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| Ministry of Forests and Range |
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| Inventory
Projection |
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Tree Volume Estimation
Gross Volume
Gross tree volumes are based on the taper equation developed by Dr.
Tony Kozak. Taper equations were adopted as the standard for gross volume
estimation due to the ease of determining volumes of stumps, logs and
tops of any dimension. Inventory samples such as VRI ground samples,
permanent and temporary sample plots (PSPs, TSPs) use the 1994 biogeoclimatically
(BEC) based taper equations and there are coefficients available for
most species at the BEC zone level. Timber cruising for appraisal purposes
uses taper equations based on the old system of forest inventory zones
(FIZ).
Net Volume
Deductions are made for losses due to decay, waste and breakage. There
are two different systems in use to estimate loss for inventory purposes:
net factoring is used in VRI ground samples and the biogeoclimatically
based loss factors are used in the pre-existing permanent and temporary
sample plots that are in turn used in the fitting and calibration of
stand level loss models used in the VDYP7 growth and yield model. The
1976 FIZ and PSYU based loss factors are used in timber cruising for
appraisal purposes.
Decay Loss
Volume deductions are made for all wood that is weakened by decay. This
includes all advanced and final stages of decay and some forms of incipient
decay.
Waste Loss
Volume deductions are made for non recoverable sound wood associated
with logs or trees that have less than 50% sound wood volume.
Breakage Loss
Breakage is an estimate of felling and yarding damage and refers to
sound wood volume not merchantable due to broken pieces too small for
economical handling.
Net Factoring
Net factoring is the process used by the ground sampler to estimate
the amount of sound wood less decay on a log-by-log basis for a tree.
Allowances for decay are made only for visible decay in terms of consistently
measurable or detectable features.
Loss Factors
Both the (1995) BEC based and 1976 loss factors are based on similar
data and models. The BEC based factors represent an improvement on the
1976 factors due to its biological basis to the stratification, the
larger datasets and the use of regression in the fitting of the model.
Trees are grouped by risk of decay as evidenced by visible pathological
indicators. The percent loss deductions amount to average percent decay
by species, age class, dbh class and risk group and are based on empirical
data collected from destructive sampling. Both sets of loss factors
are considered to be biased due to subjective location and tree selection.
Destructive Sample Database
The Ministry of Forests and Range manages a database of
over 85,000 destructively sampled trees collected across the province
between the early 1950's and the present time. All trees sampled prior
to the early 1990's were subjectively selected but since that time,
all sample trees were selected without bias and with known probabilities
of selection. The dataset consists of two types of trees: 84,000 trees
were sectioned into 2 to 5 m long sections for decay measurements and
35,000 trees were used for taper measurements. A number of trees were
used for both taper and decay measurements. The taper equation was fit
using the taper sample trees and undamaged decay sample trees. Data
collection is ongoing in the form of Net Volume Adjustment Factor sample
trees.
Statistical Adjustments
Since the estimates of gross and net tree volume are obtained from models
and tools based on subjectively selected sample trees, they contain
an unknown amount of bias in their volume estimates. Unbiased sampling
can be used to correct the volume bias through statistical adjustments.
Net Volume Adjustment Factor (NVAF)
The VRI uses the NVAF to correct for the all the inherent biases in
the tools used to estimate net tree volume with the exception of breakage.
This includes the error in the gross volume estimates produced by the
taper equation, the sound wood estimates from net factoring and for
cruiser measurement error. The NVAF is mandatory for a VRI and is calculated
from an unbiased minimum sample size per management unit of 55 live
and 5 dead destructively sampled trees.
Statistical Taper and Loss Factor Adjustments
BEC taper equations and the decay and waste loss factors have been statistically
adjusted where unbiased destructive sampling has been conducted. Where
VRIs have not been conducted, there remains a need to correct biases
in tree volumes for timber appraisals and timber supply purposes. Data
requirements are higher than an NVAF sample and can amount to 100 to
200 sample trees per species. Statistical adjustments are available
for the Queen Charlotte Islands, the Interior wet belt and the Golden
TSA. The sample trees used for NVAF are also available for this purpose.
Software
The interactive and batch tree volume compiler program, TREEVOL, compiles
gross and net tree volumes the using BEC taper and loss factors at a
variety of utilization specifications.
Software can be found here.
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