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It’s all about density

Angus Gordon, New Zealand Tree Grower November 2008.

Hautapu Pine Products tree farm is situated on the north bank of the Orua river east of Apiti and adjacent to the Ruahine range. The majority of the property is a series of terraces and rolling land that are covered in outwash gravels from the adjacent Ruahine range and a considerable depth of volcanic ash top soils. It ranges in height from 600 metres to 900 metres above sea level and is an interesting example of the application of managed forests using regimes ‘outside the conventional square’ to target a particular product.

Round wood posts

It has been described by one of its directors as the ‘life after nigra’ solution. This is for the production of high quality, small diameter roundwood for Hautapu Pine’s post and pole production plant at Taihape. The ‘life after nigra’ comment refers to the fact that 10 years ago the majority of the round wood that they produced was from old Forest Service Pinus nigra plantings. These were being used by the new corporate owners of these forest estates at a prodigious rate. It was a premium resource that had taken between 40 and 50 years to grow, but was limited in its volume and was not being replanted. So the writing was on the wall to find a comparable resource grown by someone else, grow your own resource or miss out on quality roundwood feedstock.

In reality Hautapu Pine has adopted a compromise approach with the aim of approximately 30 per cent of their feedstock coming from their own forest farm, and the rest being out-sourced. They are targeting the roundwood post and pole market as opposed to sawn post material, with a reasonable quantity of their production currently heading into the grape industry.

High stocking rate

Their regime is highly stocked, with initial planting densities of 4,000 stems per hectare being planted in the first two seasons. From year three onwards the planting densities were dropped back to 3,000 stems per hectare, all of which were planted by a planting machine towed behind a tractor. On contours that were not capable of being machine planted, hand planting was undertaken, with a stocking rate of 2,500.

Their target diameters are a 150 mm diameter at breast height and 100 mm at 13 metres height at harvest, with a sheath of higher density restricted growth rings surrounding the bottom portion of the log. These regimes have no management after planting, apart from spraying for dothistroma.

Mixture of species

In all, 580 hectares of the property is to be planted for roundwood production. A further 100 hectares above 750 metres in altitude is planted in Douglas fir, 10 hectares in cypress, mainly lusitanica, and two hectares in Eucalyptus nitens as an experimental roundwood planting.

In the pine resource, a proportion of radiata hybrids has been included in the planting programme, and in the last planting season 12 different higher wood density clonal families have joined the mix. There is one more planting season to go before this property becomes fully stocked.

The one factor that must be noted about this site is its proximity to the Ruahine ranges and the effect that this has on the local climatic

conditions experienced there. Since the beginning of their planting programme back in 2000, this site has experienced two one in 50 year snowfalls, a one-in-100 year rain event, two major wind storms and the usual long winters with out of season frosts that can occur any day of the year.

Matching the resources

The observations that can be made about the relative success or failure of any of the planting here is that it is similar to many other farm plantings in the Middle Districts region. The first,and probably the most important, is that for trees to stand upright and remain growing, an unimpeded and relatively free draining root zone is needed. Over the majority of this property the soils consist mainly of volcanic ash lying over alluvial greywacke gravels. In most cases these are extremely well drained, and in general there is a good match between the soil resource and the intended land use.

However, in a few cases this is not been the case. One example is on the terrace adjacent to a gorge where the eucalypts were planted. It had previously been a damp pasture site with a component of moisture loving species. Before planting it had open drains pulled through with an excavator alleviating much of the water ponding issues. However it has not appeared to lessen any perching of water tables that exist due to imperfect subsoil drainage.

The trees planted at 1.5 metre spacing withstood two snow storms very well with no damage and had grown to approximately 10 metres in six years when the August 2008 easterly wind storm struck. The result has been almost total devastation due to uprooting of the root plates. This appears to be due to a restricted root penetration into the subsoil zone, the relative lack of height of neighbouring trees being half that of the eucalypts, and the exposure of the site to the easterly wind. Gusts reached 150 km an hour at ground level in other parts of the district.

Toppling

A second example is where there was considerable toppling of radiata pine from heavy snow on a damp site with shallow top soil and a sedimentary sub-soil with restricted drainage. This is a common occurrence in all of the elevated areas in this region that have compacted mud stones and silt stones as the parent rock of their sedimentary soils.

The common factors for tree toppling are −

  • Relatively fertile sites that had been farmed
  • Restricted drainage of sedimentary soil types and their plastic nature when wet
  • Spray releasing, removing the reinforcing effect that the roots of competing species would have had on the topsoil had they been present
  • The young age of the stand with little mutual protection being offered by adjacent trees
  • Wind, snow or wind and snow in combination.

This toppling issue is experienced by many exotic species other than radiata pine. It would appear that it is the inability of these species to sink their anchoring roots into anaerobic sub-soils on ex-pasture sites and their fast initial growth that leaves them vulnerable to toppling before canopy closure.

Return after 15 years

This property is an example of work in progress, in terms of optimising tree genetics and stocking rates, and it will be interesting to see what the mix evolves into. One important thing to note is that the return comes in 15 years after planting, which is rare in terms of conventional forestry regimes. This has considerable advantages in terms of the return on initial capital investment. It is an area where there is real potential for farm foresters to adapt and evolve their regimes and their own species mixes to achieve earlier returns for themselves than − plant, prune, thin and then harvest at 30 years.

However the key is to find a particular market and then provide the requisite wood quality needed for the intended end use. For many farm foresters it would be a case of integrating up the supply chain towards the end user and stepping into fields of expertise that they may not be familiar with.
In the case of Hautapu Pine they have integrated themselves down the supply chain to secure the feed stocks for their mill. They have made use of an already established supply chain of plant genetics and skills, which is probably easier but more capital intensive.

This property will be reported on in the future I am sure, and it will be a case of watch this space and learn.

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