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In dit proefschrift neem ik een aantal onderwerpen die betrekking hebben op de economische theorie van beheer van natuurlijke hulpbronnen onder de loep. Voorbeelden van natuurlijke hulpbronnen zijn visvoorraden, bossen en olie. Het onderzoeksveld is breed en biedt keuze uit een scala aan onderwerpen, ieder met zijn eigen problemen, eigenaardigheden en interessante aspecten. Aangezien de afzonderlijke hoofdstukken van dit proefschrift afgesloten worden met een concluderende en samenvattende sectie, zal ik deze samenvatting kort houden.De hoofdstukken 1 en 2 betreffen een inleidend literatuuronderzoek. In hoofdstuk 1 wordt het onderzoeksveld afgebakend en worden enkele basisbegrippen besproken. Twee centrale thema's zijn "duurzaamheid" en "efficientie". Duurzaamheid heeft betrekking op gelijke toegang tot natuurlijke hulpbronnen door verschillende generaties. Efficiëntie betekent het (intertemporeel) maximaliseren van een doelfunctie. Het maximaliseren van "de winst of "het nut voor de samenleving" zijn veel gebruikte voorbeelden van dergelijke functies. Na de Earth summit in Rio de Janeiro in 1992 zou "duurzaamheid" wellicht het belangrijkste thema van de twee dienen te zijn, maar er bestaat in de literatuur veel verschil van inzicht over de exacte interpretatie van dit begrip. De interpretatie is subjectief en onder andere afhankelijk van de inschatting van de toekomstige substitutie-mogelijkheden tussen de verschillende productiefactoren (bijvoorbeeld tussen natuurlijk en fysiek kapitaal), en van de mogelijkheden die in de toekomst door technologische vooruitgang geboden zullen worden.De interpretatie van efficiëntie is helder, en de mainstream van de neoklassieke economen is traditioneel gericht op dit thema. (Van meer recente aard zijn modellen waarin de beide thema's gecombineerd worden.) De meeste hoofdstukken in dit proefschrift gaan in de eerste plaats over het efficiënt gebruik van (vernieuwbare) natuurlijke hulpbronnen.Het exploiteren van een natuurlijke hu

This study is an effort to understand the way various categories of social actors go about their interaction with the management of forest resources in Sudan. By providing an overview and description of the motives, perceptions, and management objectives and strategies of social actors, the study tries to contribute towards better understanding of the social, economic and cultural factors influencing the resource management. The specific objectives of this study include:(1) Investigation of how social actors interact with forest resources in an era of rapidly changing conditions.(2) Get a better understanding of how changes in management objectives and strategies are influencing and are influenced by knowledge processes.(3) Analyze present forestry education's curriculum.(4) Explore possibilities of incorporating what we learn during the course of this study to propose a model for development of curriculum for forestry education in Sudan.This dissertation is written on the basis of empirical data collected from various individuals, groups and institutions involved in forest resources management in Sudan. The field work for the study was carried out mainly in the Central region of Sudan. However, the author's own experiences and those of other officials who worked in other locations, made the scope of the study not limited to geographical boundaries of the region.For the data collection, a combination of methodological instruments such as discussions, semi-structured interviews and participant observation, were used iteratively. Besides, taking a social actor as a unit of analysis, the study has used a hybrid of the systems' and the actors' perspectives as the main analytical tool.The main findings of this study indicate that:First, whereas, a forest is the unit of concern for officials, it is the tree which draws villagers' attention more than the forest as such. However, in general social actors see trees and forests as sources and signs of life. Besides, various a

Sahelian rangelands are subject to degradation caused by physical and human factors. This report presents the results of recent research in Burkina Faso on methods for regenerating such rangelands. It begins with an overview of existing regeneration methods. Quantitative ecological field work conducted at two representative sites to understand the regeneration processes, to determine the sustainability of these processes and to understand reasons for success or failure of regeneration technology is described. Water and nutrient (N,P and K) balances as well as vegetation dynamics were studied for different soil and water conservation technologies. A major conclusion is that degraded and crusted soils can only be regenerated by modifying the water balance. Under improved infiltration, subsequent biological processes appear to be able to transform degraded soils into productive soils. However, an improved availability of water often triggers an immediate shortage of nitrogen. Deep infiltration of water stimulates the growth of trees and shrubs which have a stabilizing effect on the overall regeneration process of Sahelian rangelands.

This project has covered one or several aspects of the life cycle of the main miombo tree species, namely Afzelia quanzensis ,Brachystegia microphylla, Brachystegia spiciformis ,Julbernardia globifloraand Pterocarpus angolensis . These aspects included natural and artificial regeneration, fertilization, artificial inoculation of seedlings and natural occurrence of mycorrhizae on field-grown seedlings. Mycorrhizal survey of pine seedlings in various nurseries and inoculation trials on pines with exotic and indigenous fungi had also been conducted. The ultimate goal was to contribute to the development of techniques needed for the silviculture of miombo tree species.Seeds of B. speciformis , J. globiflora and A. quanzensis germinated within 3 weeks in nature provided there was regular rain. Seeds of P. angolensis possessed dormancy. Providing water was not enough to induce germination. In the greenhouse a hot wire scarifier boosted their germination to more than 90% within less than two weeks. Outside rain had a clear positive influence on the germination of scarified surfacesown seeds in the greenhouse. Germination was always initiated by outside rain. All species tested were sensitive to fires. For A. quanzensis germination tests were carried out at 10, 15 and 25°C. Seedlings germinated only at 25°C. Germination failures were also encountered in the field in the cool highlands of Tanzania.Seedlings of the above named indigenous species were raised in the nursery or observed in the miombo woodlands. A number of facts emerged.A big share of carbon is allocated to the development of the taproot. In the nursery, pruning the taproot interfered with the natural survival strategy of seedlings and caused a sharp depression in seedling growth. This was observed on J. globiflora and P. angolensis . The initial growth rate of the miombo tree seedlings covered here is far higher than that achieved by pines. As observed in Morogoro on seedlings of A. quanzensis , more than 70 pe

Currently, rates of deforestation in the tropics are probably higher than ever before in the past. As a consequence, changes in the earth's physical and chemical environments are proceeding at unprecedented rates. Increasing atmospheric concentrations of CO 2 , N 2 O and other trace gases, caused by enhanced emissions from soils after forest clearing, show that deforestation in tropical areas is of global importance. Recent estimates suggest a net release of carbon from the world's tropics, due to deforestation, of between 0.42 and 1.60 Pg C yr -1(1 Pg = 10 15g) of which 0.1 to 0.3 Pg C yr -1are attributed to decreases in soil organic matter content. This carbon release from tropical areas is second only to the global release from the burning of fossil fuels (which is about 5.3 Pg C yr -1).The main objective of this thesis was to quantify the changes in soil organic carbon storage and the resulting release of CO 2 after the conversion of tropical rain forest to pasture on two contrasting soil types in the humid tropics of Costa Rica. To study changes in soil organic carbon storage, sites of an Andisol and an Inceptisol, cleared at different times in the past (deforestation sequences) were compared. A deforestation map, based on aerial photographs from the period 1952 - 1984, was made for a part of the Atlantic Zone of Costa Rica, providing a well documented history of forest clearing. Using GIS techniques, this deforestation map was combined with an available soil map to select the study sites. Analysis of deforestation patterns on the map demonstrated a close relation of deforestation rate with accessibility and soil quality.Soil organic matter levels are the result of complex production and decomposition processes. The input of carbon from grass plant roots into the soil was quantified, using pulse labelling with 14C. The pulse labelling experiment revealed that root dry matter production of an improved pasture like Brachiaria (12 Mg ha -1yr -1) was about twice th

IntroductionThe challenge for land use management in the nineties is to initiate a people- centered development process which creates opportunities for local people to make their own choices about which development strategy to follow. This need is felt in particular for upland areas where government initiated blue- print programmes for land use management have a record of failure. The major reason is that these programmes are often not adjusted to cope with the complexity and diversity of the uplands. Land use management encompasses both short-term and longterm benefits and is confronted with rapid changes. The differing role of private, state and communal lands in combination with a complex system of control and utilization makes land use management difficult to fully understand. This study responds with the development of an alternative strategy for planning sustainable land use programmes at local level and has the following objectives:- to develop a strategic model for people-centered planning of sustainable land use programmes;- to develop a flexible planning method that can serve as land use management tool at local level;- to test the feasibility of the strategy for the uplands of East Java, Indonesia.Strategic modelThe two major principles of the strategic model are:- Programmes are planned and implemented through a learning process in a local and organizational dimension. The local dimension includes a learning process of planning, implementing and monitoring of small scale programmes. The planning process is short, but planned interventions are regularly adjusted based on new insights and changing conditions. In the organizational dimension the learning process proceeds through three different phases of trial, development and expansion. In this dimension, the government can learn how to manage the strategy of implementing local level planning and how to change attitudes, norms and organizational competence of organizations in order to do so. This dimension

In the first publication in this series, a polycyclic forest management system was formulated, in which three silvicultural treatments (refinements) were scheduled in a cutting cycle of twenty years. This system, which is referred to as the Celos Silvicultural System, is developed further in this study.Selective logging is the first action of forest management. Felling limits for most species need to be raised from 35 cm to 50 cm dbh to secure future harvests. If carried out properly, logging does not cause unacceptable damage to the stand. The forest responds to logging with a slow recovery process and a refinement is scheduled one to two years after felling to release commercial species. This treatment consists of cutting lianas and poison-girdling trees without commercial value, which are either larger than 40 cm dbh or 20 - 40 cm dbh and within 10 m of a commercial tree. A second treatment is necessary ten years after the initial harvest and the third one is scheduled a few years before the second cut. These follow-up treatments differ from the first one in the selection of trees to be poison-girdled.

Soils of the "Tannes region" of the Sine Saloum bassin, Senegal, are characterized by great heterogenity in morphology and physical and chemical properties. Their caracteristics are linked to topographic position, material and hydrology.Their recent pedogenetical evolution is a consequence of the severe drought period which occurred all over Senegal since 1971. The drought has had dramatic social consequences but also consequences for the ecosystem.The rainfall deficit started and amplified supersalinization and acidification of the soils throughout the region.The surpersalinity, not widespread before 1971, has rapidly reached all soils, from the low terraces up to "glacis de raccordement".Three main processes enhanced the supersalinization:a. Vertical migration of salts by capillary rise from soil solution and shallow groundwaters, caused by intensive evaporation due to high tempratures (25 - 40°C), which leads to an upward salt flux from 8 to 9 months of the year.b. Inundations by supersaline water from the creeks and rivers, having salt concentration 2 to 3 times that of seawater (46mS/cm).C. Aeolian accumulations: Saline silt blown from bare areas is deposited in places covered with a vegetation.Because of these processes, soils remain supersaline throughout the year. Nevertheless one can observe a decreasing of the salinity during the rainy season in sandy soils of the "glacis de raccordement" and of sandy levees, and also in loam soils of the upper terrace with a smooth topography, because of saturation by rain water.Soil acidification, unknown before 1971, has rapidly spread throughout the entire Sine Saloum basin.It is due to oxidation of pyritic sediments, provoked by the long drought period, which resulted in drainage, lowering the groundwater tables and evaporation.Pyrite oxidation seems to be the most important pedogenetic process. Its importance depends on the pyrite content of the sediment, local chemical conditions (exchangeable cations, salts) and to