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Timberland Productivity

Primary Question (chapter 14): What are the status and trends of forest management practices in the South?

Related Question (chapter 18): How have abiotic factors, including environmental stressors such as air pollution, influenced the overall health of the South’s forests, and what are future effects likely to be?

In general, productivity is defined as the quantity of outputs produced from a given level of inputs. In forestry, there are several ways to measure productivity, including biomass production, the flow of values, and financial returns. The potential productivity of southern forests is examined here first in terms of biomass at both forest stand and regional scales. The feasibility of productivity gains is then evaluated by examining the financial returns to forest investments. Our analysis of timber markets (chapter 13) indicates that forest productivity is a key variable in forecasting not only harvest quantities and areas, but also the future condition of the region’s forests.

A simple measure of productivity is the rate at which timber volume accumulates in a forest stand. This rate varies with stand age. Young stands accumulate volume rapidly and later approach a stasis where growth may barely offset mortality. Productivity also varies by forest type. Physical productivity can be assessed at a broad regional scale as a ratio of net growth to total inventory volume.

Forecasts of timber production and prices are critically dependent on estimates of future tree growth, especially in pine plantations. The productivity of pine plantations varies considerably with the type of management applied. The most intensive management approaches include planting of genetically improved seedlings, applying fertilizer, and controlling competing vegetation. These practices can yield about 65 percent more timber volume when compared to plantations that are not treated after planting, and more than double physical productivity when compared to natural pine stands (chapter 14).

Timber market models have generally assumed management intensity will be increased to capture this productive potential. Indeed, increased management intensity in pine plantations is a key component of the timber market forecasts discussed in “Wood Products.” Softwood outputs are expected to increase by more than a third, and softwood inventory is expected to increase slightly between 1995 and 2040. Forest investments are forecast to increase the area of pine plantations from 32 million to 54 million acres by 2040. The area of the other broad forest types (natural pine, hardwood, and mixed stands) is forecast to decline from 149 million acres presently to 122 million acres by 2040 (chapter 13).

To test the sensitivity of timber market forecasts to assumptions regarding productivity, we examined an alternative scenario which assumed that 30 percent of expected productivity would not be attained—that is, productivity increases were set at 70 percent of current assumptions. As a result, softwood prices were forecast to increase, and the increased prices led to additional investments in pine plantations when compared with the base case. The alternative scenario produced a forecast of pine plantation area that was 12 percent higher than for the base case (for a total of 58 million acres) in 2040 (chapter 13).

This suggests that reductions in the productivity of managed stands would have the somewhat counterintuitive effect of increasing the total area of intensively managed forests. Expected productivity gains could be offset by changes in institutional factors that restrict forest management options, changes in the physical environment that would result from air pollution or climate change, or increased insect- or disease-related mortality.

Research on the effects of air pollutants indicates that acid deposition probably will not have a measurable impact on forest productivity over the next 40 years. There is a trend away from sulfur and toward nitrogen as sources of acid deposition, and effects of oxides of nitrogen are mixed. Ozone pollution, on the other hand, is expected to increase and has the potential to reduce forest productivity by up to 10 percent per year (chapter 18). Warmer conditions would worsen the damage caused by ozone, but this increased damage could be offset by increased productivity resulting from warming and elevated carbon dioxide concentrations. If, on the other hand, temperatures rise more than currently expected, forest area and structure would change, and the net effect on productivity would be negative. These results indicate the sensitivity of forecasts to assumptions about atmospheric conditions and illustrate the value of reducing uncertainty regarding these future conditions.

Forest pest-related mortality, potentially high in pine forests, is minimized by active management. Given the short rotation length (20 to 30 years typically) and active management practiced by the forest industry in the South, mortality rates would be lower on these lands; this is indeed the case (chapter 16). If a similar management style were employed on the forecast additional plantations, mortality would appear to be manageable. On the other hand, with specific exceptions such as annosus root disease, plantations allowed to grow and age without cultural treatments or thinning will be susceptible to increased pest damage and consequent reductions in productivity.