Impact of Red Oak Borer Outbreaks on Arkansas Forests

Damage from red oak borer. Photo by University of Arkansas, courtesy of Bugwood.org

Over the past 50 years, oak decline events have been reported in upland oak forests of the eastern United States with increasing frequency — and Arkansas is no exception.

A team of forest entomologists at the University of Arkansas led by Fred Stephen, in cooperation with Jim Guldin, project leader of the Southern Research Station (SRS) Southern Pine Ecology and Management unit, recently published an article in the journal Forestry that focuses on how a recent outbreak of the native red oak borer in Arkansas relates to oak decline — and how management practices might reduce this impact. 

An inch-long beetle native to forests in the eastern United States, the red oak borer causes most of its damage while in the larval stage of its two-year life cycle. The female lays her eggs on the bark of an oak; after hatching the larvae burrow through the bark, carving out galleries in the cambium and heartwood of the tree. Red oak borers cause serious timber degradation and in cases of heavy infestation, eventual tree death. Red oak borer outbreaks can be the last blow to trees predisposed to oak decline due to relatively old age, severe droughts, and other factors.

For the recent study, researchers used data from SRS Forest Inventory and Analysis (FIA) to quantify mortality before, during, and after the red oak borer outbreak that lasted from 2000 to 2003. Relative levels of red oak death were greater in oak-hickory stands than in other forest types during this period. Researchers found that red oak mortality increased as the outbreak developed in the period between the 1995 and 2000 FIA surveys. After the outbreak ended, red oak mortality declined and leveled off, but spiked in 2006 and again in 2009 in response to a general drought from 2005 to 2006 and a severe ice storm event in January of 2009.

Comparison of FIA data in oak-hickory stands during and after the red oak borer outbreak suggests that for all trees in the stand, live tree stem density (the number of trees in a given area) has declined by a third but live tree basal area (area of breast-high cross sections of all the trees in a stand) has remained the same. For red oak, in contrast, stem density has been cut by half, and basal area has also declined by nearly half. 

The recovery of live tree basal area suggests that residual trees of other species have responded by growing into the canopy gaps left by the loss of the red oaks. Mean diameter of live red oak remained the same during and after the outbreak, suggesting that mortality in the red oak component occurred across all size classes. Finally, the acceleration of red oak mortality associated with drought and ice storms suggests that stress events may act individually or in combination to deal a knockout blow to weakened red oaks.

Since drought and low tree vigor appear to contribute to red oak mortality, the authors suggest that stands at high hazard for oak decline should be managed using silvicultural treatments that reduce basal area and promote tree vigor. Consideration should be given to removal of the red oaks in intermediate and suppressed classes, and to salvage and sanitation cutting when evidence of oak decline is apparent.  Finally, regenerating mixed stands that include red oak, white oak, and possibly even oak-pine mixtures with a minor component of shortleaf pine might mitigate future impacts of species-specific mortality events.

Read the full text of the article.

For more information: Jim Guldin at jguldin@fs.fed.us

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