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Growing Stock vs. All-Live Inventory Data: Historical and Projected Trends

By David Rossi

There are several different definitions of merchantable volume in the historical FIA data. Analysts examining inventory trends need to be cautious about which definitions of inventory were used to make a market projection and how projected inventory lines up with historical data. The three definitions of merchantable volume are:

  1. Growing stock sawtimber volume: this includes only volume in the sawlog portion of trees which meet sawtimber (high grade) merchantability standards. For softwoods, the sawlog portion of each tree includes volume in trees >=9” DBH. For hardwoods, the sawlog portion includes volume in trees >=11” DBH.
  2. Total growing stock volume: this includes growing stock sawtimber in addition to high grade volume less than the minimum size requirement for it to be considered sawtimber (>=9” DBH for softwoods and >=11” DBH for hardwoods). In other words, this category of inventory includes pulpwood but omits cull volume from large diameter trees suitable for producing sawtimber.
  3. All-Live merchantable volume: this includes Total growing stock volume plus cull volume. All volume from a 1 foot stump to 4 inch top bole diameter is counted towards the All-Live merchantable volume total.

Each of these volume classes is a subset of the next. An analyst’s interest in tracking one of these inventory variables over time has implications about which sets of inventory data are used to initialize a market projection and which projected inventory variables are most useful for determining the extent to which the timber resource can be expected to expand or contract.

When a market projection is made with a growing stock .INV file, the SRTS model will use the merchandising information provided by the user (in a .PRD file and by the “PST to PPW Resid” parameter) to compute both Growing Stock sawtimber inventory volume and pulpwood inventory volume (which includes non-sawtimber grade volume that can be culled and incorporated as part of the pulpwood removals totals – thereby offsetting harvests of pulpwood-sized inventory).

The following chart shows the history of FIA data in South Carolina alongside several southwide projections of inventory computed by the SRTS model. These projections define the minimum DBH for sawtimber to be consistent with the FIA definition (>=9” DBH). For demonstration purposes, these projected variables assume that product demand remains constant at the removal numbers outlined in the v36a .INV file and that land use removals are set to 0 Cubic Feet. Further, these projections assume that 0% of sawmill residues are utilized to offset the demand for pulpwood. In this chart, the historical data is given by solid lines and the projected variables under an assumption of 0% pine cull are given by the dotted lines. We can see that when cull factors are set to 0% (meaning that no sawtimber harvest is culled to offset the market’s demand for pulpwood) projected growing stock sawtimber inventory (dotted grey line) lies well above the historical trend (the solid grey line). However, when 10% of sawtimber harvest is utilized as pulpwood feedstock (the dashed grey line), the projected growing stock sawtimber inventory aligns closely with its historical trend. This suggests that an adequate statewide cull factor for pine in South Carolina is around 10%. Note that when the cull factor is raised, there will be very little if any change in the Total Growing Stock Inventory projection, especially over the shorter term and at the start of the projection where there is little time for lower residual sawtimber harvest to contribute to total growth of the growing stock.

Using a growing stock .INV file avoids the potential to include low-grade large diameter inventory as part of the sawtimber inventory. However, in hardwood driven markets (such as those in parts of Virginia, Tennessee, and Kentucky), it may be better to include large-diameter cull as part of the harvestable inventory as this portion of volume is commonly utilized for the production of wood pellets and fuelwood. The right .INV file and projected variables to use in a market projection may depend critically on the dominant species in the wood basket of interest and the primary wood product demand that drives timber utilization over that region.