In Finland, the average conifer growing season is around 100 days. The growing season begins in the spring, when the amount of light increases. During this time, large and thin-walled tapered cells conducting water, known as tracheids, emerge in the layer close to the surface of the tree. These new layers that form early in the spring later show up as light rings in the cross section of the tree.
The tracheids that form later in the summer are smaller in size and have thicker walls than the spring cells. Summer tracheids create darker tissue that supports the tree’s structure. Together, the light springwood and the dark summerwood form the annual growth ring.
Summerwood is denser than springwood. The elasticity and resistance of wood increases with density. Tensile strength also depends on density. In the case of pine, summerwood has a tensile strength six times that of springwood.
Longer version of this article was originally published in Timber Magazine issue 2022-2023.