Temporal distribution pattern of subfossil pines in central Sweden: perspective on Holocene humidity fluctuations

The temporal variations in distribution pattern of Scots pines (Pinus sylvestris L.) have been used as an annual resolution record of past lake-level changes. Logs, preserved for thousands of years in bog and lakes (subfossil wood), were sampled from small lakes of the Scandinavian Mountains in west-central Sweden to construct a tree-ring chronology from an area where pines are sensitive to growth season (ie, summer) temperature. The chronology spans from AD 2006 to 4868 BC, with two minor gaps at 1600 BC and AD 900 and one larger gap at 2900 BC. The dendrochronological approach can provide a high quality long-term perspective on lake level fluctuations, which possibly can be coupled to changes in humidity. Submerged trunks were found in situ, embedded in sediments, and because trees from the earliest periods were not necessarily found at the deepest levels, this shows clearly that lake levels must have been lower than present at the time of tree-growth. The lake levels must have fluctuated, creating alternating conditions of pine regeneration and mortality. The fluctuating lake levels recorded are suggested to be a result of regional humidity increases and decreases, mainly governed by precipitation changes. Periods of lower lake levels were inferred at 4900—4800 BC, 3800—3600 BC, 3400—3250 BC, 2400—2200 BC, 2100—1800 BC, 1500—1100 BC, 900—800 BC, 400—100 BC, AD 50—300, AD 400—600, AD 900—1100, AD 1350—1500 and AD 1700—1800. Periods of higher lake levels are tentatively encountered at 3600—3400 BC, 3200—2900 BC, 2200—2100 BC, 1700—1500 BC, 1100—900 BC, 800—400 BC, 100 BC—AD 50, AD 300—400, AD 750—900, AD 1100—1250 and AD 1550—1700.