Greetings Lauren. To best answer you question, I think a little refresher in tree physiology is required.
Tree diameter growth is attributed to cell division within the “cambium”. The cambium is a thin layer of actively dividing cells found directly behind the inner bark of the tree, or “phloem” – this is where carbohydrates (sap) are transferred up and down the tree. To the outside of the phloem is the outer bark (dead phloem cells) of the tree. Phloem = Bark.
To the inside of the cambium are living cells called sapwood, or “xylem” – this is where water and nutrients are transferred from the root system to the photosynthetically active part of the tree – the leaves. Each spring in temperate climates, new xylem cells are added by the cambium, and new wood is formed (sapwood). Xylem = Wood. Annual rings consist of springwood (earlywood) – the lighter appearing ring made up of larger, thin-walled cells, AND summerwood (latewood) – the darker appearing ring made up of small, thicker-walled cells. Therefore, the cumulative effect of the lighter-colored springwood ring and the darker-colored latewood ring represent one year’s growth for trees growing in temperate climates (one-year worth of xylem growth).
So to answer your questions (1) the outer bark of the trees is not wood (xylem), thus should not be used to determine tree age, and (2) an annual growth ring is actually made up of two rings, the lighter-colored earlywood ring and the darker-colored latewood ring; thus, two rings in the middle of the trunk represent one annual growth ring or 1 year.
Hope this answers your question!
