1. Climatic factors
2. Biological and physical factors
3. Anthropogenic factors
The different types of vegetation that one finds in dry zones are the result of evolutionary changes which may be recent or date back into the past. They are maintained or continue to be transformed depending upon whether the system of climatic, physical, biological and human factors that determine their present state keep them stable or hasten their transformation.
It is important for the forester to be able to see the environment he has to manage in terms of this global dynamic and properly evaluate the main determining factors. This can be illustrated with a few simple examples. The forest manager will not adopt the same scenario to apply to a forest which is being degraded by a long drought and to one in which the environment is being reconstituted by reducing overgrazing, even if both ecosystems are comparable at the present time. In the same way, he must take account of the capacity of an environment to regenerate itself if he is to properly manage the duration and intensity of a fallow system.
To do this he must take a long-term perspective and thus take account of the factors which condition global transformations. Using as an example a forest subjected to a long degradation process, in one case it might lead to the constitution of a forest park (whose productivity is by no means negligible, and whose role in fertility maintenance is important), but in other cases it could be converted into land without any tree cover and with fragile soils.
Many other examples could obviously be offered to corroborate this evolutionary approach, which requires viewing forests not only in terms of a descriptive snapshot, which is essential but insufficient, but also by seeing them in terms of the history, their determining features and their potential futures.
It has been chosen to do this in a summary manner, by exploring the impact of the main climatic, biological and human factors, realizing in each case that the time scales and the spatial scales referred to are not the same, and bearing in mind that today it is the human factor which is proving decisive.
The repeated droughts in the 1970s and 1980s, particularly in Africa, have had serious repercussions. The direct effects of drought on forest systems increased mortality, made it difficult for plants to regenerate, destroyed the grass cover which is very sensitive to the lack of water, and increased the pressure (topping) on the surviving trees. This has led to a decline in production.
The indirect effects have been much more damaging to the environment:
- reduced yields have led farmers to practise extensive production methods, put into place by clearing the forests;Some plants adapt to climatic variation. In the Sahel, for example, the Acacia senegal has remarkable resistance capacity (Poupon, 1979). After many years of a rainfall deficit, Acacia senegal stands have regenerated to a remarkable degree.- as a result of the disappearance of the grass cover and water points, increasing migration and livestock mortality problems have caused pastoralists and livestock breeders to increase fodder extraction from the forests in order to feed their animals; and
- dried-out vegetation has caused more violent bush fires than usual.
Initially there is a climatic evolution which makes the forest and agricultural ecosystems fragile, followed by a serious acceleration of the degradation process caused by damage and disturbance of an anthropogenic origin. In other situations, one would imagine the possibility for forest management, coupled with appropriate water management, to regulate the erratic rainfall variations.
Brazil’s North-east region has always been affected by drought. Since the beginning of the century, there have been 27 dry years, averaging one year every three-to-four years. The notion of drought is complex. It is caused by deficits in annual rainfall or by poor rainfall distribution, and in more critical cases by both. The rainy season is in fact a season during which it might rain. It can last more than six months, while the total actual duration of rain episodes does not exceed 20 days in certain rain-deficit years. In addition to that there is a great variability in the spatial distribution of rainfall.
The annual rainfall does therefore not reflect properly the specific drought phenomenon of North-east Brazil. In one year there may be overall satisfactory rainfall, interrupted by dry periods, so that despite several attempts at sowing none of the crop cycles reach their full term, while the natural vegetation which is less demanding remains green. This is what is known as seca verde or green drought (Lebrun et al., 1995).
In the zones where annual rainfall is below 800 mm, it is not unusual to get 50 percent of the total annual rainfall of a given observation post, in one week, and 90 percent in one month. A rain-deficit year with poor rainfall distribution, may be called dry from the agricultural point of view, but yet be a good year for replenishing the reservoirs. There are therefore different approaches to characterizing the gravity of a dry period. Brazil’s North-east region is covered with a long-established and fairly dense network of 300 rainfall monitoring stations more than 70 years old.
The regions that are most seriously affected by drought are in the north, particularly the whole of Ceará, the Piauí-Ceará borderline, the Rio Grande do Norte, Paraiba and the interior of Pernambouc. The problems of Brazil North-east region are therefore more due to the spatial and temporal irregularity of the rain rather than total annual rainfall itself, and these problems must be solved by the rational use and conservation of this water.
The low flow rate is clear from the following observations: the rivers are always intermittent, and only 2-15 percent of the volume of the rainfall runs into them. The rest of the water is lost by evaporation. The water flow occurs in only a few days. The Rio Jaguaribe, in the State of Ceará with a 90.000 km2 watershed, is the largest intermittent river in the world. In dry years, the runoff is even more concentrated. In 1976, for example, 94 percent of the volume of the water ran off in only five days (Leprun et al., 1995).
The evolution of the ecosystems will be examined here very briefly, in order to emphasize the importance of certain positive and negative factors in the evolution of tree stands.
On the origin of the savannas, Schnell (1971) proposes three scenarios which are neither exclusive nor exhaustive, but which can be used as useful references:
- Natural in origin. This approach views the savannas (mainly grass-savannas) as having arisen in environments which were unable to carry abundant forest vegetation because of the poverty of the soils or adverse climates in former times. According to this approach, the savanna is the expression of an impoverished, constrained environment. However, considering such different situations as the development of savannas in the humid zones, the flora and fauna in certain savannas, the pedological structures and the entrenched existence of bush fires, the idea of a natural savanna, at least in some cases, gives way to a relic-based theory of the savanna.The two latter hypotheses are compatible and can also explain the present development of these environments. However one must avoid facile generalizations. The origin of the savannas is complex. It is highly likely that each interpretation can explain one particular case. Schnell has emphasized that: “the problem of the savannas appears to be quite diverse, depending on the conditions of the climate and the soil. It is not a matter of a savanna problem, but of savanna problems”. In most of the literature today it is admitted that the savannas have been kept in their present state as a result of fires, and that they are therefore not climax formations.- Relic in origin. This idea is based upon indications that suggest that the savannas have existed continuously since ancient times. They would have emerged in a drier period and have been maintained since by the action of fire.
- Derived origin. With this evolutionary approach, it seems possible that savannas could have a twin origin, either they derive from former closed hygrophilous formations replaced by less water-seeking species (derived savanna) or they have come from dry arborescent formations (deciduous closed woodlands, for example) in which case it can be referred to locally differentiated savannas. This type of derived savanna, mainly anthropogenic in origin, is not easy to prove, and there is little evidence to confirm it.
In Brazil, Aubréville (1961) has shown in a study on the campos cerrados that they derive from the cerradoes, which are quite dense forest formations. The campos cerrados, according to Aubréville, are open formations that have been disturbed by human exploitation, grazing and, above all, fires. In reality, fires do not occur in overly closed cerradoes, but following overharvesting, bush fires may have been able to penetrate into the forests and turn them into campos cerrados. There is also a pooling of anthropogenic phenomena, while the stands are maintained by fires.
In Mali, Nasi and Sabatier (1988) have proposed an explanation which indicates possible relationships between the different types of formations (Figure 4). There is no evolution towards dry deciduous forests, at least in the foreseeable future. The balance in the absence of forest fires is therefore found in the open woodlands.
In the Côte d’Ivoire, 65 years after the ‘fire’ trial set up by Aubréville, one can see that fire plays a major part in creating and maintaining the savanna (Louppe et al., 1995).
The importance of fires in the maintenance and evolution of these environments is a reality, and is confirmed in many situations. In East Africa, some botanists do not consider the miombo forest as climax vegetation, but rather as a ‘pyro-climax’. The climax is thought to be the dry deciduous woodland called muhulu, replaced by the miombo forest, and at the end of the regressive series by a deteriorated savanna. Table 3 gives a brief account of the stages of the recolonizing of shifting cultivation crops by the miombo forest in Zambia.
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Box 3: Miombo regeneration dynamics Lawton (1972) has summarized as follows the evolving dynamic of these open woodlands. After an area has been cleared for cultivation, the substitution species which establish themselves are Diplorynchus condylocarpon, Hymenocardia acida, Pericopsis angolensis, Pterocarpus angolensis, Syzygium guineense subsp. Macrocarpum and Vitax doniana. The two last ones form patches of cover of 4 m high which partially inhibit tall grass and the ‘eagle fern’. Under such conditions, Uapaca species (mainly U. benguelensis, U. kirkiana, U. nitida and U. sansibarica) establish themselves spontaneously. When the canopy reaches 4 to 12 m in height, the herbaceous layer has become a low and open grass cover, largely topped by leaf litter. Under these conditions, fires creep along the ground and such species as Brachystegia-Julbernardia and Marquesia macroura are able to regenerate. They will in turn, themselves top and shade the Uapaca, even if some Uapaca trees are able to survive. The first group of pioneer species becomes a coppice whose growth shall only resume after the crown cover becomes open again. Source: Lawton, 1972 |
Figure 4: Possible relationship between different vegetation stands depending upon various factors
Another example of the influence of external factors on the development of forest systems is given below to explain the dynamic of striped bush (brousses tigrées). These xerophytic and herbaceous shrub stands cover most of the Sahelo-Saharan and Sahelian zones of West and East Africa, but they are also found in Australia and Central America (Leprun and Da Silveira, 1992). Box 4 indicates the main striped bush features.
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Box 4: Features of striped dynamic bush A detailed and multidisciplinary study of different sequences of striped bush in the Saharo-Sahelian and Sahelian zones of Mali and Burkina Faso has produced the following results: All these plant stands comprise three different alternating strips (a grassy sand micro-dune, a bare clayey strip, a sandy-clayey closed wooded depression. The formation, persistence and evolution of striped bush depend on several factors including: - annual rainfall of over 200 mm and below 500 mm; The whole of the stand is highly dynamic. Under the effect of storm water the fine material of the bare layer is eroded and deposited in the runoff water from the wooded strip. The accumulation of water at this level makes it possible for the species on this strip to germinate and propagate on that layer. During the following dry season, the sand on the surface of the layer is carried away by the wind and is deposited on the grassy micro-dune which also shifts in the direction of the wind: - The speed of this simultaneous shift of the soils and vegetation varies 20-70 cm per year. This dynamic, which takes place rapidly in the north, slows down towards the south and stops when the conditions no longer subsist, particularly the edaphic conditions. Source: Leprun and Da Silveira, 1992 |
Peltier et al., (1994) provide an overview of the ecology (Figure 5) of one particular type - the striped bush in southern Niger. Pastures are an essential component in the balance of these lands. By preventing the development of grasses, grazing facilitates water seepage for the benefit of the trees.
Figure 5: The dynamics and hydrology of the striped bush
Source: Peltier et al., 1994, according to Herbes and Seghieri“The striped bush develops on the slightly sloping plateaux (0.5-2 percent). It is characterized by an alternation of bare parts on which the vast majority of the water runoff occurs and areas covered by vegetation on which the amount of water seepage exceeds the rainfall. The water seepage at the level of the vegetation strip is due to the ‘combing’ effect of the plant remnants and the grassy or woody layers which slow down the runoff rate, as well as to intense biological activity which creates a high level of macro-porosity.
“Water seepage is not evenly spread throughout the area under vegetation cover: it increases gradually from their upstream limits and reaches its maximum towards the centre, becoming zero downstream. On a balanced scrub-land, practically all the rain-water is used by the vegetation and there is very little water left to run off downstream of the plateaux. Since there is always a decantation zone upstream of the vegetation on the part that is still bare, this is colonized during the rainy seasons by a pioneer grass (Microchloa indica). The following year, if this herbaceous fringe has not been completely destroyed by livestock, the colonization process continues again by several dozen centimetres. Various species and a few trees (particularly Guiera senegalensis) develop on the Microchloa indica grass cover of year 1, whose seeds have been able to take root in the dry straw.
“During the next two or three years, other woody plants such as Combretum micranthum appear. When the pioneer front has moved several metres upwards, and the area in question is at the place where water seepage is at its maximum, more demanding species may develop such as Combretum nigricans or Combretum glutinosum, or even shade-demanding species such as Gardenia sokotensis. After several dozen years, these trees have developed as have their water requirements, but then they find themselves below the vegetation-covered area. The depth of the moisture level and the amount of water seepage has now fallen and they begin to die. Because of this gradual progression upwards and on the hillsides, the vegetation zone becomes crescent-shaped, at least when the land slopes gently... Generally speaking the crescents roughly line up, even though this type of scrub-land seen from the air shows an alternation between undulating strips covered with vegetation and bare strips, which looks like the stripes on a tiger, hence one of its names (‘tiger bush’).”
In this way the different stands in the dry zones (forests, savanna, steppes) are closely linked to one another because of climate variations, fires, herbivorous animals and the activities of man.
The degradation of the environment and deforestation are largely due to a change in human behaviour. Today, the reasons underlying these changes are mainly associated with demographic growth, progress and sometimes regression in technologies, and the implementation of inappropriate agricultural and forestry policies.
These factors compound their effects and interact negatively with other degradation factors too often. This leads to the overharvesting of forest resources, overgrazing, clearing for agricultural purposes, implementing ineffective or even damaging policies and regulations, and failure to control fires.
The population in these areas doubles every 20-25 years. With a population undergoing all-out demographic growth there will be an ever-increasing demand for resources. Household structures and prudent resource-management practices have been put severely to the test, particularly in Africa and India.
Forced migration (wars, population shifts, droughts) are forcing the people to settle in areas which are often alien to them. They bring and use their own agricultural and cropping practices, which are often more destructive than those of the local people. In Burkina Faso, for example, migrants from the north of the country due to population pressure, soil impoverishment and drought have settled in the Nazinon Nature Reserve, and have already cleared 8.000 ha for their settlement (Soto Flandez and Dilema, 1990).
In dry areas, cropping success depends upon the availability of a substantial labour force that can be rapidly mobilized for a very short period that coincides with favourable sowing and cultivation conditions. This human labour force investment remains often on the short-term, the only possibility to ensure increase in agricultural production. As long as noticeable improvements in techniques and in plant material do not revolutionize the agricultural world of most of the dry zones (particularly of Africa), no change in behaviour will take place. The too general explanations given over African fecundity “are found to be inoperative. There are African societies and African ‘fecundity patterns’ ..., the trend in all developing countries shows clearly that there is more attention given to mortality reduction than to fecundity limitation.” (Locoh, 1995)
Many forecasts based on summarized indicators at national level have contradicted the facts. However, fecundity reduction is presently taking place in Botswana, Kenya, Zimbabwe, and most probably in South Africa and Nigeria, following extensive women’s educational programmes and birth control campaigns.
Forest land clearing for agriculture is one of the first and main causes for deforestation. Beyond the demographic situation, this process is often the result of choices or ‘non-choices’ made in terms of agricultural policies (extensive agriculture, industrial agriculture). Adding up to this immediate and permanent transformation is a progressive conversion taking place, due to shortening of forest fallow periods. Soil resting periods becoming shorter, fertility maintenance is no longer assured. Deforestation is also a consequence of clearing for livestock husbandry. This phenomenon is particularly important in South America.
Overgrazing constitutes also a factor of forest deterioration in Africa and in India. Forests are one of the rare places where livestock feed is available in the dry season. Woody vegetation is then browsed and stripped of its bark, and the dried up soils are compacted, condemning regeneration establishment. Fires used in particular by livestock breeders to assist grass regrowth, are a hindrance to woody vegetation productivity. Finally, excessive trimming out of branches in the dry season, depletes the trees and boosts fire propagation.
However, the social balance of villages is based on complementary association of agriculture, livestock husbandry and forest resources extraction. As long as the animal stocking density is not in excess of the existing ecological limitations, grazing constitutes an essential part of the dynamics of ecosystems. That is why projects in Africa are presently stressing the necessity for a negotiated management of natural renewable resources. Managing sylvo-pastoral resources must in most cases, become a substitute to forest management, because if grazing is kept at a moderate level, it constitutes a favourable element in maintaining the overall balance and in achieving economic progress.
Lastly, we must take notice of the fact that forests are often seen as stocking sites for forest products that have no owners and whose extraction is free. Trees are devastated without any consideration for the long-term consequences. It is therefore necessary to undertake a reappropriation of the forest space. To achieve this, the control of the whole of social, economic and real estate aspects which are expressed in forest, agriculture and land use policies, is essential.
