[IUFRO RG 7.03 FORENT] Fwd: Dark rings in red oak logs

Any idea what would cause dark rings in red oak logs as in the attached pictures? The dark rings run the length of the logs. So far this has been seen on a number of sites in 3 or 4 counties in Minnesota. Not all red oak trees on a site have the dark rings. My first guess is that it is a barrier zone resulting from a wound or injury of some type. However it seems strange that it forms a complete cylinder that reaches the ends of the logs. I didn't see any logs where the dark ring only went part way around a log. Also there were some logs that had obvious injuries with decay and discoloration but these logs did not form rings or even have distinct barrier zones going even part way around the log. I've only looked at logs like this while in the mill, not on the site where they were harvested. At first, the thought was that these rings were the result of something happening 16 to 17 years ago. This will take more investigation, but it looks to me like the time period varies anywhere from 6 to 35 years. Also at this time I can't say if the time period varies ibetween sites, or if the time period varies from log to log on the same site. Any suggestions would be appreciated. Mike Albers Forest Health Specialist MN DNR-Forestry 1201 E. Hwy. # 2 Grand Rapids, MN 55744 (218)-327-4115