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Annual rings visible in bark

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From: James Conner City: Saint Clair Shores, MI
In my neighborhood there are many very old, huge trees. There is a particular eastern white oak that is easily six feet in diameter. I was examining it yesterday and broke off a piece of bark that was over two inches thick and I could count over 20 layers. Are these annual rings? I thought annual rings were encapsulated IN the tree. So how are these layers growing OUT from the tree? Thank you. Best and cheers, Jim
	Extension Message From: Jay Hayek Extension Specialist, Forestry Department of Natural Resources and Environmental Sciences jhayek@illinois.edu
Hi Jim: Those "layers" you have identified are a combination of outer bark (cork), cork cambium, and secondary phloem. Those layers should not be used to estimate tree age. Wood, on the other hand, is comprised almost exclusively out of secondary xylem. An annual growth ring is comprised of early wood (spring wood) and late wood (summer wood). Each annual growth ring for temperate woody species is considered as one year's worth of growth. Best, Jay
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