Virtual Museum of Canada
Jardin botanique de Montréal 
Centre for Forest Research

Dendrochronology

Good Answer

In spring, the layer just under the bark, the cambium, rapidly produces large cells in order to transport sap. As summer progresses, growth slows progressively. The tree then produces smaller cells with thicker walls, which look darker, to support and reinforce the trunk. The alternating pale and dark zones create what we call a growth ring.

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Growth rings

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