Soil carbon stock in a seasonally dry tropical forest in Thailand increases from lower to upper slopes, contrary to typical patterns. This study found that soil carbon density increased with slope position, especially in the 10-30 cm soil layer. Medium roots in the upper slopes were more abundant, particularly in the 15-60 cm soil layers. The atypical soil carbon accumulation pattern was widespread in the watershed and was influenced by root development in deeply weathered soil under dry conditions. Bamboo roots also helped stabilize soil carbon and reduce erosion.