in soils worldwide (Järveoja et al., 2020). Although primary production is limited in this region, decomposition is also constrained by cool temperatures and water-saturated, oxygen-deficient conditions, resulting in net accumulation of organic matter (Clymo, 1984). However, drier conditions associated with climate warming and/or drainage may accelerate the aerobic mineralization of organic matter (Järveoja et al., 2020; Laiho, 2006). If the rate of decomposition then exceeds production, peatlands will become a C source (carbon dioxide, CO2 release to atmosphere) rather than a C sink. An important finding from our study is that productive mires, with the greatest peat C sequestration, were affected by drainage to a larger degree than less productive mires. But, afforestation on drained mires has the potential to increase C sequestration if tree growth outpaces peat decomposition (Minkkinen et al., 1999).
These C sequestration processes are likely influenced by past management in several ways. The establishment of mire meadows was preceded by cleaning, usually of bushes but sometimes also of trees. According to the tables of the 1805 map, several of the mires that were then classified as land to be cleared had at least sparse tree cover, and this could previously have been the case also with the mires that were already managed as meadows. When trees grew back on these mires, this might have been a return to the original state, caused by ceased management, and not necessarily the result of ditching. In those cases, the mire would become a net C sink due to forestation, at least since the peak in haymaking occurred, but maybe not so in relation to the original—partly forested—state. However, for most mires, we do not know the extent of earlier forestation, and because many of the former mire meadows have also been subjected to ditching, it is not always clear whether the major cause of forestation is ditching or the abandonment of management. Regardless, the important implication of this discussion is that the effects of ceased management should be taken into account when assessing the long-term impact of ditching on mire vegetation and C sequestration, especially when it comes to productive mires that are likely to have been used as meadows but may once have been at least partly forested.
In this case study, we have demonstrated how historical maps and other kinds of archival sources can be combined with remote-sensing data such as aerial photographs and LiDAR-based DTMs to create a detailed picture of mire use over time. As the same type of sources exist for most of Sweden, there is a large potential for similar studies elsewhere. Northernmost Sweden is especially suitable for such studies because most cultivation has occurred during historical times, whereas in other parts of Sweden extensive management of these systems may have occurred before records started being kept. When Strandin Pers (2012) used historical maps to study the reclamation of mires in nine parishes distributed over most of southern Sweden, she found that some mires had been converted to arable land already before the creation of the earliest maps in the late 1600s. Along the coast of northernmost Sweden, mires could have been converted to arable land just as early, but there is evidence to the contrary. Even in the relatively early and densely populated Umeå area, reclamation of mires does not seem to have commenced before the late 1700s (Zackrisson, 1976a).
A problem when studying mires through a combination of historical sources and modern remote-sensing data is the very definition of the object of study. Here, we have chosen the definition currently in use for official Swedish statistics, which specifies that a mire is a wetland where the dominating plant communities are peat-forming, and that it is not a forest, because the production of trees does not exceed 1 m3/ha (Nilsson et al., 2019). This is a very exact definition that was not in use in the 1800s. As long as there are records, however, the Swedish word ‘myr’ has been used for peatlands, an