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Trends in Growing-Stock Volume

Historically, FIA has reported tree volumes based on the growing-stock classification. The definition of growing stock is a live tree of a commercial species that possesses, or has the potential, to produce a 12-foot sawlog. The log(s) must meet dimension and merchantability standards and have at least one-half of the gross board-foot volume in sound wood. This definition was modified in 1988. The new definition states that trees should have one-third of the gross board-foot volume in sound wood. Except for this and a few other changes, the definition of growing stock has remained constant and provides a steady benchmark to investigate trends in tree volume. The FIA data used in this report do not include volumes of trees less than 5.0 inches d.b.h. All volume data are derived from the 2001 RPA report and include the State of Kentucky, except any analysis performed using forest type. Volume by forest type was investigated using FIA data that excluded Kentucky so that the impacts of pine plantations could be reported.

Volume has increased between survey periods for both hardwood and softwood growing stock. This increase has been fairly steady, except for a slight leveling off after 1982. Between 1953 and 1999, total volume increased from 148,470 million cubic feet to 261,601 million cubic feet (table 16.11). The volume of softwood growing stock increased from 60,462 million cubic feet to 105,518 million cubic feet (table 16.12), and hardwood volume increased from 88,008 to 156,085 million cubic feet (fig. 16.15, table 16.13). The majority of this change took place between 1953 and 1982. This period accounted for 67 percent of the total increase in growing-stock volume, 91 percent of the increase in softwood volume, and 51 percent of the increase in hardwood volume.

State and Federal reforestation programs stimulated the increase in volume after World War II. Volume increases in the late 1960s to mid-1970s are a direct result of the maturing of trees planted by these reforestation projects. Data from the South’s Fourth Forest (1988) indicate a huge increase in the number of acres reforested in the mid-1950s to early 1960s, with a peak of 1.7 million acres in 1959. The increase in growing stock in tables 16.11, 16.12, and 16.13 is a direct result of this reforestation effort.

Changes in volume by diameter class

Changes in total growing-stock volume by 2-inch diameter class are displayed in figure 16.16. Note that the second to last diameter class encompasses all trees 21.0 to 28.9 inches d.b.h. This broad class explains the large bump at the end of each year’s curve. The increase in volume from 1953 to 1982 was particularly high for trees less than 14.0 inches d.b.h. This situation is attributable to the fact that trees are not included in estimates of growing-stock volume until they reach 5.0 inches d.b.h. At that time, their volume is added to the inventory and is called “ingrowth.” Since 1982, the volume in the 9.0- to 10.9-inch diameter class has never varied by more than 3 percent (table 16.14). The volume of growing-stock trees for diameter classes greater than 10.9 inches has increased steadily in successive survey periods.

For softwood growing stock, volume in all years peaks in the 9.0- to 10.9-inch diameter class (fig. 16.17). This peak was greatest in 1982, as recent surveys show a slight decrease in volume for this tree size. All years have increases in volume for each diameter class greater that 13.0 inches d.b.h. (table 16.15). For hardwood growing stock, volume peaks in the 10-inch diameter class (fig. 16.18, table 16.16). The general shape of the hardwood distribution curve is more rounded, with very large volumes in the 9.0- to 10.9-, 11.0- to 12.9- and 13.0- to 14.9-inch diameter classes. Conversely, softwood growing-stock volume is concentrated in the smaller diameter classes.

Volume of growing stock by ownership

Because NIPF landowners hold the lion’s share of the South’s timberland, it follows that the majority of the volume occurs on their land (fig. 16.19). NIPF landowners have always accounted for 69 to 72 percent of the total growing-stock volume in the South (table 16.17). The growing-stock volume for all ownerships has increased in each survey period. The increases in growing-stock volume brought about by the reforestation efforts of the 1930s, 1950s, and early 1960s are clearly seen in figure 16.19, particularly for NIPF land.

In 1953, NIPF landowners controlled 71 percent of the growing- stock volume. Since then, the proportion controlled by NIPF owners has declined due to major increases for other types of owners. Growing-stock volume on forest industry land increased 50 percent from 1953 to 1999. The volume on national forests increased 113 percent and that on other public land increased 251 percent.

Volume trends by forest type

The area of hardwood and mixed pine-hardwood stands has increased over the last 50 years, while the area of pine has declined. These changes in area caused changes in volume distributions by forest type (fig. 16.20). The volume in mixed pine-hardwood stands increased almost 8 percent between 1989 and 1999, while the volume in all hardwood stands increased 9 percent (table 16.18). The majority of the hardwood volume is in the oak-hickory forest type, which comprised around one-third of all growing-stock volume and approximately 60 percent of all hardwood volume in both years.

Southern pine growing-stock volumes increased only slightly between 1989 and 1999, from 78,671 million cubic feet to 78,785 million cubic feet. The area of southern pine timberland increased 3 percent during this period. Comparing natural southern pine stands to planted stands produces some interesting results. Natural longleaf-slash pine and natural loblolly-shortleaf pine are two of the four forest types that lost volume in the 1990s. Elm-ash-cottonwood forest and spruce-fir types also lost volume. Planted southern pine stands increased in volume by 55 percent from 1989 to 1999. These increases can be attributed to the Conservation Reserve Program. In 1989, pine plantations held 7 percent of the South’s total growing-stock volume. By 1999, plantations accounted for 10 percent of the total growing-stock volume.

Volume trends for recent surveys

As noted earlier, Alabama and South Carolina experienced completion of new surveys during the assessment process. These data were not used in the tables and figures above. In order to provide the reader with current information, these numbers will be briefly discussed throughout the chapter. Tables and figures will not be used. Individuals wishing this information should attain the publications for these States.

Alabama’s softwood growing-stock volume increased 9 percent to 12.7 billion cubic feet between 1990 and 2000. Softwood growing-stock volume increased 23 percent on public lands to 1.1 billion cubic feet and increased by 22 percent to 9.2 billion cubic feet on NIPF lands. Driven by the reduction in landholdings in the State, softwood volume on forest industry lands decreased 26 percent to 2.4 billion cubic feet. Loblolly pine was the predominate species at 8.6 billion cubic feet, an increase of 25 percent since 1990. Alabama’s softwood sawtimber totaled about 44 billion board feet, an increase of 5 percent since 1990.

Volume of hardwood growing stock in Alabama increased 17 percent to 15.2 billion cubic feet between 1990 and 2000. Hardwood volume increased 31 percent on public lands to 1.2 billion cubic feet, 25 percent on NIPF lands to 12.5 billion cubic feet, and decreased 31 percent on forest industry land to 1.4 billion cubic feet. Other red oaks were the predominate species group with 3.5 billion cubic feet. The inventory of hardwood sawtimber increased 33 percent to 45.7 billion board feet.

Merchantable volume of softwood growing stock in South Carolina increased from 8.0 billion cubic feet to 8.9 billion cubic feet, a rise of 11 percent. Loblolly pine volume increased 21 percent to 6.6 billion cubic feet, accounting for most of the increase in softwood volume. Since 1993, softwood volume on forest industry timberland increased in spite of losses of forest area under this ownership. Softwood volume on forest industry timberland rose 3.9 percent to 1.7 billion cubic feet within the State. Softwood volume on NIPF timberland increased from 5.2 billion cubic feet to 5.9 billion cubic feet. Significant reductions in the volume of slash and shortleaf pine occurred during the period, whereas the volume of longleaf pine remained relatively stable, dropping only 2.5 percent.

South Carolina’s hardwood growing-stock volume increased during the latest survey as well, from 8.6 billion cubic feet to 8.8 billion cubic feet. The increase was greatest on NIPF land in cubic-foot terms, as hardwood inventory rose 208 million cubic feet to 6.9 billion cubic feet. Forest industry timberland experienced a 23-percent reduction in volume of hardwood live trees and was the only ownership to show a loss.