Lillooet, AAC Rationale

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The role of the "base case"


In considering the factors required under Section 7 to be addressed in AAC determinations, I am assisted by timber supply forecasts provided to me through the work of the Timber Supply Review project. For each TSA determination a timber supply analysis is carried out, using a data package of information from three categories: land base inventory; timber growth and yield; and management practices. Using this set of data and a Forest Service computer simulation model (FSSIM), a series of timber supply forecasts is produced. Each forecast is based on the same set of data and reflects different decline rates, initial harvest levels, and trade-offs between short- and long-term harvest levels.

From this range of forecasts, one is chosen which attempts to avoid excessive changes from decade to decade and significant timber shortages in the future, while ensuring the long term productivity of forest lands is maintained. This is known as the "base case" forecast, which forms the basis for comparison when assessing the effects of uncertainty on timber supply.

Because it represents only one in a number of possible forecasts, and because it incorporates information about which there may be some uncertainty, the base case forecast is not an AAC recommendation. Rather, it is one possible forecast of timber supply, the validity of which¾as with all the other forecasts provided¾depends on the validity of the data and assumptions incorporated into the computer simulation used to generate it.

Therefore, much of what follows in the considerations outlined below is an examination of the degree to which all the assumptions made in generating the base case forecast are realistic and current, and the degree to which its estimates of timber supply must be adjusted, if necessary, to more properly reflect the current situation.

These adjustments are made on the basis of informed judgement, using current information available about forest management, which¾particularly during the period leading up to, and now during the implementation of, the Code¾may well have changed since the original data package was produced.

Thus it is important to remember, in reviewing the considerations which lead to the AAC determination, that while the timber supply analysis with which I am provided is integral to those considerations, the AAC determination itself is not a calculation but a synthesis of judgement and analysis in which numerous risks and uncertainties are weighed. Depending upon the outcome of these considerations, the AAC determined may or may not coincide with the base case forecast. Once an AAC has been determined that reflects appropriate assessment of all the factors required to be considered, no additional validation or precision may be gained by attempting a computer analysis of the combined considerations to confirm the exact AAC determined¾it would be impossible for any such analysis to fully incorporate the subtleties of the judgement involved.

For the Lillooet TSA, the base case projection (i.e. in the Lillooet TSA Timber Supply Analysis Addendum which corrected a land base definition error in the original BCFS analysis) indicated that the harvest level could be maintained at 650 000 m³ for 3 decades, but then must decline by about 8 percent per decade before reaching the long-term harvest level of 362 600 m³ per year ninety years from now.

The Forest Act, section 7 (3)

In determining an allowable annual cut under this section the chief forester, despite anything to the contrary in an agreement listed in section 10, shall consider

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