Ancient DNA of NW Europe reveals responses to climate change

Did trees grow on Andøya during the Last Glacial Maximum (LGM)? In AfterIce, our aim was to answer this and other questions about past environmental change in NW Europe. In contrast to the rest of Scandinavia, the northern tip of the island of Andøya, on the west coast of Norway, was ice-free during the last glacial maximum, about 20,000 years ago. Previously obtained records of macrofossils and pollen show that high arctic species grew there then. Recently, we discovered DNA of pine and spruce dated to 22 000 and 17 000 years before present, respectively. The results caused a sensation, and they were also questioned by several scientists. In AfterIce, we have conducted studies to improve our understanding of what DNA represents in terms of vegetation. We have taken new lakes cores from Andøya to check if our records of trees are repeatable, and, if so, to gain further information about the nature of the environment represented by this record and to assess the likelihood that the DNA represents trees growing at the site, rather than being an artefact. As the study of ancient plant DNA is a new technique, there are still many aspects that are poorly understood. For example, what proportion of species that grow around a lake are represented in a DNA sample from that lake? Will we only detect species growing close to the lake or also species growing further away? To answer these questions, we sampled DNA and recorded vegetation data from 11 lakes in northern Norway. We were able to detect about half of the species growing around the lake in the DNA analyses. This level of detection is comparable to or better than many conventional pollen or macrofossil analyses, and sufficient for inferring a range of features of the past vegetation. The majority of species were recorded within 2 m of the lake rim, suggesting that ancient DNA, like macrofossils, may be predominantly of local origin. Trees, when present, are key taxa in ecosystems, with many dependent species, and treeline is a critical bioclimatic boundary. Therefore, we wanted to know if our DNA methods correctly record local presence of trees, in time as well as space. In southern Scotland, the establishment of 20th-century exotic conifer plantations is well dated from forestry records and historical maps and provides the ideal study system. We showed that DNA correctly detected exotic conifers from the time when they were first present in the vegetation. Thus, DNA does not diffuse downwards in saturated lake sediments and does not give a false date for tree establishment. Further, the exotic conifers are detected by DNA only when the lakes are surrounded by forest, whether or not their pollen is present. This indicates that pollen is not likely to be the source of the DNA detected in sediment. We cored Skartjørna that has provided the best macrofossil record to date from Svalbard. In a DNA analysis of a 5.5 m long core that spanned over 8000 years, we found nearly all species previously discovered as macrofossils. This result strongly supports our contention that the information gained with ancient DNA is reliable. We also discovered six new vascular plant species, 12 species of bryophytes, and two algae with DNA. The DNA record was as informative as the macrofossil record, although they were in some aspects complementary, rather than identical. The DNA records had the advantage that more species were recorded per sample, which made then more reliable for inferring species persistence. The majority of the species that we recorded were present throughout most of the period 8500-1200 years before present, indicating that the species have been able to persist under climate change equivalent to 1-2°C mean July temperature above current values. However, as future warming is likely to reach 2-4°C above current values, we may also expect responses that cannot be anticipated by reference to records of the past. Returning to Andøya, we again found DNA of spruce and pine. Our new samples were taken from a different lake than the one originally sampled. We analysed three adjacent sediment cores and found pine in all three and spruce in two of them. In each case the findings are for the same, early part of the record and not distributed throughout the sediment cores. The chance that this result is due to laboratory contamination is extremely low, and we conclude that DNA is present in the sediment. The critical question is then how the tree DNA became incorporated in the sediment. Our results from Scotland suggest that long-distance dispersal of pollen is a very unlikely source of DNA. Other hypotheses are that the DNA is derived from trees that were growing at the site (theoretically possible but ecologically questionable) or from driftwood or from redeposition of older material within the glacial deposits. Currently, none of these three hypotheses is strongly supported by additional evidence, and the question of the origin of the conifer DNA remains open

Andøya (69 °N) is a key area for data on terrestrial flora and vegetation in Norway during the Late Weichselian (25,000 to 10,000 years (uncalibrated) B.P.). Cores covering most of this time interval has been retrieved from several lakes, confirming the e xistence of an ice-free area potentially serving as a 'cryptic' refugium at a time when almost all of Scandinavia was covered by ice. The sediments contain small quantities of pollen of tree species (Betula, Picea, Pinus); so far interpreted as deriving f rom long distance dispersal. A recent study of ancient sediment DNA in one of the lakes yielded DNA of pine (Pinus) and spruce (Picea) dated at c. 22,000 and 17,500 cal years BP, respectively, indicating that these species possibly survived in northern in situ refugia. If the source of this DNA is confirmed as locally growing trees, this means that pine and spruce were present in Scandinavia more than 10,000 years earlier than previously assumed. Similarly, recent findings of ancient DNA of thermophilic t axa in the geographically isolated arctic archipelago Svalbard indicate that species had a more northern distribution in the Holocene hypsithermal (4000 - 8000 years BP) than today. We propose to investigate the late glacial flora of Andøya and Svalbard b y: 1) using recently developed high throughput sequencing technologies to analyse ancient sediment DNA in four new sediment cores from each of Andøya and Svalbard, 2) analysing modern lake sediments and current vegetation from the two sites to evaluate th e correlation between species frequency in the vegetation, and likelihood of representation in the sediment DNA samples, and 3) assess if the methods used for extraction and amplification of ancient sediment DNA are likely to yield DNA from pollen grains within the sediments. The results of this study will increase our understanding of how species responded to climate change in the past, which is essential to forecast effects of current global warming.