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Pruning Redwood in New Zealand

A forester is shown pruning the redwood trees – second lift

June 2014, Simon Rapley, General Manager – New Zealand Redwood Company –

The forest industry in New Zealand is based on plantations of exotic softwoods, grown on rotations of between 25 and 45 years.

A significant proportion of the national estate is pruned to improve the value of the lower logs. Trees are generally pruned to about twenty feet so the first log will yield clear lumber.  Foresters weigh up the improved value of the crop due to pruning against the cost of the silvicultural treatments and the cost of carrying that expense through to harvest time. If there is an estimated net benefit, a decision to prune can be made.

Branches are generally removed with heavy duty loppers, and ladders are used to reach to twenty feet or more up the stem.

Simplified log for illustration purposes

In this simplified diagram (right) of a pruned redwood log, the center cylinder (red) contains small knots produced by the tree before it was pruned. The white hollow cylinder is produced after the branches are removed and is clear heartwood. The outer hollow cylinder (dark red) is clear sapwood.

Large areas of mostly Monterrey pine were harvested in NZ in the mid-1900s. The clear grades of lumber were sold into high value markets in Japan and America where these grades were traditionally cut from old-growth forests or stands of large diameter trees. Clear redwood lumber may fill the demand created by old-growth redwood which is now almost unavailable.

Clear heart on the left of the wall and clear commons on the right

Clear New Zealand redwood heart on the left of the wall and clear commons on the right result from pruning

The photograph on the left shows the lumber yielded by pruned redwood forests in New Zealand. Clear heart on the left of the wall and clear commons on the right.

Risk Management

NZ redwood stands established at low stockings (Approximately 400 stems per hectare) will have larger branches than naturally regenerated stands in California. Lower branches will start dying at approximately age 15, and because of their size, will most likely persist until harvest age. This means that unpruned lower logs will yield largely lower grades of knotty lumber.

This photograph shows insect entry through a dead branch and subsequent attack on the stem

This photograph shows insect entry through a dead branch and subsequent attack on the stem

 

 

 

There is some evidence of insect damage to redwood logs in NZ. It appears that boring insects enter the trees through unpruned dead branches, and can cause damage to the wood in the stem of the tree. There are also instances of secondary rot in the logs that have been attacked by boring insects.

Removing the lower branches before they die eliminates the entry point for wood boring insects.

Because of the improved value of the logs and lumber due to pruning and the mitigation of the risk of pest problems, most of the redwood growers in New Zealand (including The New Zealand Redwood Company) are pruning their forests so a twenty foot pruned log can be cut.

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