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 The Riverhead factor – a practical view of wood density

Ross Bawden, New Zealand Tree Grower May 2007.

I grew up in Riverhead – at that time a small country area on the outskirts of Auckland. It had several claims to fame including Riverhead spuds and the site of the first real traffic jams in New Zealand. The most exciting claim to fame for me however was the reputation of the 40-year-old radiata pine from Riverhead Forest. In the late 1960s radiata pine had gained a reputation as a worthy substitute for the New Zealand rimu and totara, and for the North American Douglas fir. At the time these were still the predominant timber used in government buildings.

Small for their age

I frequented the forest during Scout camps and also used it as my athletics training ground. I frequently pondered on the poor quality trees which were much smaller for their age than ones I had seen elsewhere. It must be a poor forest, I thought. Even after the Forest Service started aerial top dressing, the trees still seemed slower growing, albeit much improved on the natural model. The fact that the trees had very small branches and very close growth rings only served to emphasis what an inferior forest it must be. My interest in forestry eventuated in my becoming a forest ranger in the Forest Service where I initially carried the reminder of the Riverhead radiata reputation.

Throughout the rest of the Forest Service era and on into the world of corporatisation, I was subjected to the economics of growing forests. In a nutshell, a good site, a vigorous plant and a silvicultural regime sensitive to rapid growth was the key to gaining approval for new plantings. Clearly Riverhead Forest should have been closed down with no replanting post-harvest. Indeed a no pruning regime has been in force for many decades due no doubt to the inferior ability to grow large pruned logs.

I recall that the Forestry Encouragement Grants must have also used the above as a basis. A large private project I proposed for a grant at Wainuiomata was turned down because it would at best meet only one of the above elements. I have marketed wood from the Wellington and Wairarapa area for the last 14 years. That area in Wainuiomata would have produced exceptional quality wood even on a lower stocked clearwood regime due to its relatively poor site and inferior growth rates. It would have certainly out performed a number of other areas that I have been involved with in the southern North Island which were approved at the same time.

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The guidelines

So what are the guidelines for ensuring that you can grow a forest that will be ‘fit for purpose’ going forward under the new lumber grading rules? If you read the textbooks on New Zealand radiata pine, the rule of thumb is – the further north and closer to the coast the higher the density. Unfortunately this is quite misleading.

As a log marketer since 1993, I have spent countless hours studying logs being processed through sawmills located throughout the North Island. While I have not spent too much time directly testing log stiffness attributes at the stump, I have been able to form opinions based on my observations and on the feedback from various mills.

In general the larger the log and the wider the growth rings, then the higher the chances of that log producing low stiffness timber and inferior clearwood. This is irrespective of the forest location in the North Island, although it is probable that the base line density levels improve as one travels north.

In the southern North Island we restrict the marketing and harvesting jobs we accept to those that will produce the required quality for framing wood where the intended purchaser is aiming for that product. We are fortunate that we have an average age well in excess of 30 years in such stands and it is therefore no surprise that we have few issues based on stiffness.

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The real test

The real test however is to return to that low quality forest at Riverhead and the surrounding region. We have been harvesting several relatively large forest areas in the Greater Auckland region since 2000. As a result we have been able to form strong views based on feedback from the mills we supplied which we can relate to the actual location in the forest. In the following examples all are growing on low fertility, phosphate deficient Auckland clays, so we can assume that soil fertility is not an over-riding issue.

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Low stocking and large pruned logs

One forest was grown with great care under the low stocking and large pruned log regime of the day. Apparently very high quality S Grade logs, based on visual assessment of the log form and branch size, are simply not accepted by the three main domestic log purchasers in the area. It is not even a matter of price negotiation as the internal wood properties are simply ‘not fit for purpose’ in terms of framing.

In a second example, located very close to the first, we are dealing with a clearwood regime but with the stocking higher than the first example. This forest has consistently produced acceptable framing timber and also produces a higher quality pruned log, albeit significantly smaller in diameter.

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Unthinned forest

In another example we commenced supply from an area of 27- year-old unthinned forest. The sawmill undertook density testing and found the wood acceptable. We subsequently moved into an area that was production thinned ten years earlier. We were advised by the mill that the density had dropped below an acceptable level.

The only apparent change to the stand was the thinning which had resulted in faster growing wood.

To complete the picture, we harvested a large area of forest midway between Whangarei and Dargaville. I arranged density tests at age 24 which gave a mean of 452 – a very good result.  So is age not as significant as we thought? The reality is that it was grown on the Northern pulp CHH northern forests regime which resulted in higher stocking rates and with slower growth rates and therefore close growth rings.

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Instant feedback

Since we began we have marketed over 200,000 tonnes of structural grade logs on the domestic log market. These logs are routinely tested by the three main purchasers. While we do not get the full results, we do get instant feedback when the readings dip below the relevant acceptable level. There is a strong correlation between complaints and where in the tree the log came from, coupled with the physical attributes of the site.

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First log

We do not harvest pruned stands, therefore the first log is sold as a framing log where diameter, knot size and general quality attributes are met. This will typically be an S1 log – a log with a minimum small end diameter of 400 mm. In Dr Paul Kibblewhite’s terminology, the first log is subject to ‘funny wood’ and is renowned for its unpredictable wood quality attributes in terms of pulp production.

It would appear that the first log is the product most likely not to meet stiffness testing where other factors result in a generally lower stand quality. Where site and stocking factors are favourable, we seldom see the first log, or any log for that matter, as a problem.

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Site factors

It is very seldom that logs sourced from ridge tops or the upper slopes result in problems. It is almost inevitable that complaints can be pegged back to production from the flat areas. Superficially, stocking rates are higher on slopes whilst soil conditions and summer water availability are far superior on the flats. For whatever reason, the larger trees are found on the flat areas. Log density testing from trees sourced from ridge areas shows no significant difference between logs.

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All anecdotal evidence

To summarise – and again I stress that these conclusions are based on anecdotal evidence collected from quite large scale operations over many years. They are not based on any scientific study:

  • Fast grown trees, irrespective of geographical location, do not perform well in density testing.
  • Slower growing trees as a result of site factors or silvicultural practices generally have a sound density profile.
  • All other factors being equal, density does appear to increase and have less variability the further north the forest.
  • Individual forest variability can be quite large
  • The first log is likely to give the lowest value, however it will be quite acceptable when items 1 and 2 above are favourable.  

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