Home
Finding Your Way Within the Web Site
Glossary and Abbreviations
Contents of Heikkenen

Bark Beetles are Scavengers

a paper by Herman J. Heikkenen and Richard G. Oderwald

retired Associate Professor of Forest Entomology, Department of Entomology and Associate Professor, Forest Biometry, College of Natural Resources, Virginia Tech, Blacksburg, Virginia respectively

in The Trail Blazer, Spring,1999 a regional newsletter of the Society Of American Foresters

Bark Beetles are scavengers-symptoms of tree death. Most trees die for the simple straightforward reason that as trees grow larger, they need more water. With time, a stand will exceed the biological capacity of the site, then trees die from lack of water.

The ecological role of bark beetles is to vector wood-destroying organisms to the dead trees, thus freeing bound molecules. A dead southern pine usually disappears within ten years.

Bark beetles locate the dying/dead pines by positive flight to volatile oleoresins, especially the monoterpene alpha-pinene. A pine emits oleoresins when the abscission of the needle cluster fails. This occurs when the lack of water within a tree sup-presses cell growth. resulting in the incom-plete closure of the abscission site. The exposed resin ducts within the needle clus-ter detachment site provide conduits for the release of the oleoresins.

Normal, healthy pines do not emit oleo-resins; the intact resin ducts sequester the liquid oleoresins so that emission of mono-terpene vapors from the stomates and lenticils is unlikely.

Our stand data were obtained by Belanger during the 1970's outbreak of southern pine beetle stands of shortleaf pine in the Piedmont of Georgia. We arranged the infested stands by species composition and origin (omitting those affected by tire, lightning, windthrow, or recent logging), leaving 110 infested stands in three species-origin groups. The variables characterizing these infested stands were age at DBH, site index (SI) at age 50, and basal area per acre (BA).

Figure 1. The periodic annual increment (cubic feet per acre) is on the vertical axis and basal area (square feet per acre) is on the horizontal

A measure of desirable stand density is the BA maximizing periodic annual increment (PAI) in cubic volume. The relation of PAI to BA for a given age and site index has little change in slope across a wide range of BA (Figure 1). Beyond maximum PAI trees die. Stands maintained at BA less than maximum PAI will provide nearly as much volume growth with a lower risk or mortality.

The BA for 90 and 100% of maximum PAI for species, age, and SI of each of the 110 infested stands were derived from equations by Schumacher and Coile. The projection period of PAI was 5 years. The BA for infested stands was compared to the derived BA for 90 and 100% of maximum PAI.

Most infested stands, 108 of 110 (98%), had BA greater than 90% of maximum PAI. These stands had BA greater than the BA recommended for sound, economic forest management. Tree mortality should be expected within such overcrowded stands

The concept that desirable stand densities exist for maintaining a healthy, productive forest has long been recognized. Regard-ing sawtimber, Westveld suggested leaving the best 100 to 140 trees per acre with spacings of 18-21 feet. Davis recommended BA=SI, also a 1.7 spacing figure (BA = DBH xl. 7), both about a BA of 80.

That an optimum rate of cubic foot growth occurs over a wide range of BA has been well documented by Brender: the optimum rate occurred at BA slightly less than the SI, also a BA spread of 20 around the culmination of the PAI produced the same volume growth. Brender also found that growth in board feet could not be increased an appreciable amount by permitting BA above 80, closely approximating the long recognized rules of thumb of growing southern pine sawtimber by Westveld and Davis.

The goal of forest management is the production of forest goods and services. When goods are the primary goal, stand densities must have BA promoting growth and profits. When services such as preservation and recreation are primary, stand densities must be controlled or tree mortality accepted as a part of age and ecological succession. Bark beetles are symptoms of dead trees, not the cause. This fact should influence administrative practices, research goals, and allocation of funds. Timber losses now attributed to bark beetles are in reality either failures of silvicultural/forest management or a misunderstanding of natural processes.

(A detailed list of the contents of www.Lasting Forests.com, over 900 files, is also available.)

Return to the top .

As needed, see the Contents of www.LastingForests.com

This Web site is maintained by R. H. Giles, Jr.
Last revision: September 3, 2000