Uncalibrated radiocarbon measurements are usually reported in years BP where 0 (zero) BP is defined as AD 1950.BP stands for “Before Present” or “Before Physics” as some would refer to it.
Radiocarbon dates which have been corrected--or rather, calibrated--by comparison to dendrochronological records are designated by abbreviations such as cal BP, or calibrated years before the present.
The long and varied relationship of LTRR with 14C initiatives has continued with LTRR contributions to high-resolution studies through the 1990s and systematic efforts now underway that may eventually extend the bristlecone pine chronology back beyond its beginning 8836 yr ago as of 2009.
Calibration is not only done before an analysis but also on analytical results as in the case of radiocarbon dating—an analytical method that identifies the age of a material that once formed part of the biosphere by determining its carbon-14 content and tracing its age by its radioactive decay.
The rings are the cambium layer, a ring of cells that lie between the wood and bark and from which new bark and wood cells originate; each year a new cambium is created leaving the previous one in place.
How large the cambium's cells grow in each year--measured as the width of each ring--is dependent on seasonal changes such as temperature and moisture availability.