The International Tree-Ring Data Bank is maintained by the NOAA Paleoclimatology Program and World Data Center for Paleoclimatology. The Data Bank includes raw ring width or wood density measurements, and site chronologies (growth indices for a site). Tree-ring measurement series from other parameters are welcome as well. Reconstructed climate parameters, including North American Drought, are also available for some areas. Over 2000 sites on six continents are included.
In .crn ASCII format (and other) documented at .
From .gov site, and US government department, so one would assume a fair amount in the public domain. However, data is aggregated and includes non-US sources, so this should be clarified. (see also the related ghcn package)