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5.2 Effects of Various Forest Management Intensities on Productivity

Five management intensity classes (MICs) were developed to estimate potential pine growth and yield on FI land (Siry 1998, Siry and others 2001). MICs range from traditional planted pine management, consisting only of site preparation and planting, to more intensive approaches involving planting of genetically improved growing stock, fertilizer application, and herbicide application to control competing vegetation. MICs assumptions for unthinned and thinned stands are summarized in Table 3 and Table 4.

Genetically improved stock was assumed to increase volume by 14 percent at the culmination of mean annual increment (Siry 1998, Siry and others 2001). This increase corresponds to a 5-ft site index (SI) increase on medium sites (SI 60). The impact of 200 pounds of nitrogen and 25 pounds of phosphorus fertilizer was modeled by increasing yield by 400 ft³ during the 5-year period after treatment. The impact of competing vegetation control on yield was modeled by increasing SI by 5 ft for MIC 4 and 7 ft for MIC 5.

In thinned stands, the impact of genetically improved growing stock and competing vegetation control was modeled in the same way as in unthinned stands. Fertilizer application was assumed to take place at the time of thinning. Thinning had to remove at least 450 ft³/ac of wood volume, which roughly corresponds to about 600 ft³/ac of gross volume (wood and bark). This volume was assumed to be the minimum for economically feasible thinning. Furthermore, thinning could not reduce the basal area of residual stands below 80 ft²/ac, ensuring that sufficient growing stock remained. For multiple thinnings, a 5-year time lag between thinning was specified to capture the full response from fertilizer. These thinning assumptions reflect management objectives to provide intermediate cash flows and increase production of higher quality sawtimber. Single thinning was assumed to prevail in the Southeast. In the South Central region, multiple thinnings occurred in most cases.

Examples of planted pine yields by MICs and thinning regimes on medium sites are presented on Figure 1, Figure 2, and Figure 3. Yields in unthinned stands vary at age 25 from about 2,700 ft³/ac for MIC 1 to nearly 4,600 ft³/ac for MIC 5. The difference of about 1900 ft³/ac indicates that MIC 5 has the potential to produce almost 70 percent more volume that MIC 1. In unthinned stands, the largest increase in yield comes from controlling competing vegetation. That treatment increases yields by 600 ft³/ac for MIC 4 and 750 ft³/ac for MIC 5 at age 25. Genetic improvement increases yield by nearly 420 ft³/ac at age 25. Finally, as explicitly assumed, fertilization increased yield by 400 ft³/ac.

Yields in thinned stands vary at age 25 from about 1,900 ft³/ac for MIC 1 to 2,600 ft³/ac for MIC 5. Thinning removals for a single treatment range from nearly 500 ft³/ac to 800 ft³/ac. Thinnings produce primarily pulpwood, with the exception of MIC 5, where 35 percent of wood volume produced by the second thinning is sawtimber. Total yield (thinnings plus yield at age 25) ranges from about 2,400 ft³/ac for MIC 1 to nearly 3,900 ft³/ac for MIC 5. The difference of about 1,500 ft³/ac indicates that MIC 5 has the potential to produce 65 percent more volume that MIC 1 in thinned stands. More intensively managed stands were thinned earlier. The most pronounced yield increases resulted from competing vegetation control and fertilizer application. Fertilizing permitted earlier second thinning or increased volume in the thinning.

Thinning reduced total volume production throughout the rotations, because accelerated basal area growth of residual stands did not compensated for the loss of productive capacity removed in the thinning. The volume reduction ranged from 7 to 15 percent or from 230 ft³/ac to nearly 700 ft³/ac when compared with unthinned stands in the MICs at age 25. Thinning also shifted the diameter distribution to the right, implying that thinned stands grow less timber, but that its quality and value are higher. While the share of sawtimber in total volume in unthinned stands ranges from 32 to 48 percent at age 25, in thinned stands it ranges from 45 to 76 percent.

Table 5 compares unthinned planted pine yields by MIC on medium sites with empirical yields used by the Subregional Timber Supply (SRTS) model (Abt and others 2000) and the 1993 RPA, and yields recorded in the 1997 FIA survey of Georgia. SRTS yields rely exclusively on empirical values developed directly from FIA data, while RPA yields rely on FIA data as well as on yield curves developed during past RPA assessments. This analysis of the planted southern pine growth and yield indicates that projected plantations yields are much higher than historical FIA data. Increases range from 15 percent (for MIC 1) to 94 percent (for MIC 5) above current SRTS empirical data for average sites at age 25. Projected yields are also greater than those used in the last RPA modeling efforts. Furthermore, projected yields, with the exception of the youngest age class, are consistently higher than yields from the most recent FIA Georgia survey.

In summary, intensified management of planted pine provides substantial opportunities for increasing timber growth, yield, and quality. Fertilizer increases yield by 400 ft³/ac per treatment; genetic improvement increases yield by nearly 420 ft³/ac; and competing vegetation control increases yield by up to 750 ft³/ac. These treatments applied together have the potential to exceed traditional yields (MIC 1) by 70 percent and SRTS-FIA and the last RPA yields by nearly 100 percent.

Information about effects of various management intensities on natural forests productivity is limited. FIA based empirical yields developed for the SRTS model indicate that average annual growth rates for natural pine across all sites can be as high as 86 ft³/ac, followed by oak-pine (54), upland hardwood (47), and bottomland hardwood (44) (Abt and others 2001, Siry and others 1999). These results also indicate that FI natural pine yields can be nearly 20 percent higher than NIPF yields. The estimated average annual growth rates in natural stands are lower than those of planted pine stands, which range from approximately 109 ft³/ac (MIC 1) to 183 ft³/ac (MIC 5).

In comparison with pine management, hardwood management in the South has been neglected. The range of active management approaches varies, but managed stands rarely achieve growth rates that are much higher than those in unmanaged natural stands (Robison and others 1998). Research results indicate that treatments including herbicide application, fertilization, enrichment planting, and thinning have the potential to substantially increase hardwood stand productivity (NC State Hardwood Research Cooperative 2001, Groninger and other 1998, Lockaby and others 1997, Meadows and Goelz 1999a, 1999b).

The area of hardwood plantations is very small. It is estimated that there are about 200,000 acres of hardwood plantations in the South (Dvorak and others 1998). FI owns about 60,000 acres of hardwood plantation (Goetzl¹). In addition, the industry established about 12,000 acres of hardwood plantations with short rotation intensive silviculture (SRIS). These plantations are managed on up to 12-year rotations. Management treatments include intensive site preparation, plantation of genetically advanced seedlings, complete competing vegetation control, and high-intensity fertilization. Genetic improvement increases yields by up to 25 percent per rotation.

Hardwood plantation establishment in many cases has been difficult and expensive. Earlier plantations had growth rates similar to natural hardwood stands, with the exception of cottonwood plantations along the Mississippi River (Robison and others 1998). Progress in genetic improvement, propagation, and silviculture appears critical for hardwood plantations to increase the production of high-quality and uniform wood. Hybrid poplar plantations in the South already can grow substantially more timber than natural hardwood stands (Alig and others 2000).

¹ Goetzl, A. March 23, 1998. AF&PA southern forest management intensity survey. Data summary and survey results. Unpublished report. On file with: American Forest and Pulpwood Association, Washington, DC.