Why have Swedish primary forests persisted? Their locations, naturalness, and physical representativeness of managed secondary forests

Context Primary forests, with little to no direct human impact, can act as baselines to understand the impact of land use. However, due to extensive conversion for land use, their location, naturalness, and characteristics remain poorly characterized. Objectives This study aims to present a new map of Swedish primary forests, understand their persistence in a heavily used landscape, and assess their suitability as baselines for studying land use impacts. Methods We mapped 384 primary forests across Sweden, using historical records to assess their naturalness. To evaluate factors influencing their persistence, we analyzed their accessibility by measuring proximity to historical (1939) roads and timber floating rivers, and their blockiness. Lastly, we determined their suitability as baselines by comparing their topographic characteristics with those of managed secondary forests. Results Most primary forest land (93.6%) is located at higher elevations, associated with low productivity and accessibility, while 6.4% is in lowland areas. Over one-third of the lowland productive primary forest land is topographically similar to the typical Swedish forest landscape. These forests are more distant from the 1939 road network and more frequently in blocky terrain than non-primary forest land. Conclusions Three types of primary forests persist: low-productivity mountain forests, lowland forests in complex terrain, and those far from historical roads or in blocky landscapes – highlighting the role of physical and economic factors in their persistence. A sizeable portion of lowland primary forests are representative of managed secondary forests, making them suitable natural baselines for studying ecosystem impacts of land use change.