Executive Summary

As part of the provincial Timber Supply Review, the British Columbia Forest Service has examined the availability of timber in the Arrow Timber Supply Area (TSA). The analysis assesses how current forest management practices affect the supply of wood available for harvesting over the next 200 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 indicate only 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 Arrow TSA covers a total area of 754 000 hectares, of which approximately 217 000 hectares are considered available for timber harvesting under current management practices. The area is dominated by stands of Douglas-fir, lodgepole pine, larch, balsam, spruce, hemlock and cedar. About two-thirds of the available stands are currently classified as immature.

The Arrow TSA is managed on an integrated resource basis with about 42% of the land base assigned to special management zones to account for wildlife habitat, water quality and quantity, and landscape aesthetics. Non-timber resources, notably water quality and landscape aesthetics, are particularly important to residents in the Arrow TSA. As a result, harvesting has been avoided in parts of the TSA that are otherwise eligible for harvest when current management guidelines are considered.

The analysis results indicate the current AAC of 619 000 cubic metres per year can be maintained for 7 decades without causing future shortages in timber supply. After 7 decades, the harvest level declines 10% per decade to the long-term harvest level in decade 11. The long-term level is 422 000 cubic metres per year, approximately 32% below the current AAC.

The above results reflect current knowledge and information on forest inventory and growth. However, it is important to recognize that uncertainty exists about many of the data and assumptions that define timber supply. A series of sensitivity analyses indicate that these uncertainties can affect timber supply to varying degrees.

Short-term timber supply is moderately sensitive to changes in volume estimates for existing stands. If these volume estimates were actually 10% higher than indicated in the existing inventory, the initial harvest level would be 10% higher than the base case. Conversely, if volume estimates were actually 10% lower than assumed in the base case, the initial harvest level could only be maintained for 4 decades before declining to the long-term harvest level.

Forest cover requirements and green-up periods are significant sources of uncertainty, partly because it is difficult to determine the requirements needed to meet objectives such as maintenance of water quality, but also because there is uncertainty about the length of time required for stands to reach green-up requirements.

The uncertainty about forest cover requirements could have a significant effect on the harvest forecast. If objectives for forest cover were actually 10% less than assumed in the base case (more relaxed), the only effect on the harvest forecast would be that the decline to the long-term harvest level could be delayed for 2 decades. However, if forest cover objectives were increased by 10%, the harvest rate would decline 5 decades sooner to a reduced long-term harvest level.

If the green-up periods needed to meet these forest cover objectives were actually 5 years less than estimated, there would be no effect on harvest levels. However, if stands require 5 additional years to achieve green-up conditions, the initial harvest rate would drop after decade 2 rather than decade 7, and the long-term harvest level would be approximately 6% less than in the base case.

The cutblock adjacency rules applied in the base case approximate a 4-pass harvest system whereby no more than 25% of an area may be covered by forests less than the green-up height. Sensitivity analysis examines the effect if more passes (for example, 5- or 6-passes) were required to meet adjacency objectives. If a 5- or 6-pass harvesting guideline were required to meet adjacency objectives, there would be a significant effect in the short term, with the decline to the long-term harvest level occuring much sooner than in the base case. In both cases, long-term harvest levels would be reduced as well.

There exists some uncertainty about the size of the timber harvesting land base. Any changes in the timber harvesting land base would likely have a significant effect on the results of this analysis. If the timber harvesting land base were 20% larger than assumed, both the initial harvest rate and the long-term harvest rate would be significantly increased. A 20% decrease would result in a 5% lower initial harvest rate , but would have an extreme effect on the timber supply from decade 2 onwards.

Management practices in the visual quality and watershed zones (zones 1 - 3) are associated with a great deal of uncertainty. Although the forest cover requirements for these zones are based on guidelines, they pertain to areas that have actually been accessed. Many of the areas in these zones have not been accessed yet due to harvesting and planning delays; therefore, experience in applying the guidelines has been limited. To examine this situation, sensitivity analyses investigate the impact on the harvest forecast if forest cover constraints were changed in these zones as well as how the harvest forecast would be affected if harvesting were deferred in these areas.

The results indicate that increasing forest cover requirements in these zones would have a significant effect on the harvest over the first 80 years. Decreasing the requirements would have only a slight effect. Long-term harvest levels would be only moderately affected by changes in the forest cover constraints for these zones. Both a 20- and 40- year deferral would result in an immediate decline in the harvest level to 500 000 cubic metres per year that would last for the duration of the deferral. The long-term impacts of these deferrals would be slight.

Uncertainty in regenerated stand volume estimates has no impact in the short term; however, both increases and decreases in these volume estimates have a significant impact on long-term harvest levels.

Timber supply is affected only slightly by uncertainty in minimum harvestable ages and old­growth requirements

In conclusion, the analysis based on current inventory and growth and yield information indicates that timber harvests in the Arrow TSA can be maintained for several decades without causing future shortfalls. However, meeting the management guidelines in the Arrow TSA requires a high degree of flexibility in locating and scheduling harvest. Given the high sensitivity of the harvest forecast to changes in these guidelines, any attempts to keep the harvest at the present level for a significant period of time will reduce this flexibility and present a risk to the projected harvest levels. It is important to examine these management guidelines to ensure that they accurately reflect demands and meet objectives.