DELICATE ENGINEERING IN MASSIVE TREES

Looking up it is a wonder how the huge trees support massive branches sticking out many meters from the main trunk.  It is all done by balance.  The large branches, several tonnes worth, seem like they should overwhelm the trunk.  But it can be understood if this weight is thought of in terms of two forces. And the forces are separated into horizontal and vertical components.

The vertical component, magnified by its distance from the centre, is directed down the trunk where the dead central cells easily support compressive loads.  The remaining horizontal component is directed toward or away from the trunk and pulls the tree one way or the other, but its effect is more or less balanced out by similar forces from other branches.  So, just as a boat can be balanced on its keel by supporting timbers, so can the trunk support massive branches.  To stay balanced it only needs to counteract the sum of the horizontal pull of the branches.

This balance is done by the growing outer ring of wood in the trunk.  This horizontal pull is a dynamic force, changing with the wind and shifting as the branches grow and sometimes fall. If the tree is on a hill there is a tendency for increased force in the direction of fall.

This outer wood contains fibres under tension like tent guys.  This tension is visible sometimes when milling a log.  Cutting into the upper surface, you find  the timber will peel off  and curl up like a banana.  And the rest of the log, left overnight, will form a hump as the fibres on the opposite side contract.Pureora 08 (Medium) Pureora 16 (Medium)

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