Résumé
Context Farmland on steep slopes is increasingly abandoned because it is unsuitable for most forms of modern agriculture. Succession back to forest is often slow or inexistent due to over-exploitation. Observations and measurements in Dong Cao catchment 47.9 ha Vietnam, started under farming and continued after abandonment: 20 years of uninterrupted monitoring of soil, water, land use and vegetation were integrated in this study. Objective Our aim is to identify the specific combination of soil features and agricultural practices that are responsible for fast, slow or blocked succession. We differentiate between the recovery of forest structure, relatively easy, and recovery of the original species composition, more difficult. Methods Multivariate analysis of vegetation data produced plant communities in a gradient of complexity. Using classic statistics, we sought relationships between environmental variables, land use and vegetation. Results Forest recovery failed the first 10 years, then part of the catchment developed forest. Land use explained best the distribution of plant communities over the catchment, slope and soil features were less related. Cassava cropping seriously slowed down the succession to closed forest. During abandonment soil carbon stocks (0–15 cm depth) increased with about 3% per year. Conclusion Starting from weedy thickets (2002) we distinguished two successional pathways: a positive pathway towards increased resemblance with the original Lowland forest via broken forest to closed deciduous to closed evergreen forest; a negative pathway away from the original forest species composition to degraded shrub land and low grass. Livestock was related to the negative pathway.