Abstract
In the face of decreasing harvest levels, it is incumbent on forest managers to re-evaluate all assumptions upon which the harvest levels are based. In the Cranbrook TSA there is a significant area occupied by overstocked, small diameter, and or short timber (problem forest types or PFTs) which is currently not considered to contribute to the harvesting land base. The Cranbrook TSA Timber Supply Review showed that harvest levels were sensitive to changes in operable area. These facts make investigation of the PFTs a high priority.
An inventory upgrade targeted at PFTs has been undertaken to provide reliable information on areas, stand characteristics and volumes of these stands. Seven PFT classifications identified in the Timber Supply Review have been adopted as sample populations for this inventory project. The following objectives have been met:
Undertake sampling of PFTs in an unbiased manner;
Use the results to develop an inventory attribute adjustments strategy;
Reassess subject stands for classification as problem forest types; and
Develop new strategies for determining the contribution to timber supply that these stands might make.
Selected in an unbiased manner from the PFT stands across the total Cranbrook TSA land base were 154 polygons. Of these, 137 polygons were sampled (with replacement) with a cluster of five fixed area cruise plots. Compiled cruise summaries represent ‘truth’ values for the evaluation of the accuracy of the forest inventory for seven strata as defined in the sampling plan.
Ministry of Forests (MOF) forest inventory estimates of stand attributes and computed site indices and volumes were compared to compiled cruise ‘truth’ values. This allowed a determination of the accuracy of the PFT subset of the existing inventory. The table below presents age, height and volume comparisons between inventory and ‘truth’ values. Those shown to be significantly different are in red.
Where inventory volumes predicted by the Variable Density Yield Prediction System (VDYP) were found to be significantly different from cruise compiled values (in red in the table above), the most appropriate method of improving those estimates was identified. The adjustment strategy was implemented across the whole TSA land base and PFT contribution recalculated. The new distribution of PFTs in relation to the rules used in Timber Supply Review I to define contribution to the net harvesting land has implications for timber supply. The possible impacts of the redistribution are provided in the table below.
The adjustment strategy increases the net land base by 4,594 ha (4.8%) and indicates that there is available a volume of 3.7 million acres over and above that previously believed (an increase of 35% on the PFT land base).
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Updated April 12, 2007
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