Trees produce rings every year due to seasonal changes in growth speed. Using these rings, scientists in the field of dendrochronology can calculate the age of trees independently of other methods.
Two bristlecone pine trees, Prometheus and Methuselah, are known based on their ring counts to have lived for over 4,800 years – significantly older than the worldwide flood that YECs claim occurred roughly 4,350 years ago. But by cross-matching patterns based on multiple bristlecone pine trees in the same area, the timeline can be extended back even further. For example, a ring pattern of "thick, thin, thin, thin, thick, thin, thick, thick, thin, thin" found in the inner portion of one bristlecone pine might be found in the outer portion of another such tree nearby. Using this method, scientists can date trees back over 11,000 years ago – far longer than the 6,000 years allotted by YEC. The supposed date of Noah’s flood around 2350 BC makes this even more problematic, since no trees would have survived.
YECs generally try to cast doubt on this data by pointing to some species of trees that occasionally make more than one ring per year. However, fellow YEC John Woodmorappe has acknowledged that there is no evidence for this in bristlecone pines:
"Could the weather patterns right after the Flood, probably quite different from those of recent decades, have triggered flushes of multiple ring growth in the BCPs of the White Mountains, California—the ones that form the inferred 8,000 year chronology? This seems unlikely, as BCPs already grow in a variety of montane environments in the western U.S., yet none of them is known to have ever produced more than one ring per year."
In fact, missing bristlecone pine tree rings occur with relative frequency, so if anything, the trees’ estimated ages are slightly underestimated. Woodmorappe also reviewed the cross-matching process and found no errors in the methodology. The only explanation he was able to make up to explain the cross-matching data is some sort of phantom "migrating ring-disturbing event" – a far-fetched idea that he has absolutely no evidence for, and which still wouldn't explain why individual bristlecone pines contradict the flood date by hundreds of years.