Utilization of Beetle-Killed Southern Pine
George Woodson – Prepared under contract with the Forest Service, U.S. Dpearment of Agriculture Forestry Associate Professor, Wood Utilization, School of Forestry, Louisiana Tech University, Ruston, LA.
United States Department of Agriculture, Forest Service, General Technical Report WO-47.
It is commonly believed that felling beetle-killed timber is more dangerous for workers than felling sound green timber. While this may be true for material in advanced stages of deterioration, it does not appear to be true in general. This author's observation of a logging operation in a beetle infestation in Mississippi disclosed no substantial breakage during the felling of 100 trees (in various stages of crown condition from green needles to being devoid of needles but having small branches intact). Significant amounts of bark were lost, and in many cases tree-length stems were devoid of bark. This might mean a loss in revenue from bark sales or fuel value for the mill owner, but it does not appear to create unusually hazardous working conditions.
Loss of bark would be important where logs are stored under waterspray until they can be processed. Brodie and DeGroot (1976) reported weight gains during waterspray storage averaging 21 percent for loads of beetle-killed logs and 10 percent for loads of sound green logs.
Sometimes it becomes necessary to fell beetle-killed trees and leave them on the ground (cut-and-leave). Special circumstances (enough volume and easy accessibility) sometimes make it practical to utilize cut-and-leave material even after it has been left several months. One instance has been noted in central Louisiana where summer-killed trees (June) remained on the ground and were used the following April. The loggers only salvaged the butt log of each tree (32 feet).³
In other situations, salvage sales of beetle-killed timber offered by National Forests have drawn for bids. Various reasons are given for this, including poor timber markets or insufficient volume in specific infestations to attract buyers.
In the South, much of the short-log logging and stick scaling has been replaced by tree-length logging and weight scaling. Considerable effort has been exerted to develop prediction equations and yield tables for estimating stem weights and sawmill lumber and residue yields from green southern pine. Guttenberg et al. (1960) claimed that scaling by weight promised equal accuracy and greater day-to-day consistency in predicting lumber yields from southern pine sawlogs than scaling by any of the usual log-rule methods. They recognized that individual mills would have to develop their own prediction factors for local conditions and pointed to the following potential advantages of weight scaling:
While the above factors may seem to be advantages of weight scaling, the uncertainty of the accuracy in converting weight to any log scale has been a major concern of people marketing logs. Guttenberg et al. (1960) established regression equations for predicting lumber tally from logs of various weights and from truckload weight. The equations were as follows:
|Board feet lumber yield =||Log weight
According to their measurements, 7 to 12 pounds of logs are required to produce 1 board foot of green lumber. On a truckload weight basis, a load weight of 10,300 pounds will yield 1,000 board feet of green lumber.
Siegel and Row (1960) developed a prediction equation to determine average weights of rough logs of varying diameters and lengths. Their formula (below) is accurate for logs between 12 and 20 feet in length and up to 22 inches in diameter:
Log weight = 0.371 D²L + 51
D = Scaling diameter (inches)
L = Log length (feet)
Prediction equations like the one just shown were developed for fresh logs from a given region and should be used only for rough comparisons. Differences in moisture content of beetle-killed logs make it unwise to apply green sawlog and pulpwood conversion factors to beetle-killed trees. For example, average weight for green loblolly pine per thousand board feet (Scribner log rule) has been reported by Williams and Hopkins (1969) to vary from 12,800 to 14, 900 pounds. McNab (1983) reported weights of 10,600 pounds per thousand board feet (Scribner log rule, Form class 78) for beetle-killed loblolly pine sawtimber in northeast Georgia. The moisture content of the beetle-killed stems was 62 percent. If a moisture content of 100 percent (normally found in green loblolly pine) is assumed, McNab's conversion factor would have estimated an average weight of 13, 086 pounds per thousand board feet, which is within the range of data reported by Williams and Hopkins (1969) for green sawlogs.
Obviously, the variation in weight-volume relationships is such that accurate predictions can be made only with specific data on moisture and bark content. Wood density varies considerably as specific gravity and mositure content change. For example, weight per cubic foot of southern pine might vary as follows:
|Density of wood at moisture content of|
|O.D. wt., |
|– – – – – – – – – – – – – Lbs./cu. ft. – – – – – – – – – – – – –|
These specific gravities may not cover the range of values for all southern pines, but they do illustrate the effect of moisture content on density. The species specific shortleaf average for loblolly and shortleaf pine is generally accepted as 0.47 based on over dry weight and green volume (Koch 1972).
The literature, as well as logic, indicate that conversion factors between weight and log scales are strongly related to diameter and specific gravity within certain geographic localities (Koch 1972; Siegel and Row 1960). Converting tapered, round logs into square-edged lumber becomes increasingly inefficient as log diameter decreases. Most sawmills measure their efficiency by the number of board feet of lumber produced per cubic foot of log. This is known as a Lumber Recovery factor (LRF) and varies considerably by mill and log diameter. If a mill manager knows the LRF and log volume in cubic feet, the volume of lumber expected can be calculated. A simple example will illustrate the effects of moisture content on weight-volume relationships. Figure 5 is an expression of the relationship:
Weight = Density X Volume
This figure provides a rapid method of determining log weight-volume relationships for known wood densities. Assume that typical green southern pine has an average density of 62.4 lbs./cu. ft. (specific gravity 0.50 and moisture content 100 percent), and that a log or a group of logs has a volume of 50 cubic feet. The weight calculation for this example would be 3,120 pounds. Likewise, if the moisture content of beetle-killed timber is only 40 percent, the density drops to 43.7 lbs./cu. ft. and the weight calculation for the same example is only 2,185 pounds. The difference would be approximately 1 ton per cunit (100 cubic feet) and would be of particular concern to the person marketing the logs if the mill were using the normal conversion factor for green logs. Had the mill manager established LRF's for beetle-killed timber, adjustments could be made in the normal weight-scaling conversion of >by the process illustrated in figure 5, where density can be adjusted. With the estimated volume, the managed could then make a reasonable estimate of the lumber output.
Figure 5 – Nomograph for
weights of southern pine logs.
Beetle-damaged timber sometimes creates special processing difficulties at the mill primarily because of property differences with green timber. Many of these problems could be avoided if the material were handled separetly. Mill managers typically lump everything together and seldom have enough beetle-killed timber to justify the expense of separation.
The bark of beetle-killed trees is so easily removed that it may jam conveyors at the debarker. Variation in moisture content may also create major problems. While moisture content of butt logs may equal that of green logs, the upper logs may be so dry that saws overheat and their teeth accumulate residues. (Chipper knives dull at a much faster rate). Figure 6 illustrates the residue buildup on an inserted tooth saw after cutting very dry, beetle-killed logs.
Figure 6A – Inserted tooth saw
with gum residue from dry logs.
Figure 6B – Inserted tooth saw with
gum residue from beetle-killed logs.
Grading of lumber from beetle-killed trees is more time consuming because of the difficulty in distinguishing between blue-stained and initially decayed wood. Most southern pine mills cut primarily dimension lumber. Since stain is not considered a defect in dimension material, the grader may overlook some evidence of decay. Lumber yields are typically lower from beetle-killed logs because the sawyer cuts larger slabs in order to remove the heavily stained (and possibly decayed) wood contained in the outer sapwood.
Widely varying moisture contents in beetle-damaged timber or mixtures of green and beetle-killed timber create substantial problems in the drying process. Material with low moisture content will be overdried if wet wood is dried to the proper moisture level. Increased permeability of wood with bluestain means that the wood dries faster than normal wood. This increased permeability also increases the rate at which wood preservatives are absorbed. With energy and chemical costs soaring, kiln operators and wood preservative plants have little margin for inefficiency in energy use for drying or in application of standard preservative treatments.
A major concern regarding the utilization of beetle-killed timber is whether it can be considered a practical and profitable raw material when fresh green material is available. Research studies have provided useful information to indicate that beetle-killed timber can be utilized for numerous wood products if harvesting and processing are accomplished soon after infestation. The following section summarizes information on the suitability of this material for various wood products.
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Last updated on Wednesday, August 09, 2006 at 01:33 PM
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