Forest, Range & Recreation Resource Analysis Table of Contents

Appendix B

Kalum South Timber Supply Analysis Report

The following material is a summary of the timber supply analysis for the southern portion of the Kalum TSA, Prince Rupert Forest Region. It is included here as an example of how a timber supply analysis is carried out and to highlight analytical considerations when dealing with an unbalanced age-class distribution typical of many of British Columbia’s TSAs.

Forest Inventory

Source material for this analysis included computerized maps, a database describing the location and nature of forest cover (size, age, timber volume and species composition) and a description of other forest values such as recreation and visual quality.

TSA Description

The southern portion of the Kalum Timber Supply Area, hereafter called Kalum South, lies within the Prince Rupert Forest Region (

Land base Inventory

The first step in timber supply analysis is to identify the timber harvesting land base by examining the TSA land base inventory. Land base information was obtained from a computer file prepared by the Ministry of Forests Inventory Branch in 1993. The file contains data about the entire TSA including the geographic location, area and nature of the forest cover (e.g., presence or absence of trees, number of trees, species, age and timber volume). Data for areas on which timber harvesting is not expected to take place, and, therefore, do not contribute to the timber supply, are also included (e.g., land set aside for parks or areas occupied by power lines, highways or town sites). Before this land base file is used for timber supply projections, data for these non-contributing areas is removed to ensure that the file represents the timber harvesting land base.

The reduced data file is derived through a computer process that identifies and removes information for non-contributing (i.e., non-timber harvesting) areas from the file. Only a single reduction is made where categories overlap (e.g., where a park area also has unstable soils).

Removal of data for areas not contributing to the timber supply does not imply withdrawal of these areas from the TSA. The Ministry of Forests still manages the entire TSA area (except certain designated lands) as a forest unit that contributes a mix of timber and non-timber values. Timber supply is managed and assessed within this context of integrated resource use.

Timber Harvesting Land base

The timber harvesting land base is the portion of the total land area of a management unit considered to contribute to, and be available for, long-term timber supply. The term does not mean that an area is open to unrestricted timber harvesting activities, nor does it mean that timber harvesting cannot occur elsewhere. Rather, it implies that forests in the area contain timber of sufficient economic value — and sites with adequate environmental resilience — to accommodate timber harvesting with due care for other resources.

Areas on which timber harvesting is not expected to occur under current forest management in Kalum South are as follows.

Figure B2 shows that slightly more than 90% of Kalum South falls under Ministry of Forests management and of that area over half (55.3%) is classified as either non-forest or non-productive forest. Figure B3 details the categories of productive forest land not within the timber harvesting land base and shows that about half (50.3%) of the productive forest is considered available for timber harvesting. More than one-third of forest land in Kalum South is considered to be physically or economically inoperable at this time.

Several types of forested areas outside the timber harvesting land base — environmentally sensitive areas, streamside buffers and old-growth reserves — are managed for non-timber values. These areas amount to 10 463 hectares or 5.2% of the total forest area in Kalum South.

Figure B4 shows the lack of balance in forest age-class distribution in Kalum South and highlights a key timber supply issue — the allocation over time of remaining old growth until currently young areas become old enough to harvest.

Timber Growth and Yield

An important part of timber supply analysis involves predicting the growth and development of individual forest stands over time. In British Columbia, yield is measured as the amount of standing timber volume (m3/ha) and growth is the change in volume per period. This volume measure reflects a minimum size limit for trees and logs that must be harvested and removed from a site (the utilization level).

Timber volumes applied to existing stands in this analysis are based on the Variable Density Yield Prediction (VDYP) model developed by the Ministry of Forests Inventory Branch; VDYP provides estimates of stand volume according to age. Timber volumes estimated for regenerated second-growth stands are based on the Table Interpolation Program for Stand Yields (TIPSY) model developed by the Ministry of Forests Research Branch. Sensitivity analysis addresses the possibility that stand volumes may be different from those predicted.

Management Practices

All forest areas in Kalum South are managed according to the “Interim Regional Timber Harvesting Guidelines for the Interior Portion of the Prince Rupert Forest Region” and the British Columbia Coastal Fisheries Forestry Guidelines. Some areas are subject to additional management guidelines to emphasize specific resource values such as visual quality or water quality.

There are three main forest management emphases within Kalum South:

Figure B5 displays the composition of the timber harvesting land base by management emphasis.

The ministry’s Timber Supply Review describes timber supply based on current management practices as implemented in plans for the area. Staff in the Kalum Forest District and Prince Rupert Forest Region define these practices in the following management assumptions.

Summary Timber Supply Analysis Results

This section summarizes the timber supply analysis results for Kalum South. The analysis uses the most recent assessments of current forest management, the land available for timber harvesting and timber yields as described earlier. These results are referred to as the “base case” because they form the basis for comparison when assessing the effects of uncertainty on timber supply. Because forest management is inherently a very long-term venture, uncertainty surrounds much of the information important in determining timber supply. The base case that follows should be augmented by a sensitivity analysis of key management assumptions. Part of the Kalum South sensitivity analysis is presented in a subsequent section.

Base Case Harvest Forecast

Figure B6 shows the base case harvest forecast or measure of timber supply for Kalum South based on current management assumptions. The initial level of 464 000 cubic metres per year is 16 000 cubic metres per year (3.3%) below the current harvest level for Kalum South. After the first decade, the harvest is scheduled to fall by 10% to 418 000 cubic metres per year for the decade 11 to 20 years from now. The harvest level that is sustainable over the long term — 400 000 cubic metres per year — is reached in the third decade.

The estimated unsalvaged losses to fire and wind of 9500 cubic metres per year have been subtracted from all harvest forecasts shown in this summary.

Several criteria were used to define this base case harvest forecast. The initial level attempted to maintain the current harvest level for Kalum South (480 000 cubic metres per year) for at least the next 10 years, to decline by a maximum 10% in each of the following decades and to avoid harvest shortfalls below the long-term level. If the current harvest level could not be maintained without causing serious future harvest shortfalls, as was the case, the initial harvest level was lowered until a maximum drop of 10% to the second decade could be attained without causing a future timber supply shortage. (A higher initial harvest could be attained by allowing a reduction of more than 10% after the first 10 years.) The long-term harvest level was defined as the harvest that will match growth so that harvesting can continue at a constant level in perpetuity (Figure B7). A continually declining growing stock would signify that timber is being harvested above the productive capability of the land.

The long-term harvest level is about 5% less than the maximum average growth rate of timber in Kalum South. This loss in efficiency of timber production occurs because stands are not harvested exactly at the age that maximizes long-term volume production (i.e., the age of maximum average growth) due to forest cover objectives and practical limitations on scheduling harvests.

Timber productivity in managed second growth is expected to be higher than in existing timber. Growth estimates used in this analysis result in a long-term harvest level about 22% higher than in existing unmanaged stands.

The harvest forecast includes a small shortfall 140 to 150 years from now. At that time, the volume of available timber — growing stock that is older than minimum harvesting age and is not needed to maintain forest cover objectives — is at a minimum. Of the forest cover objectives discussed previously, visual quality objectives place the greatest limits on timber availability. The effect of forest cover objectives on timber supply are examined in the following section using sensitivity analysis.

Age class composition over time

The charts in Figure B8 show how the age composition of the forest within the timber harvesting land base would change over the next 250 years under the base case harvest forecast. Currently, the timber harvesting land base comprises almost equal areas of forest at or above minimum merchantable age and areas younger than merchantable age. (Figure B8a) Most of the merchantable forest is older than 300 years. Over time, most of the very old forest is harvested and the majority of the area remains 140 years old or younger (Figures B8b, c, d, e, f).

These charts include streamside buffers and old growth — a total of 2306 hectares assumed to be 300 years old or more — that are excluded from harvesting under current management but would otherwise be included in the harvesting land base. This area is included to highlight that some old growth will continue to be present in areas available for timber harvesting. The charts do not include environmentally sensitive areas, non-merchantable stands and physically inoperable areas that also contain old-growth forest. Forest cover objectives for visual quality and old-growth forest (older than 150 years) require that additional area remains in old-age forest, which can be seen in the bars representing forest from 160 to 300 years old.

Timber supply is most limiting in the decade from 140 to 150 years from now. This might not be expected since a significant area at that time is older than the average minimum harvesting age of 110 years. However, most of the timber available for harvest in 140 years time comes from poorer quality growing sites. Minimum harvestable ages for poorer sites are 125-140 years and timber volumes on these sites are not expected to be as high as on better sites. Together, these factors mean that timber availability is at a minimum about 150 years from now (Figure B8d).

Alternative initial harvest levels and harvest flows over time

The base case harvest forecast shown inFigure B9 compares two harvest flow alternatives to the base case. If the initial level is set at the current harvest level of 480 000 cubic metres per year and maintained for 20 years, a fairly significant shortfall appears between years 130 and 150. If the initial harvest is set at 480 000 cubic metres per year and then reduced immediately in 10 years to the long-term level (a reduction of 80 000 cubic metres per year or 16.7%), there is a small shortfall 140 to 150 years from now, as in the base case.

These harvest flow alternatives all assume that existing and future timber will be grown and harvested at a maximum rate. A consequence of this policy is that all alternatives show at least minor shortfalls.

It would also be possible to reduce the harvest immediately to the long-term level. Based on current knowledge and management practices, surplus merchantable timber would exist beyond both the forest cover objectives of current management and timber harvest targets.

Sensitivity Analysis

Sensitivity analysis was carried out to examine uncertainty about three key forest management assumptions in the Kalum South timber supply analysis:

The analysis illustrates the sensitivity of harvest forecasts to some variables and their relative insensitivity to others. The sensitivity analyses provide indicators of risk to be considered in determining allowable harvest levels and information pertinent to determining priorities for future data collection.

Uncertainty in existing timber volumes

Estimates of timber volumes in existing stands are subject to uncertainty because they are based on extrapolation of statistics from a sample of stands to all stands in an area and on inventory classifications which themselves contain uncertainty. The volumes are more accurate when averaged over large areas than they are for estimating volumes in a specific stand.

Figure B10 demonstrates that timber supply over the next several decades is very sensitive to uncertainty in existing volume estimates. If existing volumes were 20% greater than those used for the base case, the current harvest level of 480 000 cubic metres per year could be maintained for almost 80 years before declining to the same long-term level as in the base case. Conversely, if existing volumes were overestimated by 20% for the base case, harvests would need to decline immediately from the current level to avoid severe harvest shortfalls in future years.

One alternative would be to decline immediately to 347 000 cubic metres per year for the next 100 years before rising to the base case long-term level. This would be an immediate drop of 133 000 cubic metres per year (28%) from the current level and would be 53 000 cubic metres per year (13%) below the long-term level. A second alternative would be to start at 10% below the current harvest at 432 000 cubic metres per year, then decline by 10% per decade to 315 000 cubic metres per year and remain at that level for almost 60 years. The initial harvest level could be higher for this alternative if the allowable rate of decline were greater (e.g., 15% rather than 10% per decade). The higher initial harvest in the second alternative is achieved only by declining to a level below the harvest in the first alternative for many years. These two harvest flow alternatives illustrate the trade-off between short- and long-term benefits and costs that exists when harvests must decline in the near future to a level that is sustainable in the long term.

One method for determining whether actual volumes are higher or lower than the estimates used in the base case forecast is to compare the estimates with field data such as timber cruises. An average volume from 50 recent timber cruises in Kalum South was 476 m3/ha. The average volume projected to be harvested over the first 20-year period in this analysis is 583 m3/ha. The average cruise volume is, therefore, about 18% less than the existing volumes indicated in the base case. Since the timber cruises referred to were not designed as a statistical sample of all forest types in Kalum South, this comparison cannot support a conclusion that existing volumes are overestimated by 18%. However, the evidence suggests that actual harvested volumes may be lower than estimated in the base case.

This sensitivity analysis shows that timber supply over the next several decades is very sensitive to uncertainty about volumes in existing mature stands. Kalum Forest District staff believe the existing mature volumes used in the base case overestimate actual yields. This uncertainty does not affect the harvest level that could be sustained over the long term.

Uncertainty in the method used to represent adjacency objectives

The “Interim Regional Timber Harvesting Guidelines for the Interior Portion of the Prince Rupert Forest Region” require that trees in a harvested area must reach a specified green-up height before adjacent areas are harvested. To ensure that harvesting does not become overly concentrated in a watershed, the guidelines set a maximum limit on the overall area that has not yet reached the green-up condition. Specifically, the regional harvesting guidelines require that watersheds be harvested over a minimum of three passes, whereby up to 40% may be harvested in the first pass and 30% may be harvested in each of the remaining two passes. This analysis employs a forest cover requirement to represent this watershed-level guideline and to approximate the adjacency guidelines. Because the forest cover requirement is an approximation, it is not certain how accurately it reflects how adjacency and the three-pass system guidelines affect timber supply. Four or five passes may be required to meet adjacency objectives in some situations.[115]

The three-pass harvesting guideline applied in the base case does not restrict timber supply; therefore applying a forest cover requirement to mimic a two-pass harvesting system does not increase timber availability.

If a four-pass guideline (a maximum of 25% of the harvesting land base may be younger than green-up age) more accurately reflects harvesting restrictions that are required to meet adjacency objectives, timber supply would decline slightly relative to the base case 10 to 20 years from now (Figure B11). If harvests continue for 10 years at the initial base case level, harvests would have to drop to 406 000 cubic metres per year in the following decade.

Figure B11 also shows the effects of a five-pass harvesting guideline. While the guideline would allow harvests over the next 10 years at the base case level, it would severely limit timber availability 11 to 20 years from now.

If a three-or four-pass forest cover guideline for areas under general integrated resource management meets adjacency objectives, timber supply would not differ significantly from the base case. If more restrictive guidelines were needed, short-term timber supply would be substantially reduced.

Uncertainty in forest cover objectives for old forest

Forest cover objectives for old forest used in this analysis follow guidelines requiring that at least 12% of each major forest ecosystem remains in old-growth condition. The purpose of the guideline is to protect biological diversity. Since management for biodiversity is a relatively new forest management objective of which knowledge is limited, the 12% figure may either over- or underestimate the old-growth area actually required to protect biodiversity. This section examines the effects on timber supply of this uncertainty.

A portion of the 12% old-growth objective is met by forest in environmentally sensitive areas, non-merchantable forest types, streamside buffers and old-growth reserves that are not in the timber harvesting land base. In addition to these areas, a forest cover objective for old growth was applied to represent a requirement that at least 1680 hectares (about 1.6% of the harvesting land base) remain above 150 years of age at all times. Figure B12 shows the effects on timber supply if this objective is removed and if it is doubled to 3360 hectares. Removing the old-growth forest cover objective would allow timber harvest to begin at the current level (480 000 cubic metres per year), then decline over the subsequent 30 years to a long-term level slightly higher (1000 cubic metres per year) than in the base case. Doubling the old-growth objective would require an immediate decline in harvest to 423 000 cubic metres per year, about 12% below the current level and then to a long-term level 4000 cubic metres per year less than in the base case to avoid harvest shortfalls below the long-term level.

Timber supply, therefore, shows low to moderate sensitivity to uncertainty in the old-growth objective. Harvests over the first 30 years change by 5%-9% compared to the base case. Over the long term, the changes are about 1% when the objective is either removed or doubled.


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