The debate about the expansion of agriculture in forest areas and the conservation or reforestation of head watersheds is still going on in montane Southeast Asia but in a rapidly changing context. Tremendous change occurred in the highland agrarian systems of northern Thailand during the past decades, leading to new farming practices, an increased diversity of stakeholders concerned by land management issues, and new relationships between villagers and national policies (decentralisation of resource management, shift from forest exploitation to conservation, etc.) and international conventions. In this context, the debate about the true participation of rural people in managing local renewable resources is taking central stage. New conceptual and practical tools to understand rural change in a more distributed, inclusive and interactive way have also emerged. System approaches relying on collaborative modelling are used to facilitate communication, knowledge sharing and the exchange of points of view among different types of stakeholders about a common resource management problem. The iterative and evolving Companion Modelling (ComMod) approach relies on multi-agent systems and makes use of the synergistic effects between role-playing games and computer agent-based models to co-construct simulation tools with stakeholders used in the joint exploration of possible future scenarios of their choice as part of negotiation processes leading to concrete action plans. In the past three years, such a ComMod process has been implemented in the head watershed of Nan province to understand the effects of recent change in forest management on the agrarian system and to mediate a land use conflict between foresters and Hmong herders. A preliminary diagnostic-analysis showed the influence of increased forest conservation efforts on the dynamics of deforestation in the local Hmong agrarian system. These land use dynamics were represented in a spatially explicit computer-assisted