees) from among trees from different populations, ideally from throughout the range of the species. Furthermore, this should be an ongoing rolling program of multiple-trait selection, in which more and more intensive selection is imposed through the addition of selection criteria for new traits while, at the same time, new sources of genetic stock are continually added as new accessions enter the program. These new accessions should come from further germ plasm exploration and from breeding programs and so continually broaden the genetic base of the planted trees. Molecular genetics techniques can be used to ensure that genetic diversity is maintained in the clones used for commercial production.
Selection procedures will vary depending on the product and the situation. For example, for indigenous fruits, rapid progress will be made if indigenous knowledge can be used. Usually rural people know which are the best individual trees in their area for yield, fruit size, or flavor. Thus, as with temperate apples, pears, etc., people can be asked to report the existence of superior trees, so reducing the task of screening large numbers of trees. On the other hand, for medicinal trees, it is more likely that a chemical screening process will be required, but the magnitude of this task can probably be reduced by starting on a population basis, since it is likely that trees from certain environments will be richer in the required metabolites. Meanwhile, for timber trees, log size and straightness are the first selection criteria. Various forms of "plus-tree" (i.e., elite genotype) provenance and progeny selection are well known. To these have recently been added some "predictive tests" which can be applied in the nursery as a procedure for mass screening from genetically diverse seedling populations.
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