Topographic controls on lateral expansion and large-scale patterns of peatlands in the northern boreal landscape

Despite their importance in global carbon and hydrogeochemical cycles, large-scale spatiotemporal analyses of the lateral expansion and landscape patterns of peatlands have been scarce. This has impeded our possibility to scale-up important peatland processes and properties, such as carbon accumulation to the landscape level. Here we combine landscape-level analysis of ten mire chronosequences to study lateral expansion rates, with an in-depth analysis of mire morphometry in a single chronosequence, to quantify controls on peatland distribution patterns. All ten chronosequences are located along the Swedish coast of the Bothnian Bay Lowlands, and span an age range of 0-9000 years of post-glacial land-uplift. Our findings challenge the widespread misconception of linear mire expansion, and showcase how the extent of entire mire populations evolved over the Holocene, and under the control of upland hydro-topography. Landscape wetness, for instance, favored more rapid lateral expansion rates in relatively young parts of the landscapes. Moreover, based on the in-depth analysis of over 3 000 peatlands at one chronosequence, we found time since land emergence an important control on peatland coverage, and on the formation of large mire complexes. Topography, on the other hand, controlled peatland fragmentation and number regardless of landscape age. Altogether, our results illustrate how time since initiation combined with topographic controls influenced lateral expansion, and present-day peatland distribution patterns in the northern boreal landscape.