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The Health of Southern Forests, USDA Forest Service, Forest Health Protection, Southern Region

Stressors of Hardwood Forests

Oak Decline

The oak resource in the southern United States is significant. Approximately two-thirds of the hardwood forest is classified as upland hardwood, where a malady known as "oak decline" is most prevalent.

Oak decline has been reported in the United States for over 130 years. It is a syndrome that involves the interaction of predisposing factors such as climate, site quality, and tree age. Drought and insect defoliation escalate the condition. Pests such as armillaria root disease and the two-lined chestnut borer, which are ordinarily nonaggressive pests on vigorous trees, successfully attack trees stressed by oak decline. Decline is characterized by a gradual, but progressive, dieback of the crown (Figure 9). Mortality typically results after several years, with mature overstory trees the most heavily afflicted.

Figure 9. Poor quality crowns are typical of trees affected by advanced oak decline.
Figure 9. Poor quality crowns are typical of trees affected by advanced oak decline.

Forest Inventory and Analysis units maintain data sets that are useful in assessing oak decline on a regional scale. FIA plots were classified as meeting the definition of "upland oak" and "bottomland oak." Plots also were classified as meeting the criteria for being vulnerable to oak decline syndrome.

In 1993, analysis of data collected by FIA revealed that over 3.9 million upland hardwood acres and 0.4 million bottomland hardwood acres were affected by oak decline. Incidence, expressed as a percentage of the vulnerable host acres, was 9.9 percent for upland types and 6.2 percent for bottomland types.

Figure 10 shows upland areas that are affected by oak decline syndrome as recorded by crews of the Forest Inventory and Analysis group. Figure 11 shows bottomland areas.

Figure 10. Upland oak affected by oak decline.

Figure 10. Upland oak affected by oak decline.

Figure 11. Bottomland oak afected with oak decline.

Figure 11. Bottomland oak afected with oak decline.

For upland types, the highest incidence was in North Carolina and Virginia at over 19%. The lowest incidence for upland types occurred in Oklahoma and Louisiana with less than 3 percent. Incidence in Florida was unexpectedly high at over 18 percent.

For bottomland types, the highest incidence was reported in Florida, followed by Tennessee and Georgia. Louisiana and Mississippi have the largest bottomland resource, yet the lowest incidence (6% and 3% respectively). Sampling errors calculated for the estimates of area affected vary greatly according to the number of affected plots. Reliability of the data is low where few plots exist.

Oak decline probably will increase, particularly where oak forests continue to age. This seems to be the destiny of oak forests throughout much of the South, where harvest and regeneration occur at a low rate.

The extended drought from 1999-2002, has greatly worsened the severity of oak decline throughout much of the region, especially in the Carolinas and eastern Virginia where drought conditions are worst. (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/regional_monitoring/palmer.gif) Losses are often heaviest on dry, south-facing slopes, and on shallow soils and rocky outcroppings in the Appalachians and Ozarks .

Losses in the Ozarks are especially noteworthy because they are compounded by a highly unusual, intensely severe outbreak of the red oak borer, Enaphalodes rufulus. The drought, aging trees, high tree population density, and the borer have combined to create a catastrophic oak decline condition in this area. For more on this phenomenon, see http://www.uaex.edu/Other_Areas/publications/HTML/FSA-7055.asp .

Future efforts to improve the current monitoring procedure could be aimed at correlating oak mortality with dieback coding to see if the recognition of decline-affected plots can be improved.