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The long-term repercussions of southern pine plantations are subject to interpretation. These foresets increase the efficiency of timber production but also alter wildlife habitat.
The majority of plantation growing-stock volume is in softwoods. Of the 26,613.1 MCF of wood in plantations, 91 percent is softwood. In fact, 65 percent of the growing-stock volume in plantations is in the shortleaf-loblolly pine species group. Conversely, natural stands are composed of only 36 percent softwoods. Most of the South's hardwood volume, however, is in natural stands (Table 28).
How productive are southern pine plantations? The growth-to-volume ratio for plantation softwoods is 10.5 percent. It is derived by dividing the growth of plantation softwoods--2,544.6 MCF/yr--by the total softwood volume--24,234.1 MCF. The removals-to-volume ratio for plantations is 7.5 percent, while the mortality-to-volume ratio is 0.6 percent. Thus, plantations grow 10.5 percent of their total growing-stock volume annually, while 7.5 percent is removed each year. In 1999, growth of plantation softwoods exceeded removals. Natural stands have a growth-to-volume ratio of 4.25 percent. The removals-to-volume ratio for natural softwoods is 5 percent, while the mortality-to-volume ratio is 1.1 percent. Currently, removals of softwood growing stock exceed growth of growing stock in natural stands. Plantations are responsible for 42.8 percent of all softwood growth in the South, despite the fact that they account for only 10.8 percent of the total growing-stock volume. Mortality is also higher in natural stands, probably because management is more intensive in plantations and weak or diseased trees are harvested in thinnings before they die.
Another topic that often creates heated discussion is the contrast in diameter distributions between natural stands and plantations. Natural stands have more volume due to the large amount of area they occupy. However, the diameter distributions of natural stands and planted stands differ considerably. In natural stands, the 11.0- to 12.9-inch diameter class has the greatest amount of volume (Figure 34). The diameter class with the greatest volume in planted stands is 7.0-8.9 inches. This is the size of chip-n-saw trees. From this point on, the curve drops. By the 17.0- to 18.9-inch class, little volume remains.
The general conclusions that can be formed from Table 28 and Figure 34 are that plantations are comprised mainly of softwoods, particularly loblolly and shortleaf pines. Plantations produce more growing-stock volume than natural stands in relation to the standing volume. Natural stands tend to have a greater variety of species, especially hardwoods, and have larger diameter distributions.
Rosson (1999) found similar results in a 30-year study of Arkansas and Mississippi. He used FIA data that covered three decades (four measurement periods) and over 2,500 plots per measurement period to investigate the effects of pine plantations on species richness and species evenness for an entire State. Species richness for the study was defined as the number of species found on a sample plot. The study showed that pine plantations had a notable impact on tree species richness at the State level. In this study, Arkansas plantations had 14.1 percent lower species richness and Mississippi plantations had 28.9 percent lower species richness than natural stands. Rosson reported that tree species richness declines as plantations replace harvested natural stands. Plots that had harvesting activity over the same study period experienced increases in tree species richness. Species richness on nonharvested plots increased 21.6 percent in Arkansas, and 43.8 percent in Mississippi over the 30-year period..
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