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.
Various changes in external appearance occur when beetle-killed southern pine trees are left standing. It is generally thought that tree appearance can be linked to the stage of deterioration of the wood caused by various insects and fungi. Following is a description of the changes that can be expected in the tree after insect attack.
Crown – The rate of color change (or fading) in the crown is related to the time of year the trees are attacked as well as local climatic conditions. Foliage color changes from green to yellow to red. Color changes are followed by total loss of needles, then small twigs and branches, large branches, and tops as the degree of deterioration intensifies. It is common for all levels of deterioration to be present in an infestation since the trees are normally attacked at different times and the spot may spread over a period of weeks or months. Levi (1981) makes a generalization that appearance of beetle-killed trees is a better guide to their potential utilization than the length of time they have been dead. Two major appearance classes have been suggested for judging the usefulness of beetle-killed southern pine trees for various wood products:
Class A – Trees ranging from those with needles to those with no needles but a branch and twig structure still relatively intact, and, where foliage exists, color ranging from green to yellow green to red.
Class B – Trees ranging from those with smaller branches and twigs breaking off to those with completely broken tops and generally no foliage.
Figure 1 – Class A trees.
Figure 2 – Class B trees.
Stem – Stems of discolored trees usually show small yellowish-white masses of pitch, called pitch tubes, marking the points where adult beetles have bored directly through the bark. Sometimes pitch tubes do not form and the only evidence of attack may be reddish-brown boring dust in bark crevices and on spiderwebs and foliage at the base of the tree. Close examination of the bark will often reveal numerous small holes about the size of birdshot caused by young adults emerging from the bark. Soon after beetle attack, the bark also loosens and is easy to peel from the stem. Many timber markers and procedurement people use the presence or absence of bark as a guide for determining the suitability of SPB-killed trees for wood products, but this is not reliable since the rate of bark loss vaires widely.
Inner bark – The appearance of the inner bark of beetle-killed trees is characterized by winding S-shaped galleries. Eggs are deposited in niches on either side of these galleries and hatch into small grubs. The curved shape of SPB galleries clearly distinguishes them from those made by any other pine bark beetle in the South (fig. 3). The excavations of beetles and their larvae in the cambium and inner bark criss-cross one another and contribute to the death of the tree through the girdling action.
Wood – The wood in standing SPB-attacked trees usually becomes heavily stained from blue-stain fungi carried by the southern pine beetle. The tree's water conducting system is plugged by the fungi and this, when combined with the girdling action in the cambium, is certain to result in death of the tree. The tendency of blue-staining fungi to associate with wood parenchyma tissue of the sapwood (usually found in the wood rays) creates an appearance of radial streaks or wedge-shaped areas fanning out in the sapwood. These characteristics patterns are easily seen on the ends of sawlogs or pieces of pulpwood. Numerous small pin holes and larger sized borer holes often give the wood a distinctive and pleasing appearance for some speciality wood product uses. These holes are created by other beetles (ambrosia beetles and woodborers) boring into the sapwood after bark beetle attack and are the most numerous in the outer portions of the sapwood. Figure 4 illustrates the appearance of lumber with distinctive patterns that would be typical of material obtained from the outer sapwood of such trees. The presence or absence of these patterns would depend on the stage of infestation by the bark beetles and associated insects.
Figure 3 – S-shaped galleries
of southern pine beetle.
Figure 4 – Appearance of lumber
produced from beetle-killed trees.
A number of wood properties are of particular importance in determining the suitability of beetle-killed southern pine for various products.
Specific gravity – Because specific gravity is so useful in predicting other properties and projecting the value of products, it is of primary interest in utilization decisions. The rate of deterioration of SPB-killed trees is quite variable and depends on season of the year, local climatic conditions, and the speed with which infested trees are harvested after detection of insect attack. Barron (1971) reported the specific gravity reduction for increment cores taken from spring-infested trees (with monthly sampling May through October) and summer-infested trees (with monthly sampling July through December) in east Texas (see tabulation below).
|Reduction in specific
gravity for trees dead for
|Infestation time||Condition||1 mo.||3 mo.||6 mo.|
|– – – – – – Percent – – – – – –|
(with monthly sampling May-Oct.)
(with monthly sampling July-Dec.)
Summer-infested trees, felled and allowed to remain on the ground, showed the greatest reduction in specific gravity. In contrast, Walters (1982) reported no apparent reduction in specific gravity of pie-shaped wedges cut from standing and felled trees or from small, clear lumber specimens taken from beetle-killed trees dead for up to 360 days.
Walters and Weldon (1982c) reported the following average weights of wood and bark per cubic foot of material from trees killed by the SPB in summer:
|Time after kill||Standing trees||Felled trees|
|Days||– – – – – – – – – – Lbs./cu. ft. – – – –– – –– – –|
|0 (green)||64.35||– – –|
The average weight per cubic foot was greater for cut-and-leave (felled) trees than for the standing trees except for the 45-day group. The greater densities for felled trees reflect differences in moisture content. And since the moisture contents were not given, the data cannot be analyzed for changes in specific gravity.
Moisture content – Approximately half of the total weight of green southern pine wood is accounted for by water. When expressed as a percentage of dry wood, this translates into a moisutre content of 100 percent. Moisture affects the weight, strength, shrinkage, machinability, and other important properties of wood. In beetle-killed trees, the comparison of weight-scaling factors (normally used for green trees) is of primary concern.
Substantial amounts of water are lost almost immediately after a tree dies, but the amount of drying depends on the lenght of time since death, environmental conditions, and location in the tree. Barron (1971) reported that moisture content is highest at the base and decreases with increasing height in beetle-killed trees. The opposite is true in green southern pines. In green trees, moisture content increases with height, and upper logs always contain a higher percentage of moisture than butt logs (Koch 1972). Patterson et al. (1983) reported on moisture content for beetle-killed trees by time since death and location in the tree. The data indicated rapid loss of moisture in the upper stem within the first 2 months, but substantial moisture remained at the base of the tree. Barron (1971) reported that the moisture content of beetle-killed trees dropped 22 to 53 percent within the month after attack, and the loss was slow thereafter.
Data collected by the author from beetle-killed trees in southwest Mississippi further verified this rapid loss of moisture.² The trees were harvested approximately 3 months after infestation was detected by aerial survey. Moisture content samples were collected from each tree at the top of the butt log (25 feet) and at the merchantable top. Tree crown condition and moisture contents (dry basis) were as shown in the tabulation below.
|Crown condition (color)||Moisture content at top of|
|Butt log||Upper log|
|– – – – – – – – – – Percent – – – – – – – – – –|
|Green-fade (color changing)||36.6||33.0|
|Red-thin (some needles missing)||33.9||29.2|
|Black (devoid of needles)||32.1||27.6|
Mechanical strength – Strength properties of small, clear specimens from beetle-killed southern pine have been reported by Sinclair et. al. (1979a) and by Walters (1982). These studies indicated a 10-percent reduction in bending strength (modules of rupture) within 60 days after foliage fade. Modulus of elasticity, an indication of material stiffness, was not as sensitive as modules of rupture, but some reduction occurred. Mean reductions of 19 percent in modulus of rupture and 11 percent in modulus of elasticity were noted for wood taken from trees standing two full seasons (20 months) after foliage fade in the Piedmont and Coastal Plain in Virignia.
In an east Texas study, Goehring (1980) processed southern pine trees, left dead on the stump for 12 months, into 2 by 6 structural lumber to evaluate the effects of delayed salvage on stress grades. These results revealed that work to maximum load and modules of rupture properties were significantly reduced by the time the tree had stood on the stump for 1 year. The study revealed that lumber cut from these trees could safely perform as No. 3 grade, but that structural utilization should be restricted to floor and wall stems in which other members may help share the applied loads. Extreme care should be exercised in placing lumber cut from trees in this stage of deterioration into critical structural applications such as trusses.
Results of toughness tests on beetle-killed southern pine by Sinclair et al. (1979b) indicated that most of the loss in toughtness occurred during the first warm season following death of the tree. Reductions of 30 to 40 percent in toughness values at 12 months since foliage were reported.
Permeability – The capabiltiy of wood to allow passage of fluids under pressure is called its "permeabilty." Southern pine sapwood is more permeable than most species and, therefore, less troublesome to dry and to treat with wood preservatives. Heartwood is much less permeable than sapwood because most pits in the heartwood are aspirated and because the extractives further block the flow.
It is generally believed that southern pine sapwood infested with blue-stain fungi absorbs more water and preservatives than noninfested wood. These fungi obtain their nourishment primarily from the materials stored in parenchyma cells in the sapwood and cause little damage to the cell wall structure. The fungi occupy the ray cells primarily, and the increased permeability is due to a breakdown of ray parenchyma and some direct penetration of the fungal hyphae through tracheid cell wall. Since blue-stain fungi are deposited when the southern pine beetle attacks, utilization guidelines for such material must consider the increased permeabiltiy and how it affects the product or process.
Effective utilization of beetle-killed timber requires prompt attention to salvage the material as soon as possible. This is not always easy to bring about. Beetle infestaion sites are often small and scattered, and woodland managers hesitate to send a logging crew to a site for such a small quantity of timber. At other times, mill managers are concerned that beetle-killed timber might cause processing problems.
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Last updated August 2018
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