ts 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, and predominantly for wetland habitats, where there could be some trees but not closed forest (Svenska Akademien, 2020). An analysis of the wetland terminology of a group of dialects of northern Sweden supports this definition (Wiklund, 1992: 97–99). In future studies, one should explore how the term ‘myr’ has been applied in Swedish land surveys over time and across Sweden. In this study, we base the definition on the current interpretation of the word and our own experiences of the maps and aerial photos that we have studied, concluding that what was called ‘myr’ in 1880 and today is essentially the same.
Another crucial concept of our study is the total mire area, which we used to calculate the proportions of mire managed for different purposes. Defining the total mire area as the sum of all mire meadows, peat pits and unused mires indicated on the enclosure map from 1880, is likely a conservative estimate of what occurred before mire management began. Indeed, mires mapped in 1880 could exclude mires that once existed but that had been converted to other land classes earlier. However, when we checked all available historical maps from both Abborrtjärn and surrounding villages, we did not find any indications of such changes. In 1880, all cultivated fields of the study area were located close to the farmsteads on sandy soils with varying proportions of clay, according to the map tables, and thus not on peatland. Unlike Strandin Pers (2012), who was able to detect fields that once had been mires from their names, ending in ‘fen’ (sw. ‘-kärr’) or ‘bog’ (sw. ‘-mosse’), we found no such cases on the maps of Abborrtjärn, neither in 1805 nor 1880.
Another land class that could potentially have included mires in 1880 is ‘land suitable for cultivation’ (sw. ‘odlingsmark’), but in our study area, those areas were mostly located next to existing fields on sandy soils. Other kinds of land use were indicated on the 1880 map, such as ‘gravel pits’, but they were unlikely to have affected any mires. In other words, although we