AgroforestrySystems 19: 131--137, 1992. © 1992 KluwerAcademic Publishers. Printed in the Netherlands.

Improving multipurpose tree and shrub species for agroforestry systems

F. OWINO International Council for Research in Agroforestry, Nairobi, Kenya

Key words: multipurpose trees and shrubs, agroforestry, choice of species for improvement research, provenance trials, selection and breeding

Abstract. With the aid of an example of ICRAF's tree improvement research programme for the highlands of Eastern and Central Africa, a logical approach to selection and breeding of multipurpose trees and shrubs in agroforestry context is proposed. Criteria for selection of high priority species are proposed. Some species of high potential for agroforestry develop- ment in Sub Sahara Africa are proposed. The necessary sequential research steps are discussed.

Introduction

Agroforestry systems are characterised by the presence of woody perenials [Steppler and Lundgren, 1988]. The specific roles such woody perennials play in agroforestry systems has formed the basis of agroforestry systems inventory [Nair, 1987]. It is recognised that several thousand multipurpose tree and shrub species across the globe could have potential roles to play in agroforestry systems development [Burley and von Carlowitz, 1984]. Most of these multipurpose tree and shrub species with agroforestry potential exist in wild and unselected populations where they are likely to exhibit wide natural variability [Burley, 1987]. Such populations can be expected to respond well to selection and breeding thus resulting in great increases in yield and adaptability to specific sites, fanning systems.

Existing tree improvement programmes are largely oriented towards large-scale plantation forestry production systems. Their goals are conceived narrowly as increased wood production. Improving multipurpose tree and shrub species for agroforestry systems should have goals which differ from the above. For example, besides total biomass production, tree root symbiont relations (particularly nitrogen fixation potential) deserve greater attention. Fodder and mulch properties of tree and shrub species should be more effectively evaluated and included as criteria for selection and breeding. The ideal tree or shrub ideotype should be clearly defined taking into account (i) tree-alternate crop interactions and (ii) intended tree management practices as components of agroforestry systems.

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Goals of multipurpose tree improvement for agroforestry systems

The first goal in improving multipurpose tree and shrub species for agrofor- estry systems should be to improve (through breeding) high priority species for specified agroforestry technologies taking into account (i) farmers' needs for tree products and services (ii) compatibility with companion crops and (iii) sustalnability and enhancement of land productive capacity [Owino, 1990]. The second goal (which should apply to all tree improvement pro- grammes) should be to produce the maximum amount of tree product and/or services in the shortest possible time thus enabling maximum economic returns.

It is important that researchers keep sufficient focus on the first stated goal to ensure that trees and shrubs are evaluated and improved in the context of agroforestry. In this regard, it is recommended that research steps and considerations in improving multipurpose tree and shrubs species for agroforestry technologies, as presented in Fig. 1, be adopted.

Choice of priority species for improvement research

Several thousand species of trees and shrubs could have potential roles to play in both traditional and such recently proposed agroforestry technologies (like hedgerow intercropping) the world over. For example, over 1,000 species of agroforestry potential have been described in ICRAF's multi- purpose tree database [von Carlowitz e t al., 1991]. Needless to stress the need to adopt rational methods of arriving at manageable sets of priority species to be the focus of improvement research within clearly defined agro- ecological zones and for specific agroforestry technologies within these zones.

Three primary considerations should guide the choice of the set of species for research concentration in any given situation viz (i) well-perceived potential of the species to solve identified land use constraints (e.g. fodder shortage, fuelwood shortage, soil infertility etc. and combinations thereof), (ii) site adaptation and comparative growth performance, and (iii) farmers' preference and/or adoption potential.

A list of criteria to be used in the choice of high priority species for improvement research is suggested in Table 1. Some species of high agrofor- estry development potential in Sub Sahara Africa are indicated in the Appendix.

Research steps in improving multipurpose trees for agroforestry systems

It is proposed that sequential research steps for improving multipurpose trees for specific each agro-ecological zones and farming systems as depicted in

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Table 1. Criteria for selecting high priority tree species for agroforestry research and development.

(i) Adaptabifity considerations (a) Adaptability and good growth performance in broad agroecological zones (b) Adaptation to specific soil conditions (e.g. Acidic soils) (c) Adaptation to constraining soil water regimes (e.g. drought resistance in arid lands)

(ii) Agroforestry technology consideration (a) Compatibility with alternate crops (both below and above the ground) (b) Special products and services (e.g. fodder, nitrogen fixation) (c) Tree management requirements in specific agroforestry technologies (d) Freedom from pests and diseases (including those of alternate crops)

(iii) Species-specific potential gains from improvement research (a) Existing genetic variation as basis for further improvement (b) Potential gains through improved silvicultural practices (c) Economic justification on long-term improvement programmes

Fig. 2 be followed. The starting point should be on ecoregional zone-based agroforestry technology diagnostic and design exercises (D&D). This results in the definition of agroforestry technologies with the greatest potential to solve land-use constraints in specific zones. As part of these exercises, a wide range of multipurpose-tree species that could play a role in agroforestry technologies are identified. The lists of potentially useful multipurpose tree species from such D&D exercises are then validated through library searches and/or use of specialized databases such as the ICRAF's multipurpose tree database and similar databases of other institutions. These steps provide the basis for starting general and agroforestry technology-specific species trials within defined zones. Within Sub Sahara Africa, such trials are implemented within the framework of ICRAF's Agroforestry Research Networks for Africa (AFRENA). The most outstanding ten or so species identified from such trials are then included in co-ordinated regional species and provenance trials. Such co-ordinated trials could address research issues as: range-wide genetic variation, comparative provenance adaptability and performance, and provenance × environment interactions etc.

Beyond the above stage, selection and breeding research should be concentrated on best provenances of three to five best species for each broad agro-ecological region (see Fig. 2). Two points should be stressed with regard to the selection of species for selection and breeding research. Firstly, it should be recognised that comprehensive breeding programmes call for long-term commitment of human and other resources to the extent that it would be unrealistic to cover more than five species for each broad agroeco- logical. Secondly, early attempts should be made to address two preliminary issues (i) justifying the proposed breeding programmes in economic terms, and (ii) rationalizing the breeding strategy on the known genetic system of the species. This second issue is particularly important when dealing with the many potentially useful species which are currently little known to science.

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ADAPTIVE RESEARCH PHASE / STRATEGIC RESEARCH PHASE ADAPTIVE RESEARCH PHASE

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Fig. 2. Successive stages in multipurpose tree improvement.

An example of an improvement research programme for the highlands of East and Central Africa

The sequential steps in Fig. 2 have been followed since 1987 in the recently started multipurpose tree improvement programme for the bimodal rainfall highland zones of Eastern and Central Africa. Agroforestry technology D&D exercises were undertaken in Kenya, Uganda, Rwanda and Burundi. A zonal synthesis arrived at in late 1987 involving multidisciplinary teams of scien- tists from the four countries. As part of this zonal synthesis, a tentative list of potentially useful multipurpose tree species was proposed. Subsequently, the list was updated in consultation with databases, literature, and other experts. The updated list formed the basis for the establishment of both general and agroforestry technology-specific species trials in all four countries. Beginning in early 1988, network scientists established general species trials involving some 144 species and provenances. In addition, they established agroforestry technology-specific species trials (e.g. for hedgerow intercropping, boundary planting, scattered trees on farms, etc.), involving some 10 species. Arising out of this assessment a tree-improvement research programme was initiated for five top-priority species viz Sesbania sesban, Markhamia lutea, Grevillea robusta, Leucaena diversifolia and Calliandra calothyrsus.

One of the species which showed great promise across the defined zone, for planting as scattered trees on farms, was Sesbania sesban. Provenance trials in Kenya, Rwanda, and Burundi identified Kakamega provenance of tiffs species as the best for the zone. A decision was therefore taken to concentrate selection and breeding work on this particular provenance.

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In addition, farmer surveys were conducted within a small part of the zone (Western Kenya) to determine how farmers ranked S. sesban and it was confirmed that this is among the top five species preferred by farmers and that its major attraction is the ability to improve soils (by nitrogen fixation) and rapid production of fuelwood.

In the meantime, it was determined from agroforestry technology-specific trials that S. sesban is not suitable for hedgerow intercropping due to its poor coppicing ability. This finding helped to focus breeding orientation to superior trees for scattered tree planting on farm, rather than densely planted trees in hedgerows.

Based on such analyses, a S. sesban breeding programme was started in 1989 with the main criteria of selection and breeding being (i) rapid growth, (ii) straight stems (iii) freedom from knots and (iv) light crowns. Mass selection for a base breeding population was completed in 1990. Half- siblings have already been evaluated at three different locations and two seed orchards are already producing semi-commercial quantities of seed. In parallel to this, a comprehensive study is in progress (in collaboration with the Soil Science Department of the University of Nairobi) to identify the most effective Rhizobium strains for S. sesban and to study tree genotype x Rhizobium strain interactions.

Acknowledgements

The author acknowledges valuable comments and suggestions by various colleagues at ICRAF, especially Marcellino Avila, Peter von Carlowitz and Francis Esegu. Financial support from the Republic of Germany's BMZ Grant No. 4-3001-60.100470 is appreciated.

Appendix

Some high priority species for agroforestly development in sub-Sahara Africa

Unimodal rainfall plateau of Southern Africa Faidherbia (Acacia) albida Calliandra calothyrsus Cassia spectab ilis Leucaena leucocephala Gliricidia sepium Sesbania sesban

II. Bimodal rainfall highlands of Eastern and Central Africa CaUiandra calothyrsus Leucaena diversifolia Sesbania sesban Markhamia lutea

Grevillea robusta Casuriana junghuniana Gliricidia sepium

III. Semi-arid lowlands of West Africa Fraidherbia (Acacia) albida Prosopis africana Gliricidia sepium Cornbretum aculeatum Acacia nilotica Parkia biglobosa Butyrospermum parkii

IV. Humid lowlands of West Africa Paraserianthes falcataria Cassia siamea Gliricidia sepium Calliandra calothyrsus Inga edulis Tephrosia candida Flemingia cardifolia Pentanclethra macrophylla Acioa barterii Dialium guinense Pterocarpus mildbraedii

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References

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Nalr PKR (1987) Agroforestry systems inventory. Agroforestry Systems 5:301-318 Owino F (1990) Small-scale farmer oriented strategy for evaluation and improvement of

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Steppler HA and Lundgren BO (1988) Agroforestry: now and in the future. Outlook on Agriculture, Vo117 (4): 146-152

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Von Carlowitz PG, Wolf GV and Kemperman REM (1991) Multipurpose tree shrub database: An information and decision support system. User's manual and diskettes, Version 1.0. International Council for Research in Agroforestry, Nairobi, Kenya