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Science based redwood clone selection

Wade Cornell, New Zealand Tree Grower August 2012.

Redwood has become a bit more popular as a species to plant in recent years. If your aim is to plant redwood as erosion control on a hillside and never consider harvesting, or if the trees are being planted purely as a carbon forest, the wood quality of harvested timber will be of little or no interest to you.

However, growing redwood for timber means you have to consider redwood’s two most important wood properties – stability and durability. Without these there is unlikely to be much of a market for the timber when it is harvested.

Clonal redwood selection should be for stability and durability, and fortunately scientific protocols exist for making good selections. However, some tests are rather expensive and there is still much work to be done to make good selections for growing redwood clones.

Stability of the wood grown in California and New Zealand is generally acceptable, but cannot be taken for granted. Estimates are that between one and two per cent of redwood genotypes have unstable wood. Professor Bill Libby was very unhappy when his top performing clone from the Russell Reservation, which included all the Kuser clones, tested as being unstable. Bill Libby and others have observed some New Zealand grown redwood which is unstable.


Durability is a much trickier devil to tie down. Class one durability in redwoods which are less than 30 years old is rare, but this is the age at which most New Zealand plantations are expected to be harvested. Redwood timber becomes more durable with each successive formation of year’s heartwood.

In California, logs are at least 60 years old when harvested, but even then may not be durable. Darker coloured heartwood is used as a convenient guide to grading for durable boards coming out of a mill. Unfortunately this does not equate to absolute durability, especially in younger redwood.

A scientific trial, in which 14 redwood clones were tested at Scion by soil block method, also compared depth of dark or red colour to see how accurate this might be in assessing potential clones. The two darkest genotypes rated lower than some of the lighter coloured ones. If this is typical, then a large number of clones chosen by just looking at the colour without scientific testing, are likely to be deficient.

More trials

A recent study using the same clones grown in California and New Zealand has shown inconsistency, with less durability in New Zealand. This was a very limited trial and requires further work, especially to understand how and why this variation might be happening. It is unknown how much variation there could be between sites within New Zealand. This further complicates any potential assessment of not only what may be durable, but what growing conditions confer durability and consistency between regions.

A blueprint for success

Redwood clones selected for good growth and form in California have not necessarily performed well in New Zealand. Adaptation must be checked in various New Zealand environments by putting them into trials with known controls. This is the only way we can say with confidence that a specific clone will grow and perform. Some clones may have been sold without these necessary trials.

It is important for all clones in trials to have a control. A control is a known clone with wide adaptation and good growth and form which gives a basis for comparison. A good control is H-16-1, also called clone number 23, a Kuser clone which meets this criterion. Without a control there is no basis for comparison.

Older redwoods growing in New Zealand which are considered clone candidates must be put into secondary trials with controls. Many New Zealand redwoods are suspected of being inbred. Without a basis of comparison you could look at an older redwood and think it is grand, but not realise that a well selected clone could grow at twice the rate.

Seedlings vary

Seedlings are enormously variable, with the majority having relatively poor growth and form. Selecting clones from New Zealand grown seedlings requires duplicating by cloning and planting them out in various environments to check adaptation. The tree may grow well in one location but are they going to grow well in many other areas?

Beat the odds. If planting seedlings for clonal selection, collect seed from only the best trees that grow in areas which are most likely to be a similar environment. For New Zealand that means areas without fog and which can have hot dry periods. Southern and inland areas of California are the best match. Having controls in seedling trials starts you off with a good basis of comparison.

Scientific tests

Stability, durability and density need to be accurately and scientifically tested using the same clones from multiple sites around New Zealand. This is the only way in which we can say whether all of these wood properties are consistent between environments, or at least which clones may be suited to specific environments.

After selecting for growth and form, all pertinent wood properties, variations between different environments and other preferences clones can be selected based on preferences of specific markets. Western red cedar replacement is probably our largest potential market.

Choosing the clone

To choose a clone, according to Bill Libby, you would first look for excellent growth and form. This may be only one out of 20. Then apply other factors which are genetically linked and could be important to our market and conditions. As mentioned above, these factors are not necessarily the same as California’s and variation can occur with different environments. Each of these factors acts as a multiplier against your original one in 20. In other words, you may need to screen thousands of genotypes before you would satisfy every condition on this list.

A list for New Zealand should include –

  • Wood properties scientifically tested to ensure stability, short rotation durability and density appropriate for specific uses
  • Good performance in New Zealand’s windy conditions
  • Single leader replacement, important due to wind and other potentially damaging factors such as insects
  • Low incidence of epicormics or branches growing back when pruning for clear logs

Untested clones

To the best of my knowledge, it is hard to find clones currently sold in New Zealand which meet the basic criteria of having undergone scientific testing for critical wood properties, or have had their wood properties successfully tested in a variety of New Zealand environments. However it is possible that some enterprising nursery could do this very soon.

Current clone growers may have some of the right clones, but this will require steps to help find those clones. Have your clones tested – see Tree talk below – for ways in which you can collectively find solutions. You may also consider converting your current plantation by inter-planting, grafting or replacement with fully tested clones when they become available. If you intend to plant redwood clones, do not be afraid to ask for proof of scientific testing performed by a reputable independent lab.

Wade Cornell has grown redwoods since 1987 and with Bill Libby, advocated redwood trials in the mid 1990s.


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