Tree Form Size, Compression Wood and Knots

TREE FORM SIZE, COMPRESSION WOOD AND KNOTS 

Rightly, initial tree improvement programmes plaeed partieular emphasis on fast growth, forest health and good tree form (straightness of stem and the lightness and frequeney of branehing). 

Large knots drastically reduce strength and are a major cause of downgrade in lumber. They are undesirable in fibre products. Knots are very dense, some 2-3 times as dense as stem wood and are frequently above 1000 kg m . This is a consequence of compression wood formation and heavy resinification after mortality or pruning resin can account for 30% or more by weight of knot wood. Knots are hard to penetrate with chemicals and are inadequately pulped, while in mechanical pulping knots are resistant to defibration. 

The significance of branches - and knots in lumber - is out of proportion to the percentage of stem volume that they occupy. First, trees experience asymmetric loading from heavy branches such that compression wood is observed often in the immediate vicinity of knots (usually in streaks extending below the branch). The second effect arises from the stresses in the immediate vicinity of the branch or knot. 

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