In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings.
The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth.
According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings." She and her colleagues studied the apparently ringless tree (Miliusa velutina) of Thailand.
Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season.
It is increasingly difficult for prehistorians working in the twenty-first century to conceptualise the problems experienced by their predecessors, and approaches to interpretation before the 1960s are consistently criticised.
Often the borer does not reach the center of the trunk, so the total number of years must be extrapolated from the radius of the trunk.
The radius (r) can be determined from the circumference of the trunk (C=2πr), or from special tape measures that give the diameter directly.
Today, scientists have painstakingly established an unbroken succession of rings extending back in time over 80 centuries (8,000 years).
Cross-dating is a valuable tool in dendrochronology and archaeology.
The 1906 ring pattern in wood Sample A (which was cut from Stump A) correlates with a 1906 ring pattern in Sample B which was cut from an older, undated Stump B. By matching up similar spaced rings in Samples B, C and D, the ages of ancient timbers can be determined.