LAUSR

Abstract Due to high rainfall erosivity and rapid land-use changes, South-East Asia is one of the hot spots of soil degradation worldwide. In recent decades, several studies showed that the expansion of rubber tree (RT) plantations into previously forested areas has caused a major increase in soil erosion. However, the effects of cropland conversion to RT plantations on surface runoff and soil detachment are still unclear. Here we show that mature RT plantations considerably increase runoff and soil detachment compared to annual crops or young RT plantations with intercrop, mostly due to the absence of understorey. Monitoring 1 m2 microplots over four years in mountainous Northern Thailand, we found that runoff and detachment increased with time since the onset of the rainy season and with rainfall height, but more so in mature RT plantations than in young RT plantations and maize. This led to much higher annual soil detachment in mature RT plantations (5.7 kg/m2 on average) than in maize and young RT plantations with intercrop (0.36 kg/m2). We identified two main factors explaining this difference: first, rubber tree leaf litter, although abundant, seemed ineffective in reducing runoff at the end of the rainy season. Secondly, the cover by low-growing plants in mature rubber tree plantation was usually sparse and provided little protection. In particular we showed that increasing the cover by low-growing plants from quasi-null cover to >31% cover decreased runoff coefficient by 32%. Our results demonstrate that afforestation by RT, at least under current management practices and on steep slopes, is overall detrimental to soil conservation but that its effects could be mitigated by the adoption of less intense weeding practices.