Mire edge is not a marginal thing: Assessing the factors behind the formation, vegetation succession, and carbon balance of a subarctic fen margin

Abstract. Peatlands are the most dense terrestrial carbon storage and recent studies have shown that the northern peatlands have continued to expand to new areas to this day. However, depending on the vegetation and hydrological regime in the newly initiated areas, the climate forcing may vary. If these new areas develop as wet, fen-type peatlands with high methane emissions they would initially have a warming effect on the climate. On the other hand, if the development starts as dry bog-type peatlands, these new peatland areas would likely act as a strong carbon sink from early on. However, although some research has concentrated on the expansion of the new northern peatland areas, there remains a significant lack of studies on the successional development of the newly initiated peatland frontiers. In this research, we combined palaeoecological, remote sensing and hydrological modeling methods to study the expansion and successional pathway dynamics in a subarctic fen margin in Finnish Lapland and discussed possible implications for carbon balance of these margin peatland areas. Our results showed that the studied peatland margins had started to develop ca. 2000 years ago and have continued to expand thereafter, and this expansion has occurred in non-linear fashion. In addition, the wet fen-type vegetation persisted in the studied margin for majority of the development history and only the dryer conditions after the Little Ice Age instigated the fen-to-bog transition. However, a notable part of the fen margins in the Lompolovuoma and Lompolojänkkä basins has remained as a wet fen-types, and their persistence was likely caused by the hydrological conditions in the peatland and in the surrounding catchment. Our findings show a large variation in the peatland expansion and succession dynamics even within a single peatland basin. Although changes in climate conditions had initiated the fen-to-bog process in some margins, some had remained in the wet, fen stage showing resilience to allogenic forcings. Thus, when estimating the peatland carbon stocks, and predicting the future trajectories for peatland development, this heterogeneity should be taken into account to avoid errors caused by over-simplification of peatland lateral expansion dynamics.