Quantification of peat volume change in Northern peatlands : A study of mires capacity to swell and shrink and its relation to mire age and land management

Abstract: Peatlands are important ecosystems that provide ecohydrological functions related to carbon storage and cycling, water quality, flood attenuation, and groundwater recharge. One key characteristic that gives peatlands these functions is the capacity to swell and shrink upon wetting and drying, commonly referred to as peat volume change. This property of peat volume change is closely related to the fluctuations of the water table and has a buffering effect on the water table depth relative to the peat surface, which acts as an important control on many ecohydrological functions such as carbon cycling, vegetation composition, and biogeochemical processes. In an attempt to fill a gap of knowledge, this thesis investigated peat volume change for multiple Northern peatlands close to Umeå, Sweden, using groundwater level and mire surface level data obtained during the summer of 2021. The objectives were to investigate the temporal trends and characteristics of changes in the water table and peat volume at the studied site and to determine how peat volume change capacity differs for mires of different ages, as well as different land management such as natural, drained and restored peatlands. It was found that old (older than 2000 years) mires have a significantly smaller peat volume change capacity compared to young mires (younger than 1000 years), as well as smaller specific storage, indicating that factors that change as the peatland evolves are important for the ability to expand and contract. It was also found that the relationship between the mire surface and water level was linear for some mires but not for others, including drained and old mires. For the drained mires this could be explained by very deep water tables compared to the natural mires, however, they did not stand out among the natural mires concerning peat volume change capacity. The comparison between a restored mire and a drained gave ambiguous results. It was also found that the specific storage, which is directly related tothe compressibility of the peat, was greater during drying conditions compared to rewetting conditions, highlighting peatlands ability to maintain wet conditions. The study provides a deeper understanding of peat volume change in Northern peatlands and the factors related to this phenomenon, which is crucial for further studying of peatland ecohydrology.