Lillooet, AAC Rationale

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- site productivity estimates


Inventory data includes estimates of site productivity, usually expressed in terms of "site index". Site index is based on the height, as a function of the age, of a particular forest stand. The productivity of a site largely determines how quickly trees will grow, and therefore affects expectations of the time seedlings will take to reach green-up conditions, the volumes of timber that will grow in regenerated stands, and the age at which those stands will satisfy mature forest cover requirements and reach a merchantable size or minimum harvestable age.

I am aware that site index estimates from both young and older stands may not accurately reflect site productivity. Although studies in other areas of the province suggest that some site indexes may be higher than indicated by data from mature forests, as yet I have no conclusive evidence to quantify the extent to which any potential underestimation may have occurred in the Lillooet TSA. Since site indexes are used to estimate future regenerated stand volumes, any changes will primarily affect long-term timber supply forecasts. In some circumstances, site index assignments may also have a short-term impact on timber supply, in that they can affect the estimated time required to satisfy forest cover green-up and criteria for mature harvestable age. However, sensitivity analysis conducted for this TSA where green-up periods were decreased by 5 years showed no change from the base case, and increasing green-up periods by 5 years resulted in no change in the short term supply, and only a small change in the longer term.

The CLMA submission states that the site index assignments used in the BCFS analysis are low, and urges that further study be done on site indexes for second-growth stands. The BCFS has initiated a program to improve site index estimates, through a Site Productivity Working Group, which will provide valuable information for use in subsequent analysis. In the meantime, I am satisfied that the site indexes incorporated in the BCFS analysis represent the best information available, and are suitable for use in this determination. If future studies cause changes to site indexes for this TSA, the matter will be reviewed in a future determination.

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