Lyly or compression wood
Finnish word lyly refers to the reaction wood of coniferous trees, called compression wood in English. It forms when the tree is subjected to mechanical stress, and helps to bring parts of the plant into an optimal position. This stress may be the result of wind exposure, excess of snow, soil movement, human activity etc.
Asymmetric growth is a reliable indicator of reaction wood. The cambium in the affected part of the trunk is more active on one side, leading to thicker growth rings. Branches practically always have reaction wood, since they need support to maintain their horizontal or nearly horizontal position.
There are two different types of reaction wood, which represent two different approaches to the same problem by different types of trees. In angiosperms reaction wood is called tension wood or vetopuu in Finnish (litterally ´pull-wood´). Tension wood forms above the affected part of the plant, pulling it up. It is composed almost entirely of cellulose. In conifers reaction wood is called compression wood, and it forms below the bent part, pushing it up. Compression wood is rich in lignin.
Reaction wood is usually undesirable in any commercial application, primarily as its mechanical properties are different: it breaks the uniformity of timber, and it also responds differently to changes in moisture. However, in many traditional applications lyly and vetopuu are greatly desired. For example ancient Finns used lyly for the left foot ski, while many Fenno-Ugrian peoples from Finland to Siberia used pine, fir or larch lyly for the belly of their many-wood laminated hunting and war bows. Birch or bird cherry tension wood was most likely often used for the back of these bows.
With some experience one can see the length, thickness, and straightness of lyly by looking and sometimes touching the growing tree. This is important in order to avoid unnecessary cutting of the tree.
If one is looking for lyly for bow belly, it is important that the compression side of the tree trunk is free of branches, and that the lyly straight, without any turns or spiralling. After felling the tree, lyly can be clearly seen thanks to its reddish colour.
In the old days, Finns and Sámi people would go to the forest and bind young pines in order to create lyly. This would ensure future supply of this important material.