Degrading uplands in the rainforest region of Madagascar: Fallow biomass,nutrient stocks,and soil nutrient availability |
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| Authors: | Erika Styger Erick C M Fernandes Harivelo M Rakotondramasy Eric Rajaobelinirina |
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| Abstract: | Soil fertility restoration depends on natural fallows in the slash-and-burn system of eastern Madagascar. In the Beforona-Vohidrazana
study zone, none of the fallow species are able to withstand the slashing, burning and cropping frequencies of 3–5 years.
Eventually soils are abandoned for agriculture. Along the degradation sequence, this study quantifies fallow biomass, nutrient
stocks and soil nutrient availability of four dominant fallow species Trema orientalis, Psiadia altissima, Rubus moluccanus, and Imperata cylindrica. At 3 years, the shrubs Psiadia and Rubus were more productive (11–14.4 t/ha aboveground biomass or AGB) than the tree species Trema (8.5 t/ha). Only after 5 years did tree productivity (24.7 t/ha) exceed that of shrubs (17–20 t/ha). Imperata’s biomass stagnated at 5.5 t/ha after 3 years. A sharp decline in fallow productivity was observed with advancing fallow
cycles after deforestation. While Psiadia produced highest AGB in the second fallow cycle (C2) being 100%, C1 achieved 89% of that, C3 74%, and C4 only 29%. With the
ability to propagate vegetatively and to accumulate important amounts of nutrients in roots, Rubus and Imperata, both exotic and invasive species, showed improved adaptation mechanisms towards frequent disturbances compared to the two
indigenous species Trema and Psiadia. Available soil nutrients P, K, Mg were highest under forest and declined rapidly with increasing fallow cycles. Ca and pH
rose momentarily in the first fallow cycle before declining with advanced soil use. Al concentrations increased steadily with
time. As lengthened fallow periods are not practical, there is a need to intensify upland systems based on improved nutrient
cycling, targeted inputs, fire-less land management, and land use diversification. Allowing regrading tree and bush fallows
to accumulate biomass (above- and belowground carbon) will significantly improve Madagascar's greenhouse gas mitigation contribution. |
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