The wood in these rings once laid down remains unchanged during the life of the tree.
This is very useful as a record of the radiocarbon concentration in the past.
It is calculated on the assumption that the atmospheric radiocarbon concentration has always been the same as it was in 1950 and that the half-life of radiocarbon is 5568 years.
For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made.
The amount of carbon-14 gradually decreases through radioactive beta decay with a half-life of 5,730 years.
So, scientists can estimate the age of the fossil by looking at the level of decay in its radioactive carbon.
The ensuing atomic interactions create a steady supply of c14 that rapidly diffuses throughout the atmosphere.
Using very old trees (such as the Bristlecone Pines in the western U. A.), it is possible to make measurements back to a few thousand years ago.
Plants take in atmospheric carbon dioxide by photosynthesis, and are ingested by animals.
So, every living thing is constantly exchanging carbon-14 with its environment as long as it lives. The carbon in its body will remain until it decomposes or fossilizes.
Beyond that timespan, the amount of the original C formed by irradiation of nitrogen by neutrons from the spontaneous fission of uranium, present in trace quantities almost everywhere.
For these samples, other dating methods must be used.