Dendrochronology is suspect because 2 or more rings can grow per year (Talk.Origins)
Two or more growth periods frequently occur during a year, so dates derived from tree rings (dendrochronology) are suspect.
(Talk.Origins quotes in blue)
1. For some trees, including bristlecone pine, ponderosa pine, and douglas fir, double rings are rare and easy to spot with a little practice. A bigger problem is missing rings; a bristlecone pine can have up to 5 percent of its rings missing. Thus, dates derived from dendrochronology, if they are suspect at all, should indicate ages too young.
What Talk Origins does not tell you is that the extra rings make up at least 20% of the total, this leaves an error of at least 15% too old.
Both extra and missing rings are detected by comparisons with other trees. This is a somewhat subjective process, since even trees of the same species growing side by side do not produce absolutely identical ring patterns. The fact that there can be extra rings and missing rings only increases the subjectiveness of dendrochronology.
For most of the dendrochronological record, dates are determined from more than one source, so errors can be spotted and corrected.
Morris's reference was to the dating of single trees by their ring count and does not refer to the entire dendrochronological record.
The problem is that comparisons between trees are somewhat subjective, since even trees of the same species growing side by side do not produce absolutely identical ring patterns. Therefore finding errors is also subjective, adding to the overall subjectiveness of dendrochronology.
Furthermore, conventional dendrochronology does not consider the after effects of the Genesis Flood. Climate instabilities immediately after the Flood would probably have resulted in additional rings across the entire dendrochronological record.
2. Dendrochronology is in rough agreement with carbon-14 dating, so even if it is off, it is not off by much -- certainly not by orders of magnitude, as young-earth claims would require.
Not surprising since dendrochronology is used to to calibrate carbon-14 dating. Not only that but carbon - 14 dating is used in tree ring matching. As a result the two are not independent dating methods, but are actually mutually dependent.
- Reference: Tree ring dating (dendrochronology)