Vegetation can be used to infer the potential climate feedback of peatlands. New studies have shown the recent expansion of peatlands, but their plant community succession has not been studied. Although generally described as dry bog-type vegetation, our results show that peatland margins in a subarctic fen began as wet fen with high methane emissions and shifted to bog-type peatland area only after the Little Ice Age. Thus, they have acted as a carbon source for most of their history.