Increasing Timber Inventory to a Sustained Yield Goal

By Thembi Borras

Developing a sustained yield goal is in part based on site quality, which conveys the relative productivity of a land area. According to the Soil Survey Report for the western part of Mendocino County, the potential annual production from a fully stocked stand can be as low as 245 board feet (bf) per acre per year to as high as 2,050 bf/acre/year. Although different for each soil complex, often the potential annual production from a fully stocked stand of redwood and Douglas-fir is closer to 750 bf/acre/year. The sustained yield goal can be determined given the potential annual production and a selected long-term annual growth rate that can be maintained while protecting forest related values.

For example, if the potential annual production on the Jones Family Forest is 800 bf/acre/year and the Jones Family selects a long-term annual growth rate of 4%, the sustained yield goal is 20,000 bf/acre. This is determined by the following formula: 800 bf/acre/year = (.04/year)*X; X=20,000 bf/acre. The Jones Family Forest is 200 acres. Therefore, the sustained yield goal is 4,000,000 bf on the entire Jones Forest.

Critical to this exercise is to compare the sustained yield goal to the current standing volume and the current annual growth rate to the long term annual growth rate.

Continuing with this example, based on information from a timber cruise, the Jones Family Forest supports a current standing volume of 2,000,000 bf or 10,000 bf/acre. The Jones Family now knows the starting point, which is 2,000,000 bf, and the goal, which is 4,000,000 bf. At this point, a Potential Harvest Schedule can be developed, which will reflect how quickly this goal is reached based on the volume harvested at each entry, the responding growth rate and the frequency of entries.

By cutting less than growth over several cycles, the inventory of the stand will build and eventually the sustained yield goal will be attained, at which point, the periodic growth can be harvested without ever depleting the inventory.

The inventory can be viewed as “principle” and the growth as the “interest” earned on that principle. A truly sustainable forest allows the perpetual harvest of the interest without ever having to touch the principal.

The Soil Survey Report for the western part of Mendocino County is available at http://www.ca.nrcs.usda.gov/mlra02/wmendo.html. A portion of this production was gleaned from an unpublished paper by Craig Blencowe entitled, Craig Blencowe: Building up the Forest.