An e-publication by the World Agroforestry Centre
AGROFORESTRY A DECADE OF DEVELOPMENT 
|
|
An e-publication by the World Agroforestry Centre |
|
AGROFORESTRY A DECADE OF DEVELOPMENT |
|
|
section 5 Chapter 16 Jeffery Burky Introduction Trees and shrubs occur in a wide variety of land-use systems. For simplicity these systems can be grouped into 11 major categories. 1. Natural vegetation management Although considerable research has been undertaken on the management of tropical rain forests, relatively little attention has been given to the natural tree and shrub associations of drier zones. Nevertheless these communities offer a significant and often the sole source of plant materials, especially for fuel and fodder; often they contain species that could have great potential as planted exotics for other sites. Generally there is more information and experience of tropical and subtropical trees than shrubs from the points of view of ecology, distribution, inventory, use and management. 2. Industrial plantations These are large areas created and managed intensively, usually with exotic species, for the production of timber to supply sawmills, pulpmills, veneer factories, chipboard plant, etc. The plantations are usually owned and managed by state enterprises. 3. Community woodlots These are small areas of 5-10 ha created for the benefit of village or town communities (where they are often referred to as "peri-urban plantations"), often by the state, and more recently by the communities themselves in some form of social forestry. They may be on state or community land and may yield timber, poles and fuelwood plus occasionally fruit, fodder and other products. There are often difficulties of management, protection and distribution of benefits. 4. Farm woodlots These are small plantations of less than 10 ha, often much less, that are established by the individual farmer for the production of poles, fuel, fodder and possibly other products; multipurpose trees are thus desirable. The products supply the farmer's own needs with excess for sale and such woodlots may be established on unused or degraded land with a view to rehabilitating it. 5. Trees in crop land Individual trees may be left or planted randomly at wide intervals in productive agricultural land to supply wood, fuel, fodder, fruit, honey and shade. 6. Alley farming In this group of systems, one or more rows of trees are planted alternately with several rows of agricultural crop plants and the trees are hedged, coppiced or pollarded frequently. The decline in value of crop yield caused by the loss of land occupied by trees should be compensated or exceeded by the fertilizing effect of the tree leaves and other ways of soil improvement by trees, and by the value of tree products (poles and fuel). This is becoming the most widely recommended agroforestry system (see Kang and Wilson, this volume). 7. Linear planting This includes the planting of one or more rows of. trees, with or without subsequent management, along farm borders, river or stream banks, or along roads, railways or canals. They can provide the usual services and benefits. 8. Shelterbelts. The are belts/blocks consisting of several rows of trees established at right angles to the prevailing wind. They are also known as windbreaks and have significant effects on micrometeorological factors up to several times their height away from the edge. The species, age composition, canopy density, height and profile are all important determinants of their effectiveness. They also produce valuable by-products. 9. Sequential cropping Trees and agricultural crops may follow each other on the same piece of land in planted "fallow" systems in which the trees restore the soil fertility. Taungya is a system whereby trees are planted, often at close industrial spacing, together with intercrops of agricultural species, the agricultural crops being grown for up to three or four years. 10. Silvopastoral systems Silvopastoral systems involve the incorporation of tree and shrub management and animal husbandry. The trees may be used for fodder production, shade and pasture improvement. Intensity of the operations can vary from extensive range management in dry zones to intensive trees-over-pasture in areas of higher rainfall. 11. Protection forestry, land rehabilitation, reclamation The use of trees in these roles encompasses many technologies. Protection forestry generally requires the management, through natural regeneration, of existing indigenous vegetation, which requires protection from grazing and damaging exploitation above all. For the reclamation and rehabilitation of degraded land the function of trees is primarily for soil conservation or improvement, coupled with production of (mainly) wood and fodder. The arrangement of the planted trees (which can be combined in agroforestry systems) should follow these major objectives. For virtually all these systems a multipurpose plant would be considered more useful than a species fitted for only one purpose. In fact there can be very few species that, if they are used at all, are not used for several purposes, products, benefits and services. There has been considerable discussion of the definition of multipurpose trees (see the views of several specialists compiled in Burley and von Carlowitz, 1984) but the concept is now well established, largely as a result of the interests and activities of ICRAF. The common abbreviation is MPTS, which can imply "Multipurpose TreeS" or "Multipurpose TYees and Shrubs". The attention to MPTS has developed in parallel with the growth of social forestry programmes and the research and development of agroforestry systems that can be used to meet the objectives of social forestry and integrated rural development programmes. The incorporation of MPTS into land-use systems requires Significant changes in the attitude, understanding and co-operation of professional foresters, horticulturists, agronomists and various groups of extension workers.
The products and services derived from trees and shrubs are manifold and vary between societies and environments, but they can be summarized simply as follows: Products Products Environmental benefits Socio-economic benefits
Exploitation is often used in a pejorative sense to indicate the utilization of a person or object for one's own selfish ends, but in fact this is the implication of man's use of multipurpose trees and shrubs. We seek species and populations that can provide the many benefits in the several land-use systems indicated above. Potential is taken here to indicate cryptic, possible value for such uses and to exploit it requires knowledge of hitherto hidden values; this necessitates research that is specific to sites, managerial systems and end-use processes. An idealized, complete research programme for a new land-use system or site is summarized in Appendix 1. Clearly not all of these stages are required for each site or species; many species are already at an advanced stage of genetic and managerial development either by researchers or by line managers an.d farmers. However, the appendix acts as a checklist for researchers and managers initiating development of new areas or systems. Not all of the stages need to be conducted sequentially; some can be undertaken in parallel or telescoped together. Nevertheless, the determination of an optimum system and combination of species requires co-operative research and implementation between a range of authorities and disciplines. The major, specific stages and problems related to the incorporation of the MPTS themselves can be grouped into the following principal topics — genetic variation, germplasm supplies, assessment of multiple products and services, and crop management. Genetic variation Number of species The large number of tree and shrub species already recorded as promising or possible (some 2,000 species were listed in Burley and von Carlowitz, 1984) is at once a potential benefit and a problem. No one species is likely to be the "wonder species" for all sites and purposes but it is difficult to cope with such large numbers in formal species trials even on efficient research stations interested in a single major product. It is infinitely more difficult to cope with large numbers on less well endowed stations or in on-farm research. Preliminary screening is thus needed and the climatic matching systems collectively known as homoclimal comparisons (and quantified by, e.g., Booth, forthcoming) permit the reduction of the number of species worth consideration for trial on a given site type. The several data bases developing within ICRAF itself will formalize and often quantify data on sources, uses and characteristics of MPTS. However, there will always be a need for elimination and proving trials of reasonable numbers of species (20-30) on each major site type. Centrally planned co-operative trials on many sites, such as the Oxford Forestry Institute (OFI)'s international trial of dry-zone hardwoods, will allow the estimation and explanation of genotype-environment interactions and the extrapolation of results to untested sites if the environmental conditions are known. In this international network seed and herbarium material from 25 Central American species were collected (see Table 1 and Hughes and Styles, 1984); seeds are being distributed to some 60 sites in more than 25 countries. An indication of the suitability of these species for different uses in their natural range is given in Table 2. Not all of these will be found in all exotic sites and not all are equally important on any individual site, but they indicate the potential of this group of species. Similar sets of species can be found elsewhere (e.g., the many promising Australian and African Acacia species) and these must be compared. The collaborative (zonal co-operative) programme of ICRAF (COLLPRO) will undertake species evaluation for common site types while national programmes deal with specific locations. Variation within species For naturally widely distributed species, natural selection (and often man's interference) causes genetic variation between populations; for species that have been managed as exotics for several generations artificial and natural selection in the planting sites may cause the evolution of land races. All of these different sources (termed "provenances" by foresters) should be tested for each site and management system, but clearly they compound the problems of species trials with large numbers of sources. Nevertheless, the correct choice of optimum seed source offers the major and simplest step in genetic improvement and the determination of the most productive or acceptable system. Later tree-breeding efforts are wasted or reduced in value if the original germplasm source is not optimal.
Little species and provenance research has been conducted on MPTS other than the initial exploration and evaluation of arid-zone species by CSIRO, CTFT, FAO/IBPGR, NAS, NFTA and OFL1 There is great potential for national organizations and multilateral programmes to conduct systematic research on both indigenous and exotic species, including local land races that have been manipulated genetically, whether consciously or unconsciously, by local farmers. Current emphasis is on provenance variation of Acacia, Eucalyptus, Leucaena and Prosopis species, while among the most exhaustive exploration and evaluation of a single species are the studies on Leucaena leucocephala (Brewbaker, this volume) and the OFI international study of Gliricidia sepium (Hughes, 1986, 1987). The available seed sources of the latter are listed in Table 3. Preliminary assessments of field trials in several locations demonstrate considerable differences between provenances in form and growth rate up to two years.2 In a trial of 10 sources in Costa Rica, Salazar (1986) observed significant differences between provenances in seed size and shape that were related to altitude of seed source; after 60 days heights ranged from 35 to 47 cm. All the earlier evidence from international and native provenance trials of industrial species such as tropical pines and eucalyptus confirms that large intraspecific variation occurs in wide-ranging species. The same situation is being demonstrated in MPTS and variations can be expected to be enhanced by the existence of local land races that developed in exotic and natural locations under man's selection. With the exception of Leucaena species (covered in a separate chapter in this volume by Brewbaker) and Prosopis species (see several papers in Felker, 1986), for most MPTS currently in use or under research it is perhaps premature to consider selective tree breeding, but the short sexual and managerial cycles allow rapid progress to be made once individual selection begins. The principles are no different from those of industrial tree species, nor indeed from those of agricultural and horticultural species, and they were outlined for MPTS by Burley (1987). Evidence of intra-population genetic control of one set of characteristics (biochemical) of fodder trees was found in Prosopis by Oduol et al. (1986); 14 half sib families representing several species showed intra-class correlations of pod sugar (0.3-0.4) and pod protein content (0.04-0.6) depending on planting site. All of the above are examples of the potential of MPTS but, with the exception of Leucaena, they are preliminary indicators only. As highlighted by Brewbaker (this volume), the widespread attack by psyllids on Leucaena leucocephala and the species intolerance of acid soils stress the value of interspecific hybridization within a genus and the importance of evaluating a wide range of other species and genera. Germplasm supplies and certification The great interest in development of a wide range of MPTS causes a corollary problem in supplies of appropriate germplasm. Even assuming the correct genetic source has been identified, it is often difficult to obtain guaranteed supplies with the internationally recognized certificates of health, origin and physiological and physical quality. An indication of the systems of certification available was given by Jones and Burley (1973), and the problems of tropical and subtropical (developing) countries in implementing such systems were described by Burley (1986). Sources of potential suppliers of a large range of MPTS were provided by von Carlowitz (1986). They show a remarkable variation in ability or inclination to provide appropriate seed-source information.
Evaluation of multiple benefits For industrial plantation species the most important features affecting quantity or quality of products are well known and standard methods of assessment exist (for example, height, diameter, form, taper, branch dimensions, forking or wood density). For MPTS, however, many products and services need evaluation and, for some, appropriate techniques are either not yet developed or at least not known to the forestry-trained researchers who are currently conducting the bulk of the tree-related research; these include fodder-quality characteristics, soil-improving capacity, nitrogen-fixation ability, and the effect of the tree or shrub on agricultural crops or animals. Market values may not exist for some of these traits, while others may be negatively correlated so that an improvement in one causes a decline in another. This causes particular problems for the tree breeder who then needs to develop selection indices, but rapid and simple assessment procedures are also needed for the evaluation of MPTS in species, provenance and management trials. In addition to anatomical, chemical and morphological features, the traits of interest include phonological and physiological characters of the tree or shrub and chemical and biophysical evaluation of its effects on the soil. Many of these traits were described by Burley et al. (1984). The effects of MPTS on microclimates are more complex to evaluate (Huxley et al., forthcoming). When using MPTS as energy sources there is a need to estimate and partition the amount of biomass produced within and between individual plants. Classical industrial forestry methods of mensuration are not applicable to MPTS used for biomass production since they have small heights and diameters, multiple stems and branches, and a significant component in their leaves, particularly if they are to be used for liquid and gaseous fuel or chemical and dietary feedstock. There is an urgent need to develop sampling and statistical prediction methods to partition biomass among stems, branches, leaves and roots while extrapolating from small plots or individual trees (often open-grown) to large areas. Evaluation in agroforestry systems The initial screening and evaluation of tree and shrub species, commonly called elimination, is concerned principally with the ability of the species to survive the natural and managerial conditions of the planting site in the first few years. This stage is often carried out using standard forestry approaches with replicated designs and multi-tree plots, good protection, weeding and other site improvements. However, MPTS must also be evaluated in the longer term to estimate their performance in agroforestry systems, both on research stations and on farms. There is therefore a need to develop designs to test for early intercropping and resource sharing, to examine the effects of trees and shrubs on the soil sustainability, and to study the response to managerial treatments such as coppicing, lopping and pruning. Such designs are currently being developed by ICRAF, particularly for its Agroforestry Research Network for Africa (AFRENA) programme. When the effectiveness of these designs is determined, and the results of systematic research become available, individual workers and institutions will be able to evaluate and capitalize on the great potential of MPTS to enhance man's survival and welfare. Without rational research and development of acceptable species and systems the current enthusiasm for MPTS runs the risk of alienating farmers and development agencies by failing to provide socio-economically acceptable packages.
Booth, T. H. Recent developments in homoclime analysis to assist species selection. In T. Darnhofer and W. Reifsnyder (eds.), Proceedings of the International Workshop on the Application of Meteorology to Agroforestry Systems Planning And Management. Nairobi: ICR AF (in press). Burley, J. 1986. Problems of tree seed certification in developing countries. Invited Paper, 18th IUFRO Congress, Ljubljana, Yugoslavia. In: Proc. Div. 2, WP S2.03-14, 112-123. ——.1987. Strategies for genetic improvement of agroforestry trees. Lead Paper, Session 8,IUFRO/ISTS Workshop "Agroforestry for Rural Needs", New Delhi, India, February 1987. Burley, J. and P.O. von Carlowitz (eds.). 1984. Multipurpose tree germplasm. Nairobi: ICRAF. Burley, J., P. A. Huxley and F. Owino. 1984. Design, management and assessment of species, provenance and breeding trials of multipurpose trees. In R.D. Barnes and G.L. Gibson (eds.), Provenance and genetic improvement strategies in tropical forest trees. Commonwealth Forestry Institute, Oxford and Zimbabwe Forestry Commission, Harare. Felker, P. (ed.). 1986. Tree plantings in semi-arid regions. Amsterdam: Elsevier. Hughes, C. E. 1986. Protocol for the international provenance trials of Gliricidia. Oxford: Oxford Forestry Institute. ——.1987. Biological considerations in designing a seed collection strategy for Gliricidia sepium (Jacq.) Walp. (Leguminosae). Commonwealth Forestry Review (in press). Hughes, C.E. and B.T. Styles. 1984. Exploration and seed collection of multipurpose dry zone trees in Central America. International Tree Crops Journal 3: 1-31. Huxley, P.A., E. Akunda, T. Darnhofer and D. Gatama. Tree/crop interface investigations: some comments and preliminary investigations. In T. Darnhofer and W. Reifsnyder (eds.), Proceedings of the International Workshop on the Application of Meteorology to Agroforestry Systems Planning and Management. Nairobi: ICRAF (in press). Jones, N. and J. Burley. 1973. Seed certification, provenance, nomenclature and genetic history in forestry. SUvae Genetica 22(3): 53-58. Oduol, P.A., P. Felker, C.R. McKinley and C.E. Meier. 1986. Variation among selected Prosopis families for pod sugar and pod protein contents. In P. Felker (ed.), Tree plantings in semi-arid regions. Amsterdam: Elsevier. Salazar, R. 1986. Genetic variation in seeds and seedlings of ten provenances of Gliricidia sepium (Jacq.) Steud.3 In P. Felker (ed.), Tree plantings in semi-arid regions. Amsterdam: Elsevier. von Carlowitz, P. G. 1984. Rapid appraisal methodology for selecting priority multipurpose tree species and criteria for determining status and research needs. In J. Burley and P. von Carlowitz (eds.), Multipurpose tree germplasm. Nairobi: ICRAF.
Major steps and technology components in tree and forestry research to develop appropriate land-use systems Background research Natural vegetation management technologies Plantation technology Basic or facilitating research
1 Editors'note: see the full names of these institutions listed at the beginning of this book. 2 Personal communication from Janet L. Stewart, Oxford Forestry Institute. 3 Editors'note/The 1986 ICRAF"Master List" of MPTS, verified with the Royal Botanical Gardens, Kew, U.K., cites Gliricidia sepium (Jacq.) Walp. as the correct (current) taxonomic nomenclature for the species. |
