Abstract

An outdoor workshop, “Are My Pine Trees Ready to Thin?”, was developed to help small private landowners decide when their CRP pine plantation was ready to be thinned. The goal was to train landowners to measure their own plantation and make a preliminary evaluation. This half-day workshop did not replace professional advice by a forester but helped landowners decide when to contact a forester. This workshop is easily adaptable for other species in different timber economies.

The CRP program was initiated in 1985 by the United States Department of Agriculture to protect topsoil from erosion. Many farmers converted marginal cropland into pine plantations under this program. Approximately 1.2 million acres of CRP pine plantations have been established nationwide (308,000 acres in Mississippi). Many of these plantations will soon be ready for the first thinning.

The workshop trained landowners and foresters how to measure and quantify five characteristics a pine plantation needed to have before it should be thinned. Sampling techniques, measurements, and terminology were simplified to improve landowner comprehension. The five plantation characteristics measured and their target values (in parentheses) were: stand density index (density > 55%), natural pruning height (pruning >18 feet), average tree DBH or diameter at breast height (diameter > 6 inches), average heights of dominants and codominants (height > 40 feet), and basal area growth rate (growth rate < 11%).

The decision of whether to thin or not was made with specific knowledge of these five characteristics, rather than on stand age, appearance, or pulpwood prices. Plantations with all 5 characteristics above target value should be thinned. Plantations with one or two characteristics on the borderline or below target do not have to be thinned immediately. The landowner may wait a few years for better pulpwood markets. Plantations with all characteristics below target value should not be thinned.

Workshops were held in 36 locations throughout Mississippi from 1999-2000. Written evaluations from the 812 attendees indicated this training would improve their forestry income $6.7 million.

Introduction

The Conservation Reserve Program (CRP) is the federal government’s single largest environmental improvement program (USDA 1997). The CRP program was initiated in 1985 by the United States Department of Agriculture, Farm Services Agency to encourage farmers to establish permanent land cover to protect marginal cropland from erosion. Plantations of loblolly pine (Pinus taeda) qualified as a permanent land cover in the southeastern U.S. Farmers received annual rental payments plus half the cost of establishing plantations (Dorell et al 1993). The average rental payment in Mississippi for CRP Pine Plantations has been $45 per acre per year for a 10-year period (Londo 2000). CRP funded the establishment of approximately 1.2 million acres of CRP pine plantations nationwide and 308,000 acres in Mississippi (Londo et al 2001). Most of these plantations began with the 1986-1990 sign-up periods and will soon be ready for the first thinning.

Proper timing is the most important management decision landowners can make for their pine plantation. The first thinning sets the stage for the future sawlog production. Timing of the first thinning should be determined by plantation characteristics and the landowner’s objectives. Mississippi has traditionally been a sawlog economy. Sawlogs are frequently worth 5 or more times the pulpwood value. The Mississippi Timber Price Report (Daniels 2001) shows a drop in pulpwood prices once thinning CRP pine plantations began in 1998. Pulpwood prices fell from a high of $17 per ton (Daniels 1998) to $6 per ton (Daniels 2001). The size of the CRP pulpwood supply bubble in Mississippi is estimated at 1 million tons per year (40,000 acres thinned x 25 tons/acre). The sawlog market has not been influenced by CRP yet and averaged $50 per ton (Daniels 2001).

In a poor pulpwood market, the first thinning becomes an important tool used to speed pine growth to sawlog size. However, some landowners view the first thinning only as an income source and resist thinning when prices are low. When pulpwood prices peak, the opposite is true. Landowners want to thin before their plantation is ready. While thinning income is important, most landowners would benefit by ignoring the pulpwood market and thin when plantation characteristics say it is ready. Timing the first thinning too soon or too late will decrease sawlog production and subsequent financial returns for the landowner.

A workshop, “Are My Pine Trees Ready to Thin?”, was developed to train landowners and foresters how to determine the time when a pine plantation is ready for the first thinning. The workshop provided hands-on training in plot sampling and tree measurements. Data averages were then compared to target values for five plantation characteristics. They were stand density, natural pruning height, DBH, height and growth rate. Faculty in the Department of Forestry at Mississippi State University conducted 36 workshops over a two-year period testing and perfecting the workshop. The workshop, as well as the target value for each characteristic will be described.

Workshop mechanics

Each workshop was hosted in a county by the County Extension Agent in conjunction with the local County Forestry Association (CFA). The Extension Agent distributed brochures and handled all mass media advertisement. The CFA collected registration fees and paid for refreshments and other program costs. The program was delivered by Extension Foresters with help from recruited consulting and industry foresters in the area.

The first hour of the workshop was a review of information about pine growth and development, reasons for thinning, and methods for thinning pine plantations. Following this lecture period, each landowner then received training on use of a diameter tape, clinometer, compass, and increment borer.

Under the guidance of a forester, landowners were broken into groups and dispersed through the plantation. Measurement plots were laid out in a systematic grid using compass and pacing. Participants were encouraged to collect data on at least ten plots, scattered throughout the plantation. This was a minimum number of plots needed to get a representative sample. A tally sheet was provided to record data for 10 plots in a way that made hand calculations easier.

A double sampling technique was used to collect data. The first sample was a 1/100^th acre circular plot measuring DBH of each tree and number of trees per acre. Within each plot, a single sample tree in the dominant or codominant crown class nearest plot center was measured for total height, natural pruning height, and basal area growth. The sample tree selected was also preferably free from any serious defect.

DBH was measured with a diameter tape and total height with a clinometer. Natural pruning height (height to the first live limb) was measured using an 11 ft. pole that is marked into 1-foot increments. A normal person holding this pole, with arm fully extended, could measure heights to about 18 ft. Stem radial growth was measured from an increment core of wood taken horizontally through the central pith of the stem of the sample tree at breast height. Width of the growth rings for the last full three years was measured. Radial growth was used to estimate basal area growth.

Table 1 was developed using the sample tree DBH and radial growth to estimate future basal area growth Predicting future wood growth using past growth is “a reasonable postulate for a 3-5 year span” (Avery and Burkhardt 1994). Bark growth for the three-year period was assumed to be negligible. A simple interest rate was used because a straight-line best describes tree growth in young unthinned stands (Grosenbaugh, 1958). The calculations used to generate Table 1:

Basal area (BA) was expressed in square feet per tree while DBH and radial growth are in inches.

current BA = [DBH]² * .005454
future BA = [DBH + (2 * radial growth)]² * .005454

BA growth rate expressed as % per year:
BA growth % = [(future BA – current BA) / (3 years * current BA)] * 100%

In some plantations, the number of forked, diseased, or ice-damaged trees was also collected. Ice storms in 1994 and 1998 damaged many pine plantations in North Mississippi. This additional information can be important for determining stand health and thinning recommendations.

After measuring the sample plots, groups returned to calculate averages for DBH, trees per acre, total height, pruning height, and growth %. Average values were then recorded on Table 2 and compared to target values to determine if a pine plantation was ready to thin. Plantations with all 5 characteristics above target value should be thinned. Plantations with one or two characteristics borderline or below target do not have to be thinned immediately. The landowner may wait a few years for better pulpwood markets. Plantations with all characteristics below target value should not be thinned.

Plantation target values

DBH

We recommend at least an average DBH of six inches before thinning. Average diameter is important, because trees must be at least 5" DBH to be sold for pulpwood (Traugott 2000). Trees smaller than five inches DBH typically won’t be cut. Consequently, thinning plantations when only the larger trees are big enough to cut for pulpwood may result in high grading of the stand (Traugott 2000). Thinning larger diameter trees also produce more volume, increasing the money generated by thinning.

Stand Density

Stand density was evaluated by plotting average DBH and trees per acre on Figure 1. The plotted point was compared to a “thin – wait” decision line for loblolly pine.

Points above the line were dense enough to warrant thinning. This “thin – wait” line represents the density where mortality in a plantation is expected to begin. The line is derived from combinations of average DBH and trees per acre that equate to 55 percent of the maximum Stand Density Index value (SDI) for loblolly pine (Reineke 1933). Fifty-five percent of maximum SDI is where density-related mortality (self-thinning) can be expected to begin (Dean and Baldwin 1996).

A stand density index value of 55% was set as a general target. Thinning a stand prior to this density may be desirable if a landowner is interested in maintaining high stand vigor and rapid individual tree growth. Delaying thinning beyond this density may be desirable if a landowner is willing to risk some mortality in order to improve stem quality and accumulate more volume. In addition to stand density index values, the decision of whether to thin or not must also take into

consideration the other plantation characteristics discussed in this paper.

Tree Height

Trees should be at least 40 feet tall before a plantation is thinned (Traugott 2000). Experience indicates that a total height of 40 feet will produce the minimum 30-foot merchantable length needed for tree-length mechanized thinning. The top ten feet will be cut off to achieve a 3-inch top. Double stacking stems shorter than 30 feet on a log truck prevents trucks stacked full to be close to the maximum haul weight. Hauling pulpwood in underweight trucks can subsequently drop stumpage prices.

Natural Pruning Height

Natural pruning needs to be at a minimum height of 18 feet. Since pines are shade intolerant, their branches die from the ground up as trees become crowded and over topped (Traugott 2000). Waiting until branches die to 18 feet will result in a clear 16-foot butt log for future harvests.

Natural pruning is most important in the butt log since it is the largest and most valuable log in the tree. Live limbs produce knots in the wood, decreasing strength and subsequent value as lumber. Thinning before natural pruning occurs will allow limbs to live longer and get larger in diameter. Future quality sawlogs may become pulpwood. Mechanical pruning can be used to remove limbs, but most landowners do not have the time or money to invest in this kind of operation.

The 18-ft minimum here is a much lower standard than the 25 ft reported by Nebeker et al (1986). This height would achieve 1.5 clear logs. Natural pruning to 25 ft is fine if pines are 42 feet tall. This equates to a 40% live crown ratio, the lowest ratio recommended for young pines (Nebeker et al 1986).

Basal Area Growth

We recommend thinning plantations once basal area growth drops below 10% per year. Basal area (BA) growth is the annual percent change in stem cross-sectional area at breast height. BA growth is one of the most important factors to consider before thinning. Slow growth is an indicator of poor tree health and increased risk of loss to southern pine beetles. BA growth is also an important financial measurement because it is closely related to volume growth (Wenger 1984). In general, trees growing 10% or more each year are producing enough wood to justify waiting to thin.

The minimum acceptable growth rate can change with the economy. If landowners can reinvest thinning income at a much higher rate of return, they may choose a higher minimum. Landowners would thin earlier. If lower rates are expected from reinvestments, landowners may delay thinning.

Workshop evaluation

Workshops were held in 36 locations throughout Mississippi from 1999-2000. A total of 812 landowners and foresters owning 254,789 acres attended. Attendees returned written evaluations indicated this training would improve their forestry income $6.7 million. Many of Extension Forestry’s regular clientele stated this workshop was the best forestry program they ever attended.

Summary and conclusions

Thousands of acres of CRP pine plantations are approaching the time for the first thinning. Proper timing is the most important management decision landowners can make for their pine plantation. The first thinning sets the stage for future productivity and value.

The “Are My Pines Ready to Thin?” workshop was very successful at training landowners to determine when their CRP plantation should be thinned. The decision of whether to thin or not was made with specific knowledge of DBH, density, total height, natural pruning height, and basal

area growth. Threshold levels for each of these factors were provided to indicate the need for thinning. Workshops were simple to conduct and much appreciated by landowners and foresters alike. Modifications for other species in different regions of the world could be easily made.

References

1. Avery, T. E.; Burkhart, H.E. 1994. Forest Measurements 4^th Ed. McGraw-Hill, Inc, New York, NY. 408 p.

2. Daniels, R. A. 2001. Mississippi Timber Price Report. May/June 2001. Mississippi State University, Extension Service Publication. 4pp.

3. Daniels, R.A. 1998. Mississippi Timber Price Report January/February 1998. Mississippi State University, Extension Service Publication. 4pp.

4. Dean, T.J.; Baldwin, V.C.; Jr. 1996. Growth in loblolly pine plantations as a function of stand density and canopy properties. For. Ecol. and Manage. 82:49-58.

5. Dorell, T.E.; Straka, T.J.; Moulton R.J.. 1993. An economic analysis of federal forestry cost share allocations 1985-1992. Efficiency of allocation related to forest productivity, economies of scale, and type of forestry practice. P. 561-562. In . Proceedings of the Society of American Foresters 1993 National Convention.

6. Grosenbaugh, L. R. 1958. Allowable cut as a new function of growth and diagnostic tallies. J. Forestry 56:727-730.

7. Londo, A.J. 2000. The effects of forest fragmentation on forest management for Mississippi private non-industrial forest landowners. p. 116-124. In: Proceedings of the Forest Fragmentation 2000 Conference. Sampson Group Inc. Alexandria, Va.

8. Nebeker, T.E.; Hodges, J.D.; Karr, B.K; Moehring, D.M. 1986. Thinning practices in southern pines-with pest management recommendations. Tech. Bull. 1703. New Orleans: U.S. Department of Agriculture, Forest Service. Southern Forest Experiment Station. 36p.

9. Reineke, L.H. 1933. Perfecting a stand-density index for even-aged forests. J. Agric. Res. 46(7):627-638.

10. Traugott, T.A. 2000. Are my CRP pine trees ready to thin? Mississippi State University, Extension Service. Publication 2260. 7p.

11. USDA 1997. The conservation reserve program. U.S. Department of Agriculture. Farm Services Agency PA-1603. 40p.

12. Wenger, K. F. 1984. Forestry Handbook 2^nd Ed. John Wiley and Sons, Inc., New York, NY. 1335 p.