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Slash-and-bum cultivation and natural resource management: evolution and perspectives of three mountainous rural areas of Eastern Madagascar This thesis was supported by an increasingly multidisciplinary team, and lay within the framework of a research project (Terre-Tany/BEMA), whose aim was to provide reliable, useful information to those responsible for the development of the east coast of Madagascar. This information will help to plan and implement measures aimed at making permanent improvements to rural living conditions. When it comes to managing natural resources, the traditional slash-and-bum culture predominates on the eastern slopes of Madagascar. Known locally as "tavy", this system produces rain-fed rice. Several factors reinforce the view held locally that tavy is a vital element in the lives of the Betsimisaraka, the region's dominant ethnic group: rice is the staple diet of the Madagascan population; the tavy system has been passed down from their ancestors, who are considered to be the "intellectual guides" of the current generation; and finally, the undulating landscape and the frequency of very heavy rainfall make the production of irrigated rice difficult. Because it is often seen by outsiders as an incomprehensible, ecological scandal, a precise scientific analysis of the impact ofthe tavy system, and more globally, of the impact of natural resource management by villagers was required. The thesis also considered methods of achieving sustainable development. In brief, the thesis focused on these points; research was restricted to the local environment, and the villagers* opinions were also canvassed. As well as considering the findings of several close colleagues, this thesis used three in-depth methodological approaches to produce a relatively wide-scale analysis of local conditions, thereby coming up with sound, workable proposals. First of all, it uses phytosociologic and phytoecologic methods to show how secondary vegetation encroaches on fallow land after slash-and-bum has occurred To achieve this, modem floristic ordination methods were used, as well as a valuation system for ecological vegetation information; both were based on diverse statistical analyses. Next, it considered the disruptive influence of land clearance, slash-and-bum and letting land lie fallow. In particular, it looked at the resulting loss of fertility, and examined the constituents ofthe vegetation, of the biomass located both aboveand underground, and of the nutrients. Semi-structured sociological population surveys aimed to show the interests of user groups, their role and their part in resource management. This allowed for a better understanding of the main socio-cultural mechanisms needed to bring about behavioural change, which would ensure sustainable development and at the same time, be socially acceptable. The results of the ecological analyses show the decisive nature of the first slash-and-bum deforestation. On the one hand, this is because thereafter, the land will normally be strictly used for agricultural purposes. It is also because the greatest loss of vegetation species and nutrients occurs at this stage of the tavy degradation process. The repetition ofcultivation cycles one rainy season for rice and sometimes a second season for less demanding products like manioc or sweet potatoes brings about a reduction in fertility which is strongly linked to the length of the fallow period which follows (five years on average), and to the topographical conditions, Floristic analyses show four general secondary vegetation types which display regressive succession: peri-forest fallow land which still contains coppice shoots and other typical forest elements; shrub fallow land, in which some secondary ligneous species are still present; productive Inderal fallow, in which certain intrusive, non-indigenous plants, such as Lantana camara or Rubus moîlucanus provide sufficient biomass for cultivation; and finally, degraded ruderal fallow land, found in localised spots on the most fragile sites, peaks and steep slopes where agricultural cultivation is not possible. As fer as fertility is concerned, this thesis confirmed the importance of looking at the soil and vegetation system in tropical countries in its entirety, especially if the biomass is a determining factor in site productivity, as is true of the tavy system. There seems to be some general correlation as regards the elements contained in the soil and vegetation: the fewer elements found in the soil, the higher their concentration in the vegetation (for example, potassium in the study area). The amount of biomass produced by fallow land due for burning showed a correlation with elements well-known in fanning circles nitrogen, phosphorus and potassium. These three elements, but especially phosphorus, also seem to be important for agricultural yields. On the other hand, soil in the final stages of degradation, such as fern thickets or anthropic savannah land (Aristida sp.), differs notably in its lack of exchangeable

Culture sur brûlis et gestion des ressources naturelles: évolution et perspectives de trois terroirs ruraux du versant Est de Madagascar