Soil organic carbon stock in restored and natural forests in northern Thailand

In order to increase understanding of the role that tropical forest restoration might play in mitigating global climate change, soil organic carbon (SOC) was determined in a chrono-sequence of plots in northern Thailand; from a site undergoing unassisted natural forest regeneration (control); through 3 sites undergoing forest restoration by the framework species method, aged 2, 7 and 11 years since tree planting (R2, R7, R11); to a nearby area of relatively intact forest (NF). Forest restoration greatly increased SOC stocks compared with pre-restoration data, predicting a return to NF levels in less than 21.5 years after commencement of restoration activities. However, SOC stocks, measured in soil pits dug down to 2 m depth, did not increase in sequence with forest development, as expected: control 205.8 tCha-1; R2, 254.4; R7, 251.1; R11, 161.8 and NF, 244.9. The incongruously low SOC in the 11 year-old restoration plot might be explained by the persistent, overriding, effects of land use history reducing SOC in the lower soil layers. Per cent organic carbon declined with soil depth, following reliable power functions (R2 0.92-0.97): %SOC=k.DEPTHp (k=7.7 to 22.2; p=-0.41 to -0.80). Comparison with other studies showed that forest restoration by the framework species method sequestered more soil carbon than monoculture plantations in the same region.