Topography and Time Shape Mire Morphometry and Large‐Scale Mire Distribution Patterns in the Northern Boreal Landscape.

Peatlands are major terrestrial soil carbon stores, and open mires in boreal landscapes hold a considerable fraction of the global peat carbon. Despite decades of study, large‐scale spatiotemporal analyses of mire arrangement have been scarce, which has limited our ability to scale‐up mire properties, such as carbon accumulation to the landscape level. Here, we use a land‐uplift mire chronosequence in northern Sweden spanning 9,000 years to quantify controls on mire distribution patterns. Our objectives include assessing changes in the spatial arrangement of mires with land surface age, and understanding modifications by upland hydrotopography. Characterizing over 3,000 mires along a 30 km transect, we found that the time since land emergence from the sea was the dominant control over mire coverage, especially for the establishment of large mire complexes. Mires at the youngest end of the chronosequence were small with heterogenous morphometry (shape, slope, and catchment‐to‐mire areal ratios), while mires on the oldest surfaces were variable in size, but included larger mires with more complex shapes and smaller catchment‐to‐mire ratios. In general, complex topography fragmented mires by constraining the lateral expansion, resulting in a greater number of mires, but reduced total mire area regardless of landscape age. Mires in this study area occurred on slopes up to 4%, indicating a hydrological boundary to peatland expansion under local climatic conditions. The consistency in mire responses to spatiotemporal controls illustrates how temporal limitation in peat initiation and accumulation, and topographic constraints to mire expansion together have shaped present day mire distribution patterns. Plain Language Summary: Peatlands store nearly one third of the global soil carbon, despite covering only around three percent of the land surface. Open mires, which are characteristic peatland types at high latitudes, represent an important peat carbon store. Few studies have explored how mire patterns in the landscape change over time and space. This knowledge gap has limited our ability to estimate and scale‐up mire properties, such as the peat carbon store, from the individual mire to the entire mire landscape. Here, we study the mire patch distribution in a landscape that covers nine thousand years of landscape development. Using this aging landscape, we can separate temporal controls on mire patterns from spatial controls related to local topography. We found that mire cover was mainly controlled by age, while the abundance of mires and their fragmentation was defined by local topography, for example, through the catchment‐to‐mire ratio and the slope of the surrounding upland areas, which define the limits for mire expansion upland. Our results provide an important step in understanding the spatial and temporal controls that give rise to present mire distribution patterns. Such information can further support landscape‐level estimations of mire properties and functions, such as the long‐term peat carbon store. Key Points: Mire cover and shape are linked to the formation of large mire complexes, while abundance and fragmentation are driven by topographyAreas of recent isostatic uplift hold small mires, while diverse initiation and expansion lead to heterogeneous mire patches in older areasScaling up mire properties to the landscape level requires an understanding of spatiotemporal controls behind mire distribution patterns [ABSTRACT FROM AUTHOR]