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Oak Decline

Oak DeclineOak decline is a gradual weakening of the tree over several years followed by death. Outer branches die and leaves grow only near the trunk. Oak decline is not well understood but is thought to be a complex mix of tree age, site, location, weather, soils, fungi and insect attacks.

"Oak decline is a normal part of ecosystem processes in aging upland hardwood forests.  Dieback and death is an expected result when physiologically mature oaks come under stress.  It is a normal function of root disease fungi like Armillaria mellea and insect pests like the two-lined chestnut borer to preferentially attack, kill and decompose weakened members of the tree canopy.  As expected and natural as these results may be, many forest values including wildlife, timber and recreation will be influenced by oak decline.  These effects will occur on large scales, given that incidence ranges from 28 to 57 percent in landscapes dominated by vulnerable hardwood forest types.  Whether these effects are positive, negative or neutral depends on the importance that oaks are deemed to have in the ecosystem."  (From Survey of Hardwood Decline/Mortality on Three National Forest Ranger Districts in the Southern Region by Dale A. Starkey, Steven W. Oak, and Paul Ishikawa, Jr., USDA Forest Service, 1995.)

Oak decline's potential wildlife habitat impacts include reduced mast yield and quality, reduced oak regeneration capacity and altered species composition in subsequent stands.  Estimates of current and 5-year projected mast yield in an affected stand in the George Washington National Forest, Virginia show a 41% current reduction and a 58% projected reduction when compared to potential mast yield without decline.  Decline for black and scarlet oaks intensifies more rapidly over time than for white and chestnut decline.  Chestnut oak has the lowest mortality rate of all oak species and hickory in the southern region in 1985.  Even trees with severe decline are assumed to worsen only slightly and remain alive with an acorn yield reduction from 66% current to 75% projected.  Wildlife habitat managers should consider oak decline in inventory procedures and resource management planning.  From Oak Decline Alters Habitat in Southern Upland Forests by Steven W. Oak, Dale A. Starkey, and Joseph M. Dabney, 1988 Proc. Ann. Conf. SEAFWA.

OAK DECLINE IN TENNESSEE:

Oak decline is weakening and killing increasing numbers of large mature oaks, especially the red oak group.  During the latest forest survey period ending in 1999, 5.2% of oak sawtimber died, much of it due to oak decline.  Mortality for the 1989 survey, which included severe drought years, was 6.6%.  This disease will become more common as these forests continue to mature.

MANAGEMENT OPTIONS - SHORT TERM:

Landowners have several options to promote healthy hardwood forests including oak.  These can be customized to suit any forest, depending on the amount and distribution of dead and unhealthy trees.

  • Cut and sell declining red oaks and other oaks, especially red oak borer brood trees.  Cutting these trees along traveled roads, trails or in recreation areas will reduce hazards to people.
  • Remove mid-story trees of less desirable competing trees to encourage growth of oak seedlings and saplings.
  • Cut small (approximately 1/2 are) groups of trees.  The combination of more sunlight striking the ground and possibly heavy acorn crops from stressed trees in small areas of severe oak decline can increase the number and success of oak seedlings.
  • Where heavy decline and mortality are widespread, cut patches or whole stands to give oaks a better opportunity to re-grow from stump sprouts and to give oak saplings the sunlight they need to thrive.
  • Use prescribed fire to encourage growth of oak seedlings in mature stands and to reduce the number of less fire-tolerant competitors.

MANAGEMENT OPTIONS - LONG TERM:

For areas with less severe mortality/decline and areas that may be vulnerable to decline, the following options may be useful.  Hazard rating systems (see Range of Risk for Oak Decline chart below) can help identify vulnerability

  • Improve the existing forest.  Selectively cut smaller trees from the mid-story, thin the forest by cutting out some of the larger trees, and even use prescribed fire to develop oak seedlings and saplings.  These treatments also reduce the number of less desirable competitors.  Where appropriate, cut red oaks in favor of quality stems of less-susceptible trees like white oaks and hickory.
  • Start a new forest.  Cut all trees in declining areas if there are enough well-distributed and well-developed oak saplings (10 feet tall) in the understory.

RANGE OF RISK FOR OAK DECLINE:

Low Risk
High Risk
Adequate growing-season moisture Acute summer drought for 2-3 years
No recent spring defoliation Recent spring defoliation
Physiologically immature (Pole-size, <50 years old) Physiologically mature (sawtimber, >50 years old)
Mostly white oaks Predominantly red oak group
"Rich" site (site index > 70) "Poor" site (site index < 70)
Mesic (moderate) site conditions:  Loamy soils, few rocks; deep (>18") soils; coves, terraces, bottoms, lower slopes; north and east aspects Xeric (dry) site conditions:  Rocky soils; shallow (<18") soils; ridges or upper slopes; south and west aspects.

HAZARD RATING

Field sampling of oak decline conditions was conducted on the Hiwassee and Nolichucky Range Districts of the Cherokee National Forest.  Hazard rating used forest type, stand age, oak site index and condition class (a measure of predominate size class, density, stem quality and damage conditions limiting management opportunities.

The result of this sampling was a predicted rating in one of four risk classes - Decline-Vulnerable, Decline-Damaged, Unaffected, or Other Damage.  Vulnerable stands have a high proportion of oak (upland hardwood or hardwood-pine forest types), a relatively low ratio of site index/stand age (a measure of physiologic maturity), and lack limiting damage conditions.  Decline-Damaged stands have the same attributes as Vulnerable stands except that limiting damage conditions are present i.e. sparse, damaged, or low quality.  Stands classified as Other Damage have the same limiting damage conditions as stands classified as Decline-Damaged but have a relatively high sight index/age ratio.  Typically, the damage is caused by fire, top breakage by ice or snow loading, wind, or other mechanical damage.  Unaffected stands lack damage conditions and have a relatively high site index/age ratio or are non-oak forest types.

Table: Field survey validation criteria for oak decline risk classes
Field Survey Results

Class Incidence   Severity
Decline-Vulnerable <50% AND Most with <33% dieback
Decline-Damaged >50% OR Dead oaks with evidence of decline present
Other Damage   Non-decline damages predominate  
Unaffected <25% AND Few with <10% dieback

This hazard rating system is recommended for stands in the Southern Appalachians where oak decline is a known or suspected forest health issue.  From Evaluation of Oak Decline Risk Rating Using the CISC Database on the Cherokee National Forest, TN by Steven W. Oak and Philip M. Croll, 1995, USDA Forest Service.