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Slash-and-burn agriculture (tavy) is the major cause of upland degradation and deforestation in eastern Madagascar. Upland degradation studies are largely based on floristic loss and have ignored the link to agriculture, the main activity on the uplands. The objectives were to analyze jointly with the Betsimisaraka farmers how slash-and-burn practices influence fallow species succession, and how current fallow/cropping regimes influence agricultural productivity and upland degradation dynamics. The study was conducted in the Vohidrazana/Beforona area, located at the margins of the Ankeniheny-Zahamena rainforest corridor. Semi-structured interviews were conducted in 9 villages resulting in 96 individual and 22 group interviews. These were complemented by 212 historical field plot inventories on 32 farms. The researchers' and farmers' knowledge systems were treated equally and complemented each other in the joint analysis. Over the last 30 years, fallow periods decreased from 8-15 years to 3-5 years. Hence, fallow vegetation is changing within 5-7 fallow/cropping cycles after deforestation from tree (Trema orientalis) to shrub (Psiadia altissima, Rubus moluccanus, Lantana camara) to herbaceous fallows (Imperata cylindrica and ferns) and grasslands (Aristida sp.), when land falls out of crop production. This sequence is 5-12 times faster than previously reported. The frequent use of fire is replacing native species with exotic, aggressive ones and favors grasses over woody species, creating treeless landscapes that are of minimal productive and ecological value. Unlike most discussions during the past 20-30 years that refer to fallow duration per se as a measure to soil recovery, our results show that fallow periods need to increase in length with each additional fallow/cropping cycle after deforestation in order to restore the soils to a similar level of productivity. Already with the third cycle after deforestation, vegetation starts to degrade quickly in parallel with agricultural productivity decline. The Betsimisaraka's fallow knowledge is very rich. Fallows are characterized by species life form, species composition, vegetation appearance, cycles after deforestation, and agricultural potential. Distinct fallow types are easily identifiable in the field and inform on the critical threshold, below which soils are lost to farming. Clear management guidelines go along with each fallow type. We recommend upland agricultural intensification and diversification based on improved soil fertility through optimized organic and inorganic inputs and fire-less land management that encourages the re-establishment of nutrient stocks. If young farmers perceive a real opportunity in agricultural intensification, the migration towards the forest borders to pursue tavy might be halted.