Lars, im not sure where you got your ideas on dendrochronology, but that dating method can now accurately measure a trees age to 8000+ years.
Tree-ring studies have been a staple of anthropological investigation in the American Southwest since the early decades of this century and in Europe since World War II. The bristlecone pine chronology of the American Southwest now exceeds 8500 years with the possibility that up to 3000 floating years will be added in the reasonably near future. The European oak and pine chronology, a composite of work done in Germany and Northern Ireland, is now over 11,000 years long.
Source: http://www.arts.cornell.edu/dendro/ajatext.html
Considering the flood was only 4000 years ago, these trees have done considerably well to survive for so long - and not to show any evidence of a flood in their rings. But that's another topic.
Seeing as most JW's dont have qualms with dendrochronology, I thought this small section of an article on corior would be of interest, to show that other dating methods are applied. For example, Ice cores.
For example, the eruption of Mount Mazama that formed Crater Lake in Oregon had been dated by the mid-1960s by radiocarbon methods to between 6500 and 7000 years ago. A direct count of layers in ice cores from Greenland, in which ash from the eruption was found, gave a date of 4401 B.C., or 6391 years ago, not all that far from the radiocarbon dates, and in fact consistent with findings from tree ring studies that radiocarbon dates tend to be somewhat older than actual dates. Human artifacts have been found under ash from this eruption. A 1979 Scientific American article said: 56
.... tests showed that tephra layers found at certain archaeological sites came from Mount Mazama.... One of the archaeological sites was Fort Rock Cave.... Under the tephra [were] found sandals made of sagebrush bark, one of which was dated by carbon-14 analysis and found to be some 9000 years old.
I'd recommend you give the whole article a read.
Oh and from johnnyc's comment about the water canopy altering dates - although a canopy would give incorrect dates for C-14 methods, there would have been other inconsistencies which we don't see. For example, besides dendrochronology - thermoluminescence dating, fission track dating, amino-acid dating, and uranium/thorium dating confirm C-14 dates for humans at the last ice age (i.e. about 21,000 years ago for the glacial maximum) within 20%. If the canopy had existed up to 4000 years ago this would not have been the case since all of the above, with the possible exception of thermoluminescence dating, are unaffected by the presence or absence of cosmic rays.
