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Forest Inventory

Primary Question (chapter 16): What are the history, status, and projected future of southern forests?

Related Question (chapter 13): What are the history, status, and projected future demands for and supplies of wood products in the South?

Another way to track changes in forests is to monitor changes in tree biomass. This approach has long been used to examine timber supply issues because it provides a direct measure of the forest’s capacity to produce timber products. It also provides insights into a forest’s capture and sequestration of atmospheric carbon.

Evaluating changes in forests has long been a principal objective of FIA. The FIA measure of inventory most indicative of total biomass is growing stock volume, measured as the volume of trees that either are or will likely be of merchantable quality. While the measure actually undercounts total biomass because it excludes certain tree species, all trees that are not yet 5 inches in diameter, and all other plants, it nevertheless serves as a useful biomass index. In the discussions that follow, “volume” refers to growing stock volume as measured by FIA.

Southern forests have accumulated substantial volume over the past 50 years (fig. 38). Between 1953 and 1999 the total volume increased 73 percent from about 148 billion cubic feet to about 256 billion cubic feet. This increase reflects the rapid growth of relatively young stands established after the 1930s. As the average age of these forests has increased, growth rates have declined somewhat. Most recent surveys show a slowing rate of accumulation for hardwoods and essentially a leveling off of softwood inventory.

The timber volume inventories of all 13 Southern States grew more than 50 percent between 1953 and 1999. Tennessee and Kentucky had the most prodigious increases, and six States—Arkansas, Kentucky, Louisiana, Mississippi, Oklahoma, and Tennessee—more than doubled their inventories over this period.

Changes in inventories reflect the combined effects of growth, mortality, and removals. Removals measure losses of inventory due to harvest and conversions of forests to other uses. Trends in timber inventories have traditionally been evaluated based on the difference between annual net growth (biological growth minus mortality) and removal rates, often expressed as the growth-removal ratio (GRR). A GRR greater than 1.0 indicates a net accumulation of volume. A GRR of less than 1.0 means that volume is decreasing.

For hardwoods, the GRR has exceeded 1.0 since the 1950s. In 1999 it stood at about 1.3, indicating that growth exceeded removals by about 30 percent (fig. 39). For softwoods, growth exceeded removals until about 1990. Since then, the GRR has been about 0.9, indicating that removals surpassed growth by about 10 percent. Models of growth and removals forecast that this situation is transitory and that softwood growth will soon exceed removals again. This is because the region is in a period of transition where pine plantations are expanding rapidly. As these young pines grow large enough to enter into the FIA inventory—forest analysts call this phenomenon ingrowth—measures of total softwood growth are expected to shift upward.

Figure 39—Average annual growth to average annual removals ratios of growing stock on timberland by species group and year, Southern United States.

Anticipated changes in GRR are demonstrated by the 1989 and 1997 inventories of the Southeastern Coastal Plain of Georgia. In 1989, the forest survey indicated that the GRR was 0.98 but also noted that the rate of tree planting had more than doubled since the previous survey. The 1997 forest survey reflected this ingrowth, and the GRR increased to 1.07. Meanwhile, the rate of planting continued to climb. Similar patterns of change are evident in Alabama and Mississippi, where recent planting activity has been high.

We forecast that softwood growth will overtake and exceed removals by a slight margin in the next few years. As a result, softwood inventories are forecast to increase steadily between 1995 and 2040. For hardwoods, on the other hand, removals are forecast to exceed growth by 2025. Thus, total hardwood inventories are forecast to peak in about 2025 and then decline to levels just exceeding current levels by 2040. Inventories for individual States are forecast to peak and then decline at different times (see discussion in the “Wood Products” section preceding and in chapter 13).

Changes in mortality over time also are important. Since the 1950s, the rate of mortality for hardwoods (mortality divided by total volume) has fluctuated around 0.7 percent per year (fig. 40). Over the same period, softwood mortality has been increasing. The softwood mortality rate stood at about 0.55 percent per year in the 1950s and 1960s, nearly doubled by the 1990s, and now stands at about 1.0 percent of inventory per year. Reasons for this increase are unclear but likely reflect several factors, including an aging softwood inventory on public and nonindustrial private land, increased susceptibility of offsite species to insects and disease, and a lack of thinning and other stand treatments in overstocked stands.

Figure 40—Mortality rates for hardwoods and softwoods, 1953 to 1999.