Landscape - Tree Decline

After a drought year like 2007, expect to see many trees in decline this year. The following is information on causes of tree decline.

Stress as a cause of tree decline

Various stresses cause reduced tree vigor and allow secondary diseases and insects to cause further decline or death of branches, roots, or sometimes entire trees. One of the major causes of stress in trees is planting them in locations to which they are not suited. Some examples:
Planting acid-loving trees in alkaline soils increases the likelihood of stress.

Planting warm-weather trees in cold climates or moisture-loving trees in droughty soils almost guarantees a life of stress.

Planting trees that require full sunlight in the shade of other trees will also cause stress. A small pine tree planted in the shade of a group of oak trees will never fully develop.

Trees that have the potential for large size should not be planted in confined locations. For example, oak, pecan or other trees that develop large tops and wide-spreading root systems should never be planted in a 4-foot by 4-foot hole in a sidewalk or street location. Such trees are generally short lived under these conditions.

Planting trees that are intolerant of air pollutants where high levels of pollution are expected is not a good idea. For instance, trees such as white pine, larch, sugar maple, white ash and yellow poplar, which are sensitive to ozone, should not be planted along freeways where traffic is heavy. Instead, trees more tolerant of ozone such as eastern redcedar, black locust or bur oak should be considered.

Planting a tree that normally grows on dry upland soils in low swampy areas is unwise. Wet area species such as pin oak, red maple, sycamore, sweetgum or bald cypress could be among the logical choices in such situations.

Another major cause of stress in trees is allowing too many of them to occupy an area. Crowded trees compete with each other for nutrients, moisture and sunlight. Not all will survive when trees increase in size. Although some trees will eventually dominate, their growth is slowed and stress is created until the final survival decision is made by nature.

Many other factors also place trees in stressful situations and eventually lead to their decline:

Periodically, extreme conditions such as severe drought or flooding occur. Disease or insect epidemics may sometimes affect enough trees that people become concerned about the situation. Heavy defoliation can be caused by insects such as the variable the gypsy moth. Repeated heavy defoliations seriously weaken trees and may well lead to problems that can show up several years later. In the United States, oak species periodically decline on a massive scale. In general, these situations have involved adverse weather factors combined with defoliation by insects or diseases. Finally, attacks on weakened trees by such things as root rot or trunk cankers or tree-boring insects result in serious decline and eventual death.

A similar situation often occurs on a smaller scale among shade trees following new residential construction. Soil compaction by construction equipment, grade changes resulting in root damage from soil removal, or root smothering by fill dirt are among many factors that cause stress. Decline may not become evident for several years.

Compaction or other alteration of the soil over the roots will occur where the children's swing set or the family dog pen is placed. Other residential yard activities may further contribute to the problem. The slightly acid soil that oaks prefer may be altered by applying lime recommended for many turfgrasses.

Alterations of natural drainage to move water swiftly away from the house also may reduce the moisture available to the shade trees.

Construction of driveways, sidewalks and streets can drastically modify the environment to which large shade trees have grown accustomed in early life. Many woodland trees are simply unable to survive the changes associated with their "urbanization."

Many stress situations such as those listed can be avoided by checking with a tree care professional or by acquiring a knowledge of the cultural requirements of different tree species and the characteristics of the area in which the trees are located or will be planted.

Symptoms of decline

Symptoms of decline may develop quickly or they may not be noticeable for years. Early symptoms include premature fall coloration, late spring leaf development, decreased twig and stem growth, leaf scorch, death of tissues between the leaf veins, and premature leaf drop. Later symptoms include dieback of larger limbs and branches; sprouting from the trunk of the tree; heavy seed crops; foliage noticeably smaller, lighter green and sometimes produced in "tufts" or "clumps" on sprout-origin tissues. The foliage over the entire tree may also look thinner in decline conditions.

Certain fungi that cause stem canker or root infections are often associated with the decline of many tree species. When trees are under stress, physiological changes occur in the roots, allowing the fungi to infect and kill them. In the autumn, clusters of mushrooms may form at the base of trees that have been infected. These growths indicate that root and trunk infections are present.

Reprinted in part from "Tree Decline: What Is It?" by John P. Slusher, School of Natural Resources, University of Missouri Cooperative Extension.