As part of the provincial Timber Supply Review, the B.C. Forest Service has examined the availability of timber in the Robson Valley Timber Supply Area (TSA). The analysis assesses how current forest management practices affect the supply of wood available for harvesting over the next 250 years. It also examines the potential changes in timber supply stemming from uncertainties about forest growth and management actions. It is important to note that the various harvest forecasts included in the report only indicate the timber supply implications of current practices and uncertainty. As such, the forecasts should be used for discussion purposes only; they are not allowable annual cut (AAC) recommendations.
The Robson Valley TSA covers a total area of 1 235 821 hectares, of which about 209 365 hectares are considered available for timber harvesting under current management practices. Total current volume of operable standing timber in the area is about 52 million cubic metres, of which 48 million cubic metres is of merchantable age. The area is dominated by spruce and balsam stands with a mix of cedar, lodgepole pine, hemlock and fir stands.
The AAC for the Robson Valley TSA is 596 377 cubic metres per year. This does not account for 3 623 cubic metres that is apportioned to woodlots.
Current forest management of the Robson Valley TSA includes recognizing areas with significant forest resource values such as aesthetics, wildlife, community water supply, and old-growth. These values have been accounted for by excluding some sensitive areas and applying forest cover requirements to the land base that is assumed to be available for timber harvesting.
Given current management assumptions, the analysis shows the current AAC of 596 377 cubic metres per year can be maintained for another 10 years without causing a reduction in future harvest levels below the long-term level. After 10 years the harvest begins to decline at 10% each decade until the long-term level is reached in the sixth decade. The long-term harvest level, which is the potential maximum that can be harvested in perpetuity, is about 351 000 cubic metres per year.
If there is no limit on how quickly future harvest levels decline and harvests are allowed to fall below the long-term level, the current AAC can be maintained for 70 years. However, this would cause a serious timber shortage after 70 years that would last for many decades. If the harvest level were to be reduced immediately and maintained at the reduced level for as long as possible, then the existing growing stock would last longer and forests in the future would be older, on average. The magnitude of these effects would depend on the amount that harvest levels are reduced. Reducing the harvest level may result in the possibility of increased loss of timber to fire, insects and disease.
Many of the data and assumptions used in the analysis are subject to varying degrees of uncertainty. As an example, the extent of the land base assumed available for timber harvesting is particularly uncertain due to potential changes through the Protected Areas Strategy, Forest Practices Code and Land and Resource Management. Sensitivity analysis was used to examine how uncertainty about data and assumptions affect the results of the timber supply analysis.
Over the first 8 to 10 decades harvest levels are most sensitive
to a decrease in the amount of mature forest available for harvesting,
tightening of forest cover requirements to account for non-timber
values and changes to volume estimates for existing timber. Short-term
harvest levels are also moderately sensitive to changes in minimum
harvestable ages but are relatively unsensitive to changes in
yield estimates of future regenerated stands. The long-term harvest
level is sensitive to uncertainty of regenerated stand yields
and the land base assumed available for harvesting. In general,
the long-term harvest level is not very sensitive to uncertainty
about other data and assumptions.