Tree Refugia: Ideas of how and when boreal plants spread to the formerly glaciated parts of the world following the retreat of the glaciers 9000 years ago are long debated. Models of the postglacial spread of boreal plants argue for dispersal from southern refugia; however,Parducciet al.(p.1083) have shown that both spruce and pine were present in small ice-free regions of Scandinavia much earlier than thought. DNA haplotyping confirmed that a remnant mitochondrial type of spruce, once unique to Scandinavia, now lives alongside the more common spruce originating from Eastern Europe. Evidence from lake cores collected from central and northern Norway indicated the survival of conifers as early as 22,000 years before the present, when apart from ice-free pockets, most of Scandinavia was covered by ice.

The use of lake sedimentary DNA to track the long-term changes in both terrestrial and aquatic biota is a rapidly advancing field in paleoecological research. Although largely applied nowadays, knowledge gaps remain in this field and there is therefore still research to be conducted to ensure the reliability of the sedimentary DNA signal. Building on the most recent literature and seven original case studies, we synthesize the state-of-the-art analytical procedures for effective sampling, extraction, amplification, quantification and/or generation of DNA inventories from sedimentary ancient DNA (sedaDNA) via high-throughput sequencing technologies. We provide recommendations based on current knowledge and best practises.