Dendrocronological age determination
Trees growing in regions with seasonal climate variations usually develop visual rings consisting early wood and late wood during every growth season. Simply expressed, thick rings are formed during years with warm and humid summers while thinner rings reflect the opposite conditions, although a number of other factors may also influence tree growth, such as sunshine, wind exposure and winter snow cover. Most trees of the same species within a restricted geographical region exhibit broadly similar tree-ring patterns in response to climate variations. This means that overlapping ring-width series from trees that lived during different periods can be connected to form continuous chronologies or reference series, which may reach thousands of years back in time. Available Swedish dendrochronologies are based on oak, beech, pine and spruce, but in some circumstances also wood from other tree species can be dated. Oak wood is common in southern Sweden while pine is the dominant species in other parts of the country. Based on detailed measurements of tree-ring series obtained from wood of unknown age, e.g. construction timber from old buildings, and matching against reference series using statistical techniques and visual inspection, the lifespan of the tree can be determined. If the last developed tree ring is preserved the year when the tree was felled can be precisely determined as an absolute age according to the principles displayed in the figure.