Annual Rings and Wood Types
When you cut through the trunk of a tree, you see a pattern of concentric circles. Those circles are annual rings (also called growth rings). Each ring tells you one year of the tree's life.
Why do rings form?
A tree grows outward, not upward. Just under the bark lies a thin layer of living cells called the cambium. The cambium divides to produce new cells — xylem (wood) on the inside and phloem (bark tissue) on the outside. This is how the trunk gets thicker.
But the cambium doesn't work at the same speed all year. In temperate regions, seasons drive the rhythm.
Spring (early wood): When spring arrives, plenty of water and nutrients are available. The cambium produces large, thin-walled xylem cells. These cells have wide cavities (lumina) and conduct water efficiently. This layer looks lighter and less dense.
Autumn (late wood): As summer turns to autumn, water becomes scarce. The cambium produces smaller, thick-walled xylem cells with narrow cavities. This layer is darker, denser, and mechanically stronger.
One annual ring = one spring wood layer + one autumn wood layer. The boundary between one year's autumn wood and the next year's spring wood is sharp, which is why you can count the rings.
In tropical regions where seasons are less distinct, trees may not form clear annual rings. Some tropical trees form rings tied to wet/dry cycles, but they are not always reliable for age determination.
What the rings tell you
Count the rings from the centre outward — that gives the tree's age (approximately). The width of a ring tells you about that year's growing conditions: wide rings mean a good year (plenty of rain, sunlight), narrow rings mean drought, disease, or competition.
Heartwood and Sapwood
Now look at the cross-section from centre to bark. You'll notice two distinct zones.
Sapwood is the outer, lighter-coloured wood. It contains living cells and conducts water and minerals from the roots to the leaves. Sapwood is functional — it's the tree's plumbing.
Heartwood is the inner, darker wood. As the tree ages, the older, inner xylem cells stop conducting water. They fill with resins, tannins, oils, and other organic compounds. These deposits make the wood darker, denser, and more resistant to decay and insect attack. The heartwood is dead but provides structural support. …