Peatlands Are Still Expanding - A Case Study from Boreal Finland

Under the present climate crisis, it is crucial that all factors contributing to the global carbon cycle are profoundly understood. In this cycle, peatlands play a major role, but their current spatial dynamics - especially regarding the expansion of peatland margins and development of new peat patches - have been overlooked. We studied the lateral expansion of Finnish peatlands over the last decades and centuries by means of radiometric dating of the basal peat of the peatland margins. In addition, mineral substrate and landscape features, including terrain topography and slope, were studied using sedimentological and fine-resolution remote sensing techniques to analyse possible causes and constrains for the lateral expansion.

Our results show that lateral expansion is still occurring all around in Finland. However, the spatiotemporal patterns vary even within the same peatland. Four out of six studied peatlands indicate substantial areal expansion during the last 150 years. On these sites, the peatland margins have expanded from the center of the peatland towards the edges in chronological order. In two other cases, the lateral expansion data were more complex and warrant additional disquisitions. The effects of mineral subsoil type and terrain topography on lateral expansion were found to be crucial in determining the variability and rate of lateral expansion. Upslope expansion was also observed.

Depending on their hydrological status, newly paludified mire margins may temporarily release large amounts of methane to the atmosphere, but soon they will turn to new long-term carbon sinks. When extrapolating these new peat areas for the entire northern circumpolar region, we demonstrate the emergence of a new carbon pool that should be considered in future earth system models. Our results further show that the long-held view, according to which all the topographically suitable areas for lateral expansion are already covered by peat, is simply false. The lateral expansion is still an on-going process that is, in addition to carbon cycling, affecting the Boreal landscapes and ecology in multiple ways.