Oak decline is a slow-acting disease complex that involves the interaction of several factors such as climate, site quality, and advancing tree age. No single cause is responsible for the decline. Trees that are greater than 70 years of age and that occur on drier sites such as shallow, rocky soils on ridgetops and south- to west-facing upper slopes are most affected. During droughts, mortality of fine roots in the upper 12 inches of the soil can initiate dieback. Secondary insects and diseases (borers, root rots, defoliating insects, and cankers) are contributing factors that cause further stress and damage to the trees.
The symptoms of oak decline are the progressive dieback of ⅓ to ½ of the upper crown leaves from the tips of the branches. Other symptoms include chlorotic, dwarfed or sparse foliage, development of epicormic sprouts on the stem, premature autumn leaf color, and foliage browning but remaining on the tree. The progression of the decline is slow, with tree mortality occurring 2 to 5 years after the initial stress. Advanced age and shorter-lived red oak seem to be more susceptible to the decline than longer-lived white oak.
Trees respond to the stress of drought and defoliation by using stored energy reserves. Once these stored reserves are depleted, the trees are not able to maintain the status quo and begin to decline. Mature, older trees may not have the capacity to resume normal growth with the return of favorable growing conditions because the tree demands more resources than it possesses or can manufacture through photosynthesis. Younger and smaller-sized trees recover more quickly and can rebuild their crowns because they require fewer resources to maintain themselves.
Site and stand factors contribute to a tree’s vulnerability to decline. Sites that are moisture deficient, usually ridgetops and south- and west-facing upper slopes are most susceptible. Overstocked stands with a large number of trees (too many for the growing space available) increases moisture stress during drought periods contributing to tree decline.
The best control for the oak decline is through prevention by maintaining healthy, vigorous and actively-growing trees. Older trees that are mature, of advancing age, and with decreasing growth rates are at greater risk to decline. Thinning reduces stocking, decreases competition among trees for moisture and nutrients, and promotes health and vigor of remaining trees.
Oak decline is a normal process of ecosystem processes in aging upland hardwood stands. Dieback and death are expected results when mature oaks come under stress. Root disease fungi and insect pests commonly attack, kill, and decompose weakened trees. Many forest values including wildlife (mast production), timber (degraded value), and recreation (visual attractiveness) will be influenced by the decline. Whether these effects are positive or negative depends on the importance that oaks are deemed to have in the ecosystem.
Unfortunately, the decline will probably continue to be a recurring problem in mature to over-mature stands, especially with red oaks of advanced ages on drier sites. Drought, an inciting factor in the decline, generally occurs every decade. Thus, forests that may be susceptible to decline should be managed so they can best withstand these stresses.