Plantation silviculture at the crossroads
New Zealand Tree
Grower May 2007
Euan Mason
What do you think of New Zealand? The classic question reputedly asked
of so many visitors has provoked a variety of answers. John Lennon
replied that after 10 minutes on the ground he thought the inside of
the terminal was just fine. I returned to New Zealand with a freshly
minted forestry degree in 1975 after a globe-trotting childhood. Dave
Elliott, then principal forester at Kaingaroa, waited a couple of weeks
before posing the forestry equivalent question – What do you think of
forestry in New Zealand?
I did not hesitate with my reply – You are not fully using the site.
Unique silviculture
There followed a careful explanation of what, at the time, made New
Zealand plantation silviculture unique. New Zealand silviculturalists
trade volume production for value in a highly productive environment,
leading them to unusual silvicultural strategies with very low stocking
rates. I explained this to an audience in the southern United States
last year. When I got to the part about pruned regimes on low quality
sites with final crop stocking rates of 80 stems per acre, a voice in
the back of the room exclaimed that was ridiculous. Many people in the
United States still equate volume with value.
Until recently, value in New Zealand was defined by large scale log
geometry, with small defect cores, large stem diameters, small branches
on unpruned logs and minimal sweep. All this was paramount in the minds
of decision makers as they crafted their silvicultural regimes.
Farm foresters leading the way
Some researchers point out that radiata pine was never intended to
produce structural products, and that the features that make it an
excellent finishing timber reduce its utility as a structural one.
Others would prefer that we switched to alternatives such as eucalypts
for producing structural wood. They may be right, and perhaps farm
foresters who frequently experiment with alternative species, will lead
the way.
New objectives for growing plantations cause us redefine silvicultural
strategies, and make silvicultural stand models obsolete.
Research at the School of Forestry aims to support forest growers as
they decide how to manage their future crops in the face of changing
objectives. If we also consider influences of climate change and carbon
sequestration, then our silvicultural strategies will change more.
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| Intensive pruning and low stockings are
typical |
New definitions
We have witnessed a dramatic change in focus over the last few years.
Late last year I took a group of forestry students on a trek into the
wilds of the Maruia, to the mainland island at St Arnaud and then on to
Nelson. Here they learned about forestry from staff at Rayonier,
Weyerhaeuser and Nelson Pine International. Nelson Pine sets a velocity
limit for logs, measured with a resonance acoustic device, of 3.1 km
per second. Velocity is correlated with wood stiffness, an essential
feature for structural lumber and wood composites. New definitions of
value require changes in silvicultural thinking and new kinds of
assessments. The initial plea from log buyers has been for longer
rotations.
Older trees, it was reasoned, would contain lower proportions of core
wood – wood growing close to the pith. Compared to outer wood, core
wood has lower density, higher microfibril angles, higher gradients in
the microfibril angles and consequently higher gradients in
longitudinal shrinkage. These features make the wood less stiff, and
less stable during drying, than outer wood. However,
long rotations come at a cost, and so there have to be better ways to
improve value.
Breeding the answer?
Breeders began to measure density and log velocity, thought to be a
measure of average microfibril angles, noting that many wood properties
are highly heritable. Breeding and perhaps clonal forestry can be part
of the answer, but silviculture is at least as important as genotype.
Arturo Bascunan, a Chilean student at the New Zealand School of
Forestry, showed that stems closer to stand edges had lower velocities,
suggesting that sway may influence microfibril angles.
Another Chilean forestry student demonstrated that higher initial
stockings of radiata pine promoted growth of core wood with higher
acoustic velocities, and more recently Matt Waghorn, also a forestry
student, mapped log velocity up stems of different breeds at a range of
stockings. He noted that while the first two metres of a stem often had
the lowest velocity, the section from two to four metres above ground
level often had the highest velocity.
Presumably, if high velocity logs are sufficiently more valuable than
lower velocity ones, managers might increase crop value by lopping off
the lower sections of stems before buckling. Mike Watt of Ensis noted
that stem slenderness was often related to velocity at breast height,
and pointed to a functional relation that might compel a slender tree
to become stiffer in order to prevent stem buckling. Matt’s results
appear to show that the ‘avoidance of stem buckling’ hypothesis cannot
explain patterns of log velocity observed up stems. Some of my personal
research indicated that distance to canopy might also influence
velocity, corroborating some theories of P R Larson from the 1960s who
suggested auxins exuded by new buds and foliage might influence
microfibril angles and therefore velocity.
No interaction
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| Stiffness of clones measured by acoustic
time of flight versus initial stocking at Dalethorpe, Canterbury
|
One fascinating theme running through all our work so far is that
genetic and silvicultural effects on velocity do not interact – they
are additive. Whatever genotypes are doing to alter log acoustic
velocity, it is unlikely to comprise the same processes that drive
silvicultural effects.
These theories do not explain why wood grown in Canterbury lacks
stiffness. In addition, some of my research found that competition from
grass in Canterbury resulted in lower stiffness, unlike between-tree
competition which increased
stiffness. For a possible explanation we need to turn to some research
conducted in the Pacific Northwest by Domec and Gartner. They compared
effects of core wood formation on structural stability with effects of
core wood on plant water relations. They showed that some important
features of core wood allow trees to minimise
cavitation – the entry of air into their water transport systems high
in a stem. Dry regions like Canterbury may grow less stiff wood because
water stresses are high.
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| Log velocity versus height and stocking for
19-year-old radiata pine on a dry site in Canterbury |
Why chase explanations
Why are we chasing all these explanations? Partly for the joy of
discovery, but mostly because forest growers are asking two questions
in the face of changing definitions of value for structural logs –
- Where and which are my high velocity stems?
- How do I create higher velocity ones in future?
They need models that represent where their stiff wood is and decision
support systems that give silviculturalists feedback about the nature
of the wood they are creating. Moreover, in my view we need models of
wood properties at a ring level, because gradients in microfibril
angles and density affect stability of sawn lumber.
Why not simply include measured effects of stocking and genotype
directly in our models? The reason why this might be misleading is that
many explanatory variables are collinear in stocking experiments, or to
put it more simply, they vary together. For example as stocking
increases then radial growth rate drops, canopies move up stems more
rapidly over time, wind loading on each stem is less and stems become
more slender.
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| The trees in the centre grew slowly because
of competition from grass but had the least stiff wood |
Much to learn
Suppose we build a ring level model of microfibril angle development
using radial growth rate as an independent variable, and a manager
decides to reduce core wood microfibril angle by retaining weeds on a
site, thereby restricting radial growth. It would make sense, and the
model would indicate that log velocity should increase. However, in my
experiment at Dunsandel, trees subjected to the most competition from
grass had the lowest velocities. Clearly there is much we need to learn
before we can produce robust silvicultural models that include log
velocity outputs, let alone estimates of stability during drying.
When we produce such models, we will need a structural log index
equivalent in function to Jim Park’s pruned log index that relates
measurable log features to log value in a structural log market. Log
velocity might give us an index of stiffness, but is it adequate as an
index of stability during drying? We clearly need log and branch
dimensions in the structural log index, and I suspect we might need to
add something more elaborate. For example, the volume weighted mean
gradient in microfibril angle within 50 mm segments of radius within a
log, given that much structural lumber is cut to 50 mm by 100 mm.
Euan Mason is Associate Professor,
New Zealand School of Forestry, University of Canterbury
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