hodological biases, due to changing definitions of mire and forest. Also, any lack of correspondence between the mire polygons defined on the 1880 map and the actual landscape would distort the results. Nevertheless, we conclude that approximately 40% of the total mire habitat area has been lost through ditching for forestry purposes. These ditches probably promoted the establishment of forests on wet sites, but it is unclear how important they continue to be in maintaining tree growth, now that high volumes of trees are keeping groundwater levels low. But, when forests are harvested and the groundwater level subsequently rises, the ditches will likely become important again to provide drier conditions for the establishment of tree seedlings. The effects are not only local, however, as drainage has been suggested to negatively affect the mire’s ability to protect downstream areas from drought and storm flooding (Menberu et al., 2016). We have no evidence of ditching after the 1980s, when drainage of new wetlands was essentially banned in Sweden. But, as long as ditches are functional, drainage will remain effective and in most cases lead to less water storage in the peat, increased annual runoff, and higher flood peaks (Holden, Chapman, and Labadz, 2004). Increased runoff also generally leads to higher levels of organic C in rivers (Asmala, Carstensen, and Räike, 2019).
Carbon (C) sequestration and storage is another important ecosystem service promoted by mires. Haymaking affected C sequestration because a major part of the plant production was removed with the hay, resulting in less litter and a slower accumulation of organic matter to form peat (Elveland, 1982). However, the most important long-term implications for climate change are connected to the C stored in peat and the conversion of mires to other land classes, especially forest, through drainage. Northern peatlands have sequestered about 270–547 Gt of C since the end of the last glacial period, which represents 20–30% of the organic C currently stored 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 effec