Chapter V: State of the Art and Tools - Biology of Forest Species

1. Survival strategies
2. Sexual regeneration or reproduction
3. Asexual regeneration or multiplication
4. Species plasticity
5. Biological diversity
6. Maintaining soil fertility

This chapter provides a simplified summing-up of the main achievements and results in the field of the biology of dry tropical forest and savanna species. It emphasizes the reproductive and regeneration capacities of this type of formation and the most commonly used improvement and substitution methods.

1. Survival strategies

It is extremely useful to be familiar with the renewal process of ecosystems and the reproduction mechanism of woody and herbaceous species in order to successfully embark upon sustainable forest management. To cope with serious water-related constraints in long dry season periods, plants adopt particular reproduction strategies, characterized by the quality, the quantity, the distribution and the type of regeneration. A traditional distinction is drawn between two main survival strategies: the ‘r’ and the ‘K’ modes (‘r’ refers to an exponential growth rate, and ‘K’ refers to the maximum biomass level), while between these two benchmark practices there is of course a wide range of attitudes.

Strategy ‘r’: this is typical of species which multiply very rapidly enabling them, under favourable conditions, to occupy the area and to mobilize the resource. But the individuals in the populations generated in this way are compelled to compete strongly for survival, leading to a high death rate. This is the case with pioneer species.

Strategy ‘K’: this is typical of species with a low regeneration rate, but whose individuals have a high chance of survival. These often occupy particular ecological niches and replace the ‘r’ strategy species which they eliminate by competition. This is the case with many timber producing species.

“The annual herbaceous species in the Sudanian Regional Centre of Endemism (Centre Régional d’Endémisme) are of the ‘r’ strategy type while, generally speaking, the woody and hardy grasses are generally of the ‘K’ type. Some more original species develop mixed ‘r-K’ strategies. This is the case with many of the woody plants in the Sudanian savannas, such as Daniellia oliveri, Isoberlinia doka, Piliostigma thonningii, etc. In their juvenile phase, these plants develop a very large perennial underground structure (often disproportionate to their aerial vegetative system) which enables them to reconstitute the aerial structures if they are destroyed by external factors (fire or drought). The strategy is therefore rather of the ‘K’ mode, and remains if the external aggression remains at a reasonable level. If, however, the pressure of external aggression becomes strong and regular, the plant eventually multiplies abundantly in an herbaceous form during favourable periods, and thereby moves over to an ‘r’ type strategy. However some plants (such as Cochlospermum planchonii, C. tinctorium and Striga baumannii) have adopted this type of development, with a large underground xylopod, as their normal mode of growth. This is a very clear-cut case of a derived character, as a result of adapting to fire. This biological form seems to be typical of the savanna.” (Nasi, 1994)

Dormancy is a complementary means used by some plants to guarantee survival. The term ‘dormancy’ refers to the stage in which germination is impossible or highly unlikely. In order for dormancy to terminate naturally, the seed must be subjected to a process which is sometimes long, often characterized by a transformation due to the effect of climate (heat, cold, humidity, drought) or to physical, chemical and/or mechanical (by digestion, rotting) scarification.

Germination can only take place when the favourable conditions are met. That is why we talk about a survival strategy component.

Box 5: Germination, dormancy termination and pre-treatments Dormancy is considered to be a germination regulatory system. It is a relative phenomenon, depending on post-harvest conditions (heating, drying, etc.), but also on the maturity of the fruit, the genotype and very likely germination conditions. It can induce a low (or zero) germination faculty if nothing is done to revive it. It characterizes the physiological state of a seed which is unable to germinate under apparently favourable germination conditions. Dormancy prevents or delays sustained, homogeneous and rapid germination. Pre-treatments do not induce the germination of seeds but make them capable of doing so subsequently when all the conditions are met. By definition, it is the treatment (or treatments) which precedes seed conservation or is applied during or after it, that puts an end to the state of dormancy. This is done by applying mechanical, chemical, physical, physiological or biological (isolated or associated) treatments. The word ‘pre-treatment’ is sometimes used wrongly because it is also applied to refer to such different treatments as coating or stripping, and plant health treatment, etc. However the purpose of these treatments is not to terminate dormancy. It should be emphasized that a wide variability may exist between seed batches and that, for one same species, pre-treatment may differ considerably in length from one seed to another or from one year to the next in the case of seed batches collected from the same tree and at the same place. Dormancy varies in type. A distinction is drawn between primary dormancy (the seed is dormant at the time of dispersal), and secondary or induced dormancy (the seed can germinate during dispersal, but a number of factors prevent it from doing so). It is often a matter of exogenous (seed coat) dormancy or endogenous (embryo) dormancy, and combined dormancy. As far as exogenous or tegument inhibition is concerned, the embryo which has been carefully stripped of its seed coat, germinates without difficulty, while the whole seed shows no sign of germination. In the case of endogenous or embryo dormancy, there is zero germinating power because even when stripped of the various structures surrounding it, the embryo is unable to germinate at the time of dispersal. Combined dormancy is the result of both endogenous and exogenous dormancy. Seed coat inhibition depends on physical, mechanical or chemical barriers. The different structures surrounding the embryo, particularly in the case of hard seeds, prevent water absorption. If the pericarp or the external seed coats are not damaged, water is unable to penetrate. This is the case with many seeds produced in arid zones (physical dormancy, which is widespread for many types of Acacia, Cassia, etc.). Inhibiting substances, both within the pericarp and in the embryo, directly interfere with germination (chemical dormancy). Abscisic acid is often mentioned in this connection. Experiments have shown the inhibiting action of external structures partly relocated in contact with the previously stripped embryo: this reintroduces dormancy. The often average-to-mediocre germination of Tectona grandis is partly explained by the presence of inhibiting substances, and probably also physical and/or mechanical dormancy (Willan, 1992). In order to put an end to exogenous dormancy or seed-coat inhibitions in seeds from tropical zones, many natural and artificial methods are used. For hard seeds, scarification by scalding is the most frequently used, but it can also be done by immersion in sulphuric acid, manual or mechanical scarification of the external teguments (by incision, slitting, piercing, very slight burning, blasting, abrasion). In nature this is done through low intensity bush-fires, mechanical action of insects, termites, bats, regurgitation after partial rumination, the action of animals’ digestive juices, the alternation of dry and wet periods, and rotting. Lixiviation with running water, pre-treatment using oxidants (H2O2 for example), oxygen enrichment of the environment, are also some of the methods used to terminate physical, mechanical or chemical dormancy. Ingrained habits and custom can, however, reduce the effectiveness of pre-treatment. For example, after pre-treatment using acid, most of the seed centres (with very few exceptions) recommend to leave the seeds to soak in water 12-24 hours. Although this does not cause losses in some species, for others it seriously reduces germination capacity (for example Acacia senegal). The time for soaking in water must be carefully studied for each individual species whether pre-treatment is used or not (for the Azadirachta indica kernels, prolonged soaking is harmful). Seed pre-treatment can be carried out before conservation (if there is sufficient manpower or time) or during cold storage, without having any marked effect on germination capacity. Pre-treatment must only be applied to seeds that exhibit strong or deep dormancy which must also be conserved under the right conditions (from harvest-time to sowing). In the case of light dormancy, seed pre-treatment is not economically justified, unless they are particularly rare, relics or valuable. In conclusion: - pre-treatment must be simple, economical, and based on the observation of nature; - post-harvest and experimental conditions to which the seed is subjected should be better explained in the literature; - description of the plant material (fruit, seed, kernel) is not always clear, creating misunderstandings; - length of the soaking in water after pre-treatment is rarely specified in detail; - it is not the percentage of germination which interests the user but the average number of seedlings which are normally constituted one month after germination; and - national centres should retain responsibility for hazardous treatment (acid). Source: Bellefontaine, 1993

Other adaptive procedures could also be listed. In particular self-fertilization (the capacity of rare species to reproduce themselves) and the level of ploidy (the higher it is the greater the adaptive capacity).

2. Sexual regeneration or reproduction

Even though much progress has been made in our knowledge of the phenology and physiology of some species, too little is known about most species, and the literature is still inadequate.

Describing and understanding the factors that condition flowering, fruiting and germination is still very difficult. This requires long periods of observation, very specific and costly experimental schemes. The most frequently mentioned factors are climate, fires and animals. The nature of the soils is quite evidently important, but the complexity of this factor and its multifaceted role make it difficult to study.

Climatic factors are the ones that have been most thoroughly studied and one can see that interannual rainfall variations explain many of the fluctuations in the phenology of woody species. The results of a survey covering eight years in northern Senegal (Poupon, 1979) confirm that:

- Some species flower in the dry season (Commiphora africana, Acacia senegal, Balanites aegyptiaca) and others in the rainy season (Grewia bicolor, Guiera senegalensis, Boscia senegalensis).

- Fruit appears during the rainy season in the case of Acacia senegal, Balanites aegyptiaca, Grewia bicolor and Guiera senegalensis and at the beginning of the dry season for Commiphora africana and Boscia senegalensis. The period of active fruit growth is always in the dry season.

- Two of the species studied are evergreen: Balanites aegyptiaca and Boscia senegalensis.

Burkina Faso has also been carrying out a more comprehensive field investigation since 1993 in the reserved forests of Tiogo and Laba. The design includes observations on plant phenology to be carried out every 15 days during five years. The observation plots, set up according to an appropriate statistical design, include some which are protected against fire and others subject to early burning. (Nouvellet et al., 1995). This study has shown that fire can induce major variations in the phenological cycles.

In Madagascar, Sorg and Rohner (1996) have been monitoring 56 species from dry deciduous forests. The species have been classified on the basis of flowering and fruiting regularity. The researchers have also given the periods for harvesting the seeds, species by species. They discuss the different influences of the soils, rainfall and length of daylight hours. The effect of the permeability of the soil varies according to the species, and the rain seems to induce the foliation process. Flowering occurs throughout the year.

Phenological studies also help to reveal certain anomalies. For example, in West Africa, Anogeissus leiocarpus only produces 2-5 percent of fertile seed, despite abundant flowering. In Burkina Faso, Burkea africana produces a great number of flowers but little or no fruit at all (Bellefontaine, 1995-a).

There are few publications that put together knowledge achieved in phenology, largely because of the extreme variability in terms of time and place. It is therefore very difficult to propose any one model, particularly since phenology is closely linked to bush fires. There are many species which produce different leaves after a fire (early young form or adult form). Do the fires affect fruiting dates? This is a plausible hypothesis, but there is no evidence to support it as yet.

As far as seed physiology is concerned, and particularly germination and conservation, much progress has been made over the past 15 to 20 years. This made it possible to set up seed banks and laboratories during 1970-1975 (DANIDA Forest Seed Centre, CIRAD-Forêt, Oxford Forestry Institute, CATIE, CSIRO, etc.), followed, 10 years later, by forest seed centres in many different tropical countries (Madagascar, Kenya, Zimbabwe, Burkina Faso, Senegal, etc.). General knowledge about seed physiology (optimum water content, dormancy termination pre-treatment, conservation conditions, etc.) has increased considerably in the past few years (Box 5). Symposiums and workshops have brought together specialists, and led to the publication of major documents. The accessibility and exchange of seeds have also been greatly facilitated by the publication of such magazines as FAO’s Forest Genetic Resources. The FAO Panel of Experts on Forest Genetic Resources, at the end of its Eighth Session (1993) recommended “that efforts be made to facilitate continued exchange of research seed-lots of forest trees and provenances among countries, and that the FAO International Code of Conduct for Plant Germplasm Collecting and Transfer be used to guide agreements between countries in this regard, in the spirit of the International Undertaking on Plant Genetic Resources and the Convention on Biological Diversity.”

Lastly, there is the importance of the role of livestock, mainly domestic herds, in seed dissemination. Many species belonging to the dry zones produce edible fruit (the shells of legumes, the fleshy fruits of the Anacardiaceae) which are palatable to livestock. The natural regeneration of Acacia spp., for example, greatly depends on straying livestock that glean the husks during the dry season.

In Africa, in the Sudanian area, the species are generally capable of regeneration by natural seeding. However the survival of the seedlings depends on the regularity and intensity of bush fires. The young shoots are often destroyed by the fires, and the sprouts then take over from compromised sexual regeneration. In the Sahelian area, natural seeding is very often disturbed by severe climatic conditions. The plants find it difficult to take root in the dry soil where the water content is low.

Furthermore, the livestock continually destroy the new shoots, and the soils become compacted by intense trampling. The strategies developed by these species are therefore more of the pioneer type, to adapt to an environment that is constantly evolving. Natural regeneration of dry savanna leads to ecological stratification, because moving northwards (from the less dry zones to the more arid zones) seed regeneration gives way to vegetative reproduction (Catinot, 1994). This assertion, which has often been verified, does not apply to all plants, and must be tuned, depending on specific adaptation strategies.

For local species, plants produced from seed generally grow comparatively slowly and require a long, sometimes very long, period of protection (closing off). Grazing exclusion periods are shorter when resource management is based on natural vegetative multiplication.

3. Asexual regeneration or multiplication

Most of the species from dry tropical zones have a strong sprouting capacity, and a less strong sucker-producing potential, for a certain period during their life. This vegetative regeneration capacity is viewed by some as a form of adapting to a highly variable environment in terms of bush fires, drought and overgrazing.

In the Guesselbodi Forest (Niger) the drought resistance of Combretum spp. and Guiera senegalensis has been explained by their ability to produce stump sprouts or root suckers, and runners from the sprouts lying on the ground. This makes up a large underground network from which the individuals finally separate. These species are therefore able to proliferate and resist fire, livestock and man (Catinot, 1994).

Five years after simple coppice logging (cutting) at Gonsé (Burkina Faso), Nouvellet (1993) estimated that 95 percent of the regeneration came from new stump sprouts, and showed that the number of stems per stump practically doubled. Another study carried out at Sikasso in Mali (Cuny, 1993-a) showed that 93 percent of the stumps following coppice cutting operations had produced shoots or suckers (Table 5). Parkan et al. (1988) have, after two years of observation, proposed a table showing the vegetative regeneration aptitude of a number of local species in Mali. Renés and Coulibaly (1988) have also contributed to our knowledge of the species which multiply through vegetative reproduction in the natural environment in Burkina Faso. Certain woody plants such as Detarium microcarpum, produce abundant suckers. On 769 stumps of Detarium microcarpum, 63 percent were seen to have produced suckers, 17 percent sprouts, 11 percent suckers and sprouts. Some suckers were observed over 10 m away from the parent stump (Kaboré and Ouedraogo, 1995).

All these observations confirm the major role of asexual regeneration in maintaining the shrub and tree component of dry forests. However, the natural vegetative multiplication of African species in the Sudanian and Sahelian areas is known only in certain specific cases, and sometimes fairly unreliably. The same applies to the Zambezian area. Furthermore, the species which do sprout are generally better known by silviculturalists than the species which produce root suckers. Recently, Bellefontaine (1995-b) tried to produce a summary overview of over 110 West African species. But the list of species which multiply by layering is very small. It would be useful to pursue this type of study and design techniques to make it possible to distinguish between the plants produced by sexual reproduction and those produced by vegetative multiplication, without unearthing the young plants.

This ability to reproduce by stump sprouts or by root suckers could be exploited widely when certain forest management activities are undertaken:

- to slightly reduce the large number of species to be treated in certain forests (for example, when the neighbouring populations have other nearby unmanaged forests to use for harvesting the non-woody products they need); and

- or when economic or technical difficulties stand in the way of establishing some species, such as production in nurseries, seed conservation, pest control, low survival rate and low growth rate after plantation, demanding long periods of deferred grazing.

Species	Nb. S (S/S)	Shoots (RS/S)	Suckers (cm)	H. DS (cm)	H. DRS SS	Percentage SRS	Percentage
Afrormosia laxiflora *	8	7	5	229	96	75	63
Annoina senegalensis	2	0	3	-	303	0	100
Bombax costatum	3	1	1	142	30	67	33
Burkea africana *	5	2	2	210	254	60	60
Butyrospermum paradoxum *	5	1	6	300	218	20	100
Combretum fragrans *	17	4	3	398	298	53	53
Combretum molle	1	0	10	-	55	0	100
Daniellia oliveri *	44	2	4	240	207	59	73
Detarium microcarpum *	94	2	4	335	312	37	87
Erytrophleum africanum *	11	7	4	393	354	82	82
Gardenia ternifolia	3	9	0	159	-	100	-
Gymnosporia senegalensis	1	1	1	56	112	100	100
Hymenocardia acida	3	6	4	124	79	100	67
Isoberlina doka *	32	2	3	334	277	28	72
Lannea acida	1	0	0	-	-	0	0
Lannea velutina	1	1	0	357	-	100	0
Marasnthes polyandra *	20	7	2	201	67	85	20
Monotes kerstingii *	30	5	4	420	165	97	27
Parinari curatellifolia *	43	1	5	221	193	14	77
Pteleopsis suberosa *	6	4	2	148	121	67	33
Pterocarpus erinaceus	4	3	2	290	276	100	100
Securidaca longepedunculata	1	10	0	60	-	100	0
Strychnos spinosa	2	2	0	310	-	100	0
Swartia madagascariensis	3	9	6	287	310	33	100
Terminalia albida	3	5	0	208	-	100	0
Teminalia laxiflora	1	0	2	-	200	0	100
Terminalia macroptera *	6	2	7	342	149	50	33
Vitex madiensis (V. barbata)	4	3	2	259	243	100	50
Xerodermis stuhlmanii (O. chevalieri)	1	0	4		215	0	100
Ximenia americana	1	0	5	-	272	0	100

Nb. S = number of stumps
S/S = average shoots per stump
RS/S = average suckers per stump
H. DS = average height of dominant shoots in cm
H. DRS = average height of dominant suckers in cm
Percentage SS = percentage of shoots sprouted
Percentage SRS = percentage of suckers sprouted
* = represented by more than 5 stumps

Source: Cuny, 1993-a

Natural layering occurs more often in gallery forests where loam can partially cover a shoot or a trailing branch (layering). However this cannot be applied on a large scale.

Kenyan farmers, who grow yam and passion fruit have opted for the Commiphora zimmermannii as the prop. A simple green stick planted in the ground rapidly takes root (Getahun and Njenga, 1990). This capacity has not been fully explored in the dry tropical zones until quite recently, but it could be researched further. The ‘large cuttings’ technique used in Costa Rica is discussed in Box 6.

Without going further into the area of cuttings in an artificial environment, we would merely mention that experiments have been carried out in Mali with the application of root growth hormones and protection under polythene sheets for moisture control. This has produced 65 percent rooting rates with Anogeissus leiocarpus (Anderson, 1994).

Further research is still needed to fully understand the vegetative multiplication capacities of species in dry tropical zones. It is also essential to know the potential of the stumps after several cuttings, to see whether repeated harvesting weakens them and then make reliable assumptions regarding the proportion of both proventitious and adventitious shoots (the latter deplete the stump, because they do not form their own rooting system). This is an important aspect on which the existence of the present management methods depend, largely based as they are on the simple coppice and selective coppice methods.

Box 6: Long cuttings In Costa Rica, at Chorotega, the peasants use a traditional technique which they call ‘long cuttings’. In March, two months before sowing, the farmers look for well-developed trees, with strong vertical shoots supported by side branches. They choose shoots that are about three years old and 15 cm in diameter, which they cut at the base just at the junction with the ‘parent’ branch (2.5 m in length) and lay them on the ground in the shade of the tree, for one week. They then set them up vertically against the base of the tree for three weeks, with the large end on the ground. They plant them in April, four weeks after cutting them, by burying them at a depth of 50 cm. These ‘vertical’ branches develop rapidly to become adult trees. They are not normal branches but shoots from adventitious buds which possess all the morphogenetic potential of the species. Thus, for example, eight years after plantation, a large Bombacopsis quinata shoot can produce a tree 20 m high, with a diameter of 55 cm, at breast height. The simplicity of this method, its low cost and its exceptionally high success rate, suggest that it offers enormous possibilities for application. Leaving the shoots for a week on the ground certainly enables the cut to heal at each end, and setting them up vertically for a further three weeks enables the mineral elements and the hormones to concentrate at the lower end. Some farmers have tried to plant the shoots immediately after cutting them, but they have always dried up, losing the bark and dying. Source: Jolin and Torquebiau (1992)

4. Species plasticity

In certain forest management plans, provision is made for local enrichment areas or small plantations. For this purpose, local or exotic species can be used. Domestication, introduction and genetic improvement programmes are for only a scarce number of species at the present time (in the tropical dry zones these include certain acacias, Azadirachta indica, Parkia biglobosa, Prosopis juliflora, etc.).

Even though indigenous species were introduced into the trials as long ago as the 1960s, it may be said that up to a recent date ~ (1980), the focus has mainly been on the silviculture and improvement of eucalyptus, and it has only been in the past 15 years that studies on woody plants in dry tropical zones have been developed.

In West Africa, many introduction tests and provenance trials have been set up in a number of countries with fairly similar ecological environments (Senegal, Mail, Niger, northern Cameroon, etc.). So far, no summary document has been produced summing up all the knowledge that has been learnt in the dry zones with a comparable ecology. Such a report would be very beneficial for redirecting ongoing research. In some cases this research is carried out at the national and local level. In Senegal, for example, behavioural trials have been carried out over the last ten years (ISRA-DRPF, 1992) and have revealed the following exotic species (with sufficiently substantial feedback to be able to carry out new trials without too many risks): Acacia bivenosa, A. sclerosperma (which are good soil fixing agents), A. hilliana (used along the coastal belt), Caesalpinia ferrea, Lysiphillum gilvum and Hardwickia binata (fodder crops of Brazilian, Australian and Indian origin, respectively), and Ziziphus joazeiro (a fruit tree that can be used for quick hedges, coming from north-eastern Brazil).

The IRBET (Burkina Faso) has produced results and opened up avenues of research for several indigenous or introduced species: Faidherbia albida, Acacia nilotica, Adansonia digitata, Ziziphus mauritiana, Tamarindus indica, Sclerocarya birrea, Anogeissus leiocarpus, Khaya senegalensis, Prosopis juliflora, P. pallida, P. cineraria, Acacia aneura, A. holosericea and A. cowleana (Billand and Diallo, 1991).

In Zimbabwe, Kenya and many other countries in East Africa, similar results are found. Not having a regional overview covering several countries, the following should be noted:

- a list of tree species deserving top priority at the global, regional and/or national level was updated at the last session of the FAO Panel of Experts on Forest Genetic Resources (1993); and

- at the regional colloquium on the In situ Conservation of Genetic Resources of Wood Species in the Arid and Semi-arid Zones held at Ouagadougou, from 31 January to 4.February 1994, 11 priority species (limited to this figure because of a lack of supplementary funding) were indicated: Parkia biglobosa, Butyrospermum paradoxum, Acacia senegal, Faidherbia albida, Acacia nilotica, Anogeissus leiocarpus, Pterocarpus erinaceus, Adansonia digitata, Dalbergia melanoxylon, Borassus aethiopum and Khaya senegalensis.

5. Biological diversity

Deforestation has been variable in time and in space, and over the last 50 years it has increased. In some regions, as a result of climatic variations, floristic changes may have also occurred. As harvesting has been intensified, some species have been threatened. For all these reasons, plant genetic resources are becoming depleted by genetic erosion and by the extinction of certain species. The flora and fauna alike are under threat.

The present network of protected areas in dry tropical zones is not sufficient alone to maintain the diversity of genetic resources in both geographical and biological terms.

It is imperative that in situ conservation actions be coupled with the many development projects which are currently being implemented throughout the world. It is important to harmonize conservation and management. In the areas that are fully protected, the ecosystem will eventually evolve towards its climax. Elsewhere, in the managed stands, it is not always possible to reconcile the demands of economic development and the maintenance of the integral tropical woody flora whose main feature is its great diversity. In these forests, the frequency of species that are little used by the local people could gradually be reduced: “Where market demand is very selective, extraction may concentrate exclusively on the best individual trees (phenotypes). In the absence of subsequent silvicultural treatment to favour regeneration and growth of desirable species and individuals, this may lead to progressive deterioration of the stand’s genetic quality. Nevertheless, with adequate control based on sufficient understanding of the ecological processes involved, logging and timber extraction as part of overall management plans can be used to assist the conservation of a wide spectrum of the principal tree species’ genetic resources.” (Kemp, 1992)

These activities to protect the flora and fauna, planned at the national level, must be tied in with regional and global plans if they are to be really effective in time (Box 7). For it is indispensable for both forest and silvo-pastoralist managers to take account of the genetic conservation of an increasingly threatened world heritage, particularly because the whole purpose of conservation is not to keep forests stationary, but to maintain an ecosystem which is constantly evolving.

The lack of information on the dynamics of mixed woody and grass formations is obvious to all. Because of our fragmentary knowledge, we are bound to act with prudence and to maintain a broad genetic base. The limits of the geographical and ecological area of most species is still poorly understood, except in the case of the few species of outstanding economic value. While the general area of distribution is unknown, even less is known about the specific area of ecological niches (which are generally genetically distinct) and risks disappearing altogether. Research on provenances and iso-enzymatic studies are therefore extremely important.

Setting out very clearly the levels and the objectives of phytogenetic resource conservation is a vital first step. Maintaining diversity at every ecosystemic, specific and genetic level, which are all closely interdependent, must very rapidly become the main concern of national political and scientific heads of research or development. Box 8 indicates the five elementary phases of in situ conservation. According to Kemp (1992), “genetic resources are associated with the various levels of diversity existing in nature, from ecosystems, to species, populations, individuals and genes. These various levels interact closely and all must be taken into account when defining conservation objectives and corresponding actions to be undertaken.”

Box 7: International programmes in genetic resource conservation A number of international groups and panels presently exist to pinpoint priorities in genetic resource conservation, draw the attention of decision-makers to urgent needs, and catalyse at national and international levels. IUCN’s Conservation Monitoring Centre gathers data on endangered plant and animal species and maintains the United Nations list of National Parks and Protected Areas; The UNESCO’s Scientific Advisory Panel on Biosphere Reserves; and the MAB General Scientific Advisory Panel, examine the world-wide coverage of representative samples of ecosystems in biosphere reserves and make recommendations on complementary action needed; The FAO Panel of Experts on Forest Genetic Resources takes stock of the situation in forest genetic resources and advises FAO on priorities for action, by region and species; The International Board for Plant Genetic Resources, examines the state of conservation with special reference to important food crops and their wild relatives, supporting also activities in the conservation of some fruit, vegetable and forage species; The UNEP, providing support to field activities in conservation projects ranging from microbes to crops and woody species, works in close collaboration with these international organizations. The above, international agencies work together under the umbrella of the Ecosystem Conservation Group and its ad hoc Working Group on in situ Conservation of Plant Genetic Resources. This latter’s terms of reference are: review ongoing and planned activities in in situ conservation in the light of recommendations of the FAO Commission on Plant Genetic Resources; UNESCO’s Action Plan for Biosphere Reserves; the IUCN Bali Plan of Action; the Tropical Forestry Action Plan; and other relevant documents and bodies; identify ways and means to strengthen action and co-operation in response to these recommendations, with particular reference to improving information flow and promoting pilot demonstration activities; advise on ways and means to link in situ conservation with other elements of socio-economic development, including areas such as ex situ conservation; resource economics; and watershed management. The Working Group recognized four major goals for co-ordinated action, of which, the following: raise awareness of the importance of in situ conservation; promote research (including inventories) on wild plants and their beneficial microbes, and natural enemies of their pests and pathogens, and their in situ conservation; organize and provide an information service; promote international co-operation. Source: FAO, 1989-b

Box 8: Operational phases in in situ conservation: The five steps I. Formulating objectives. Conservation activities must be closely linked with overall national development objectives and human well-being. The first important step in in situ conservation is to clarify objectives and needs, and to clearly define the targets of conservation (...). The clarification of needs and formulation of specific objectives should ideally be based on exploration of identified gene-pools of priority species through field surveys including biological, environmental and ecological aspects (...). There is an urgent need in many countries for national surveys, inventories and plant collections aimed at identifying important plant communities and ecologically, economically and socially important species; and at assessing their conservation status. However, in most countries enough is already known about endangered habitats and species to initiate action and priority programmes for conservation. Considering available resources in manpower and funds, there is a need for selection or choice of species to be singled out for immediate conservation. II. Selection and design of conservation areas. There can be no general rule about the optimal number, distribution, size and design of protected areas (...). A three-step process has been suggested for estimating minimum size of a conservation area: identify target or keystone species whose disappearance would significantly decrease the value or species diversity of the area; determine the minimum number of individuals in a population needed to guarantee a high probability of survival and maintenance of genetic variation in these species; using known densities, estimate the area needed to sustain minimum number. As to the minimum size of a viable population of interbreeding individuals, general estimates range from 50 non-related, interbreeding individuals for a short-term conservation, to 500 individuals for a long-term adaptation and survival. III. Harmonizing conservation with human needs. The primary focus of planning should be to reconcile conservation activities with immediate human needs. This requires the involvement and participation of local people in conservation (...). In the longer term, conservation of plant genetic resources will depend upon reshaping political and economic policies affecting their utilization, and on human employment and population growth. IV. Management. Genetic resource management can only be effective if it is treated as an integral part of land use management as a whole (...). In conserving intra-specific variation, relatively intensive management in favour of target species will often be necessary. The type and intensity of intervention will depend mainly on the complexity of the ecosystem in which the target species occurs; interrelationships between component species; and size and distribution of the conservation areas (...). In biologically more complex systems like tropical lowland rainforest, where several hundred little known species can be found within relatively small areas, management for conservation of intra-specific variation is a more delicate matter (...). To favour one species on behalf of another could be a mistake (...). Setting of priorities can only be improved by increased knowledge of the species available in nature. V. Monitoring and evaluation. Conservation activities must be continuously monitored in view of objectives set and achievements in meeting specified objectives and goals must be evaluated, so that action can be adjusted, should the need arise. Source: FAO, Plant Genetic Resources - their conservation in situ for human use, 1989-b

6. Maintaining soil fertility

Nitrogen-fixing trees (NFT) have the capacity to fix atmospheric nitrogen gas through a process of symbiotic association at the level of root nodules. These species not only meet their own nitrogen requirements but partially those of the non-NFT, crops and associated grass cover, particularly in nitrogen-deficient soils, without polluting, at a low cost, and in a sustainable manner. They have the advantage of providing protein-rich fodder. In some countries they also provide protein for human consumption: Prosopis juliflora in Peru, Acacia coriacea in Australia. Their potential is considerable and one can well understand the enthusiasm raised by the progress made over the past 15 years in becoming more thoroughly acquainted with the nitrogen-fixing process.

The main area benefiting from basic and applied research into these trees is agro-forestry, with numerous applications in terms of associated crops, wind-breaks and live hedges. Forest and silvo-pastoral managers can exploit the fall-out from this research regarding the improvement of abandoned fallow lands in forests, the enrichment of forests or range lands which are overexploited by livestock, and alley cropping for fodder production (reserves). For the forest manager or the silvo-pastoralist, the same problem arises regarding the choice of species to be given priority in terms of pre-established objectives negotiated with the neighbouring populations. This choice must take account of the local socio-economic and ecological criteria, and in particular the capacity to fix atmospheric nitrogen (which partly offsets the loss of nitrogen due to felling and partially enriches the rhizosphere of the neighbouring non-NFT).

The use of NFTs (Box 9) has produced spectacular results in some cases (for example, dune-fixation in Senegal and China, wind-break plantations in Egypt). But there have also been failures, sometimes due to soil depletion. These contradictory results can very largely be explained by the fact that, very often, the full potential of the NFTs has not been exploited.

Box 9: Main nitrogen-fixing trees used in the dry tropical region 1. Legumes Acacia ampliceps, A. aneura, A. bivenosa, A. caven, A. colei, A. coriacea, A. farnesiana, A. of the complex holosericea/neurocarpa, A. karoo, A. laeta, A. ligulata, A. nilotica (= A. arabica) A. salicina, A. senegal, A. seyal, A. sieberiana, A. tortilis (= A. raddiana), A. trachycarpa, A. tumida, A. of the complex victoriae Aeschynomene elaphroxylon, Albizia lebbeck Cajanus cajan, Colutea arborescens, C. istria, Cordeauxia edulis Dalbergia sissoo, D. melanoxylon Faidherbia albida, Gliricidia sepium Prosopis of the complex juliflora, P. africana, P. alba, P. chilensis, P. cineraria, P. pallida 2. Actinorhizanthous plants Casuarina cristata, C. cunninghamiana, C. equisetifolia, C. glauca, C. obesa Gymnostoma decaisneana