This paper considers forestry as an approach to climate change mitigation in the forests of southwest of France. In this region, little agricultural area is available to afforest, as forest already covers the major part of the land (more than 50%). However, shifts in forest management, especially longer rotation lengths, can help to enhance carbon uptake. Many studies show that appropriate tax systems implemented on carbon uptakes by trees are an incentive to longer rotation length (see Capparos et al., 2003; Enzinger and Jeffs, 2000; or Hoen and Solberg, 1994), but only a few of them attempt to assess the cost effectiveness of tax systems. The aim of the present study is to provide a measure of the cost effectiveness of a tax system, implemented on forest management in south western French forests. For a range of carbon prices between 0 and 100 euros/tC, rotations lengths vary between 51 (Faustmann rotation) and 68 years, which increases substantially carbon stocks. The marginal costs of sequestering one ton of carbon vary between 90 and 140 euros. A sensitivity analysis shows that these costs vary positively with wood prices (higher opportunity costs to extend rotation lengths) and negatively with the discount rate.