Fire and grazing influenced types of forest degradation in the Mediterranean region after Le Houerou 1992

Many studies have been made of the sequence of woodland degradation by regular burning and/or grazing in the Mediterranean region. Braun-Blanquet (1928) developed the first scheme. Le Houerou has distinguished between two series, one in the semihumid southern European Mediterranean, called the "Quercus ilex series", and another for the semi-arid north African Mediterranean region called the "Pinus halepensis series".

Quercus rotundifolia (=Qu. ilex) oak forest when burnt is transformed into cork oak (Quercus suber) dominated forest, but only on silicate soils. With further burning numerous, mainly evergreen shrubs form the vegetation (e.g. Erica arborea, Calycotome villosa, Myrtus communis) followed by rock roses (Cistus crispus, C. ladaniferus, C. monspeliensis). The final stage of this fire-determined development is a scrub vegetation with dwarf shrubs and geophytes (e.g. Helianthemum guttatum, H. tuberaria, Urginea marittima).

The sequence in dryer and more heavily grazed regions of the north African Mediterranean starts with Aleppo pine forest (with Pinus halepensis, Juniperus phoenicea) which is degraded by fire and grazing initially into a "garrigue" with rock roses and dwarf shrubs (Rosmarinus officinalis, Lavandula stoechas, Cistus libanotis). Finally, grasses of the dry steppes (Stipa tenacissima, Lygeum spartum) become dominant. The final stage of the degradation is pasture with weeds (e.g. Pega-mum harmala, Noaea mucronata).

cies for individual altitudinal zones used presently for reforestation.

Where the reforestation was successful, "forest" areas were increased, but also caused uniformity. The hoped-for economic yields have not always materialised, and no positive effects on water relations and formation of soil have so far been found. The negative side effects on the fauna and organic substances in the soil, attacks by pests and fires, etc., known from monocultures, are deplorable. With the rich Mediterranean variety of different woody species, the preferential use of exotic species would not have been required. One could have limited those for protective reforestation, e.g. on areas where a rapid protection from further erosion was required. In the long term economic success could have been sought in indigenous species (production reforestation).

| Fig. 4.1.24. Typical types of agricultural land use in countries around the Mediterranean. A Subsistence farming on a small area with mixed crops and many cultivated plant species (Central Atlas, Morocco). B Grain crops and fodder plants for the national market on large-scale farms with high input of agrochemicals and machinery (Tuscan Hills, Italy). C Mixed agroforestry with grain crops for the growers consumption, and cork (Quercus suber) for the national market (Extremadura, Spain). D Unregulated, excessive grazing with severely degraded forests and soil erosion (Central Atlas, Morocco). (Photos K. Müller-Hohenstein)

| Fig. 4.1.24. Typical types of agricultural land use in countries around the Mediterranean. A Subsistence farming on a small area with mixed crops and many cultivated plant species (Central Atlas, Morocco). B Grain crops and fodder plants for the national market on large-scale farms with high input of agrochemicals and machinery (Tuscan Hills, Italy). C Mixed agroforestry with grain crops for the growers consumption, and cork (Quercus suber) for the national market (Extremadura, Spain). D Unregulated, excessive grazing with severely degraded forests and soil erosion (Central Atlas, Morocco). (Photos K. Müller-Hohenstein)

In south European countries, the pressure on using wood from forests is also declining and recovery in the sclerophyllic and deciduous species can be observed. In north African countries, however, the pressure on the last near-natural remnants of forests is growing. It is feared that shortly the last cedar will be cut, and only sparse woods with particularly drought-resistant species, such as Pinus halepensis and Tetraclinis articulata, will remain locally.

Also, in the European/North African-Mediterranean region, it is obvious that subsistence-oriented land management in small family units coincides with a greater diversity of landscape and vegetation. Modern market-oriented management is always linked with a loss of diversity at all levels. In the recent past an agro-sylvo-pastoral balance was recommended; however, without developing precise ideas of it. Typical forms of the Mediterranean land use are shown in Fig. 4.1.24 A-D.

Desertification on the Edges of the Sahara The term desertification, coined by Aubréville (1949), has been used at least since the many years of drought in the Sahel at the end of the 1970s to describe environmental problems in dry areas. However, it is not always used with the same meaning. Today it is obvious that this term not only describes climatic stress in long-lasting drought periods, but also complex interactions which include particularly human interventions on vegetation and landscape. In the context of ecosystems in dry areas, and of the often-quoted man-made deserts, it should be stressed that, although often described as unusual, extended dry periods are a characteristic feature of dry (desert) areas. It is not only the small amount of available water after precipitation together with high temperatures, but also the temporal, episodic and spatial distribution of this precipitation which cannot be anticipated.

In the Saharan region dry periods lasting several years have always occurred, as shown by reports in historical sources, and the interpretation of old lake sediments (Nicholson 1978). The recent dry period in the Sahel had such catastrophic consequences because it was preceded by a relatively moist period, and the population (which had meanwhile grown considerably) was not prepared for this in its land use. Better medical supplies and technical innovations (deep wells to tap fossil water) improved grazing conditions and the number of animals increased continuously (Miil-ler-Hohenstein 1993). Recent meteorological data indicate large regional differences in the amount of precipitation, but a general trend of increasing drought is hardly visible.

It is incorrect to regard, in this context, ecosystems of dry areas as particularly labile systems. These ecosystems are adapted to extreme climatic variability. Autochthonous plants and animals are able to adapt in many ways, and thus are able to survive under such conditions. The human population in dry areas also knows how to maintain supplies and is prepared for variations in yield (Ibrahim 1988).

Figure 4.1.25 shows a "schematic of desertification" with the most important causes and the consequent land degradation, in dry regions. It is important to consider the discrepancy between growth of population and - despite all technological progress - the limited availability of renewable resources as well as the substitution of traditional forms of management (nomadic life style) by modern forms of grazing and agriculture on dry fields. In the last 100 years, the human population around the Sahara has grown fivefold (Goudie 1994). Today examples of poorly adapted agriculture can be found in all areas near deserts, e.g. agriculture in northeastern Syria or eastern Jordan, with precipitation below 200 mm/year, or growing animal fodder using fossil water from more than 1000 m deep in Algerian oases, or supplying drinking water for animals in the Sahelian Ferlo, where vegetation thus degrades even further.

Human influence on the vegetation in dry areas is not so much linked to the introduction of species foreign to the flora of the region, more to the disturbance of sites and the original plant cover. Thus woody plants have disappeared in many regions because of the increased requirement for energy. The naturally sparse, so-called contracted vegetation (Walter 1973) is damaged by overgrazing, shown by the decrease in cover and primary production, and a shift of the floristic constitution. Good fodder plants become locally extinct in dry areas and toxic or thorny species invade. Ultimately, loss of species is to be expected. Local changes resulting from the increased loss of vegetation are seen in remobilisation of dunes and increased number of dust storms. Also water relations of areas are affected by modern irrigation installations which result in salinisation of soils and an increase in halophytes (Fig. 4.1.26).

Without doubt, the recent deficiency of precipitation has worsened degradation, locally and regionally The desert area has probably grown by 15% in the last 100 years; between 1958 and 1975 the Sahara is believed to have extended 100 km to the north and to the south (Lamprey 1975). However, such growth of the desert is also questioned; Hellden (1991) interpreted satellite pictures and was able to show a close relationship between precipitation and the vegetation, but could not find evidence for expansion of desert areas. Questions remain: are the observed changes permanent degradation and do the processes shown lead to irreversible condi tions, or is regeneration possible? The latter is understood as the sum of processes in an ecosystem, by which lost elements may be regained and thus re-establish themselves equivalent to their original situation. This question cannot be answered yet, as there are many positive and negative interactions between climatic events, the development of vegetation and sites, all of which are inadequately known. However, in all dry areas of the Old World, there are examples of a rapid recovery of vegetation after precipitation. Obviously, the ability to regenerate in dry regions depends on the variability of precipitation and the linked natural stresses, and has

Human adaption forms

Diversification of land use

Ecosystem Arid-Semiarid

Pastoralism

High rainfall variability

Shifting cultivation

Farming-fallow rotation

Sylvi-agro-pastoralism

Traditional, seasonal run-off farming

Terraced cultivation

Frequent droughts Hazard of dune formation

Soil susceptibility to: Fluvial and wind erosion Formation of skeletic soils Salinity Alkalinity

Oasis cultivation

Traditional food storage Migration .

Desertification process

External impacts

Expansion of mechanised cash-crop farming at the cost of forest and grassland Overirrigation Overexploitation of water

Socio-economic deterioration

Destruction of human adaption mechanisms Drought disasters Famines, Undernourishment Spread of diseases Migration to towns Unemployment

Change of social norms and values Tribal conflicts Political unrest Wars

Overcultivation Overgrazing Deforestation Savanna and bush fires

Utter deforestation

Destruction of grass and herbaceous cover

Fluvial and wind soil erosion

Dune formation

Soil salinity

Soil alkalinity

Ecological degradation

Desertification scheme

Pig, 4.1.25. Scheme of process of desertification. (After Ibrahim 1988)

Stress from within

| Fig. 4.1.26. Desertification is obvious in areas where people apply inappropriate forms of exploitation. A A single tree (Commiphora africana) in the southern Sahel of Mauretania is witness to former removal of dry forests, which disappeared because of excessive grazing and movement of sand dunes. B Incorrect irrigation with water of high salt content and inadequate drainage has caused salinisation in oases which had functioned for centuries with traditional cultivation (Dakhla Oasis, New Valley, Egypt). (Photos K. Müller-Hohenstein)

| Fig. 4.1.26. Desertification is obvious in areas where people apply inappropriate forms of exploitation. A A single tree (Commiphora africana) in the southern Sahel of Mauretania is witness to former removal of dry forests, which disappeared because of excessive grazing and movement of sand dunes. B Incorrect irrigation with water of high salt content and inadequate drainage has caused salinisation in oases which had functioned for centuries with traditional cultivation (Dakhla Oasis, New Valley, Egypt). (Photos K. Müller-Hohenstein)

probably been underestimated. However, despite this, man may be causing the changes in dry areas, via global climate changes resulting from alterations to the albedo and the increasing dust load in the atmosphere, as well as to changes in soil moisture.

Destruction of Tropical Forests

Destruction of tropical forests has been discussed in public even more than the problem of desertification. Not the climatic but the edaphic preconditions, particularly the availability of nutrients, are the limiting factor. It has long been known that soils weather intensively and nutrients are leached out in uniformly high temperatures and with continuous moisture. Although litter production as well as decomposition is rapid reserves, e.g. in the form of humus, are available. Even fertilisation in the two-layered clay mineral soils is not sustainable, because they are weak exchangers. Weischet spoke as early as 1977 of a "disadvantaged tropics". The system can only remain functional if the short-circuited nutrient cycle is maintained, perhaps with the help of mycorrhizae or fine roots. The export of organic substances by harvesting results in far-reaching consequences. Exceptions exist only where primary cations are released because of the geological-mineralogical condi tions, e.g. in some regions on Java and Sulawesi (east Asia) where, with young, relatively nutrient-rich volcanic soils, permanent rice crops can be grown.

Clearing and land use have taken place in tropical rain forests for thousands of years, in India more than 9000 years ago, in Africa only 3000 years ago. The consequences of these early influences are considered to be small. Intervention over large areas began with colonial times. Of the ca. 1.5 million described animal and plant species of the permanently moist tropics - in total about 30-50 million (Furley 1993) - many thousands had probably become extinct before they could be scientifically classified. In contrast to temperate forests, "modern" forestry methods have very negative effects in the tropics, as perhaps only one single tree trunk is used per hectare and many neighbouring trees are destroyed. Even the mangrove stands in tropical coastlands are not protected for wood management. Sites of these unique plant communities are used in east Asia, as well as Equador, for breeding and cultivation of prawns.

Not only the management of forests leads to loss of species. Settlers followed the woodcutters and limited the development of secondary forests. Governments of several rain forest states saw the forests as a "valve" for the growing pop-

Fig. 4.1.27. Diversity of trees in undisturbed and disturbed tropical rainforests (top natural succession, bottom following increasing intensity of land use): a species richness as % of the maximum attainable species number and b evenness as % of the maximum attainable species number. (After Bruenig 1991)

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