Spiral compression wood is a rare phenomenon that has been reported only about a dozen times in stems of conifers. The spiral compression wood in a stem disk of Scots pine (Pinus sylvestris L.) from northern Sweden is illustrated with macro- and microphotographs of the wood structure.
The diameter of the stem disk is about 210 mm and it contains 236 growth rings. A dark band of compression wood spirals clockwise from the pith towards the cambium. The formation of compression wood has begun when the tree was 23--25 years old and had a diameter of 40 mm. The number of revolutions is six and each of them had required about 5--12 years. The width of compression wood zones decreases from 8 mm to 2 mm from the pith the to bark. There is no clear change from sapwood to heartwood. The diameter of heartwood is approximately 120 mm.
The stem is extremely slow grown and the width of growth rings varies between 0.05--1.40 mm. The growth rings in the dark spiral wood zone are 0.6 mm wide on the average. The tracheids in these wood zones are typical of compression wood: they are round in cross-section; cell walls are thick (double cell wall thickness: lumen diameter = 0.72) and there are deep helical cavities in the cell walls which can be seen easily in the radial section. The tracheid length varies from 2.2 to 2.4 mm which is shorter than in the normal wood of Pinus sylvestris.
The light wood zones between the dark spirals have very narrow growth rings which are less than 0.1 mm in width. The narrowest rings have only 24 layers of earlywood tracheids and one or two layers of latewood tracheids. Interestingly, growth rings in these zones show also wound tissue with tracheids of irregular shape.
The reason for the formation of spiral compression wood remains unknown. Apparently, the stem must have been inclined and has performed a circling movement, completing six revolutions within 160 years. Accumulating snow may have caused the stem to bend. Wound tissue in the narrow wood zones may have been caused by mechanical injury.
Key words: compression wood, Pinus sylvestris.
Correspondence: Rudi Wägenfuhr, An den Teichwiesen 4, 1109 Dresden, Germany
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