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About Husqvarna
The
Husqvarna Group is the world's largest producer of chainsaws,
lawn mowers and other petrol-powered garden equipment such as trimmers
and leaf blowers, as well as one of the world's largest producers
of garden tractors. Husqvarna is also one of the world's largest
producers of cutting equipment for the construction and stone industries.
The product offering comprises equipment for both consumers and
professional users.
Husqvarna Outdoor Products,
PO Box 76-437, Manukau City, Auckland
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The Riverhead factor – a practical view of wood density
New Zealand Tree
Grower May 2007
Ross Bawden
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.
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.
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.
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.
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.
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.
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.
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.
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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