Measuring radiata pine and forecasting yields
New Zealand Tree
Grower May 2007
Dave Crawley
Measuring radiata pine and forecasting yields has become a highly
specialised area and there is now an increasing array of software and
tools required to stay on the top of this game.
Importance of accurate yield forecasts
Realised and forecast forest values are both very dependent on log
yields by grade. Different log grades such as pruned logs, small
branched logs suitable for framing timber, larger branched export logs
and pulp logs are worth significantly different amounts. The volume and
mix of log grades actually recovered from a stand depends largely on
the nature of the stand, the age at harvest, and on the range and
priority of log grades cut. To get accurate forecasts, it is important
to get a good description of the stand, to model the stand growth
accurately through to the year of harvest, and to model log volumes
recovered by grade for a range of potential cutting strategies.
Stand measurement and yield modelling
The most reliable yield forecasts are based on stand measurements. For
radiata pine these usually involve measuring sample plots spread
throughout a well mapped stand. Stands close to harvest age often
warrant a high intensity sample with detailed tree measurements and
descriptions.
PF Olsen uses Silmetra Plotsafe to gather detailed descriptions of
sweep, branching, and other stem features. Plotsafe provides detailed
tree descriptions by measuring these features in separate but
overlapping layers. This data is modelled to forecast grade recoveries
for a range of harvest years and cutting strategies. YTGen includes
models that can apply growth information appropriately to individual
trees rather than spread growth evenly across all trees in the stand.
Younger stands often have lower intensity samples with less detail.
These samples may be collected as part of a separate mid-rotation
inventory. Atlas Forecaster is also used, to forecast yields based on
this type of data and is able to draw on individual tree measurements
of diameter, pruned height, and a sample of tree heights, to model
growth and forecast grade recoveries at the time of harvest.
Modelling log sonics for structural timber
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| The director ST300 provides a measure of
the speed of sound travelling between two probes in standing trees. |
Many New Zealand sawmills are paying significantly higher prices for
good structural sawlogs. Structural timber is largely used in building
applications and there are tightening regulations on how the final sawn
timber must perform, especially in terms of stiffness. Sonic testing is
emerging as the most reliable method for identifying the better,
stiffer structural logs. Specialised tools are required to
measure the sonic speed through logs, felled trees, and standing trees.
YTGen has just been extended to include models to forecast structural
log yields. These models were developed by the Wood Quality Initiative
and are available for their members. The models are most accurate when
based on measurements of stand density assessed from wood increment
cores taken from the stand, and standing tree sonic measurements.
Modelling pruned log quality
Clearwood is the wood without knots that has grown outside the central
knotty core after pruning. Factors that increase clearwood include
pruning on time to minimise the knotty core, and large, straight,
cylindrical logs to maximise the outer clearwood volume. The standard
measure used to indicate potential clearwood recovery is Pruned Log
Index (PLI). Higher PLI indicates a higher clearwood
component, and so higher value.
PLI can be estimated using forecasts of outer log dimensions, with
measurements taken at the time of pruning to estimate the defect core
dimensions. PLI estimation is fast and cheap, but it is only possible
where detailed pruning measurements are available. Actual PLI can also
be measured. This involves taking a representative sample of the pruned
logs and either putting them through a PLI sawmill trial, or cutting
them up into disks to measure PLI on site.
 Future
developments
It may soon become possible for remote sensing, for example from
satellites, to identify, locate and count individual stems at a
reasonable price. This has the potential to improve total volume
estimates and perhaps from stand stratification – segregating the stand
into smaller areas where the trees are similar – to improve volume
forecasts by grade, and volume forecasts for separate areas within the
stand.
It is already possible to use survey type equipment to partially scan
standing trees and record their shape electronically. If this data can
be captured and interpreted into a usable format at a reasonable price,
it could become valuable for refining tree shape equations, or as a
validation or even replacement for the current more subjective
manual tree description process.
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