In the BCFS analysis, existing volumes were estimated and projected using the Variable Density Yield Prediction (VDYP) model. This is a generally accepted yield model based on many sample plots throughout BC. However, a recently completed statistical inventory audit for the TSA found that, for the TSA taken as a whole, the mean value of the mature timber volumes as projected by VDYP from inventory information and used in the BCFS analysis were on average approximately 18 percent lower than the mean of the volumes obtained from the plots sampled in the audit. This result indicates a strong probability that the mature volumes derived through VDYP from the inventory figures and used in the analysis are underestimated to some extent. The magnitude of the implications for timber supply of this probable underestimation are uncertain, since the audit is intended to provide generalized results which are statistically reliable for the entire TSA, but which do not identify the accuracy of the volume estimates in the inventory data for a particular part or parts of, or for particular groups, species or site classes within, the TSA.
An examination of the subsample of audit plots likely to fall within the timber harvesting land base corroborated the likelihood of an underestimation of existing stand volumes on the timber harvesting land base. In addition, an earlier (1990) study of cutting permits in the TSA showed that cruise and scale volumes were greater than inventory volumes, which, although not conclusive in itself, provides some further support to the general findings of the audit. The underestimation appears to be associated in small part with inventory information on stand characteristics, but is mainly associated with the appropriateness of the data used in the VDYP model to accurately reflect the local conditions in this particular TSA, and I am requesting that work be carried out to reduce uncertainties in this respect (see below, "Implementation of decision").
From these studies there is a strong indication that the inventory volumes used in the base case harvest forecast are underestimated. Sensitivity analysis shows that a 20-percent increase in existing timber volumes within the TSA permits maintenance of the initial base case harvest level for an additional 4 decades (i.e. from 3 decades in the base case to 7 decades). I have considered this factor in "Reasons for decision".