Why what we grow matters to you

As Espen Ribe of Södra Innovation explained at our recent Fibre Education get-together, the fibres differ between species, growing locations, and between trees from the same location depending on whether the tree is dominating or suppressed by neighbouring trees. A dominant tree will have more thin-walled fibres since it will be growing faster and taking light from others around it, and altitude and latitude of the site also affect average fibre dimensions.

Even fibres within the same tree are different, depending on where in the tree the fibres come from. Fibres in a young tree differ from those in an older tree. For softwood the fibres from the outer part of the log are generally longer and coarser than fibres from the inner part.

Another important thing is that the fibres formed in a softwood tree also have seasonal variations. During winter there is no growth. In the spring the tree needs a lot of water and nutrients and produces fibres with low fibre wall thickness and a large lumen (the inner part of the tube-like structure) to maximize transportation capability. In late summer and autumn the trees prepare for winter and produce more narrow fibres with higher fibre wall thickness.

Fibre length is important since longer fibres form more bonds to other fibres, making the final sheet stronger. Likewise, the fibre cross-section affects the final product since a fibre with a thick fibre wall and a small lumen performs quite differently in a sheet than a fibre with a thin fibre wall and a large lumen. The amount of fibres per gram will be higher with thin-walled fibres, which enables more fibre-fibre bonds. Furthermore, the thin-walled fibres collapse more upon drying, which increase the load bearing fibre segments in the final sheet and therefore for instance tensile strength.

This kind of detailed knowledge comes into its own when it is time for harvesting, with Södra ensuring it has enough of different raw materials to meet its customer needs. Once the trees are harvested, knowing exactly what properties specific fibres will impart to the final sheet leads to careful segregation and mixing of recipes.

Ribe: “We keep trying to find new ways to enhance our knowledge but I do feel quite proud of what we’ve achieved so far. Generations of sustainable forestry combined with the technical work we do at our mills and in our labs means we are able to really tailor the fibres we have to achieve specific results for our customers. We learn more about the forest and how to use it every day and that’s really quite a privilege as well as an opportunity.”