Shade Tree Decline

Fall 2008

Recently the Home and Garden Information Center’s toll free number has received many calls on mature shade trees that are dying, particularly oaks.  Many callers also seem to be noticing rapid thinning or die back in the remaining trees.  Shade tree decline is a complex situation since the cause frequently cannot be traced to one single factor. Typically, one or more primary stresses cause the health of a tree to deteriorate, and then secondary pathogens and/or insects contribute to further decline. Determining causes of decline requires careful examination of the tree and growing site as well as knowledge of the tree's history. Even then, diagnosis may be difficult because the original stress may be obscure.

Symptoms
There are several symptoms that may indicate decline. Early symptoms can include: late spring flush, a decrease in twig and stem growth, premature fall coloration and leaf drop. As stress continues, foliage may appear small and pale in color. Water sprouts may grow from the branches and trunk. Thinning in the upper crown foliage becomes evident as twigs die back. Larger branches die as the decline progresses. Heavy seed formation may be another indication of decline. Affected trees may survive indefinitely while continuing to gradually decline, or death may occur in a year or two. Trees that have been declining gradually may appear to suddenly die. This can occur when already weakened trees are subjected to a particularly severe environmental stress (e.g., drought) resulting in an accelerated decline and then sudden death."

Possible Causes for Shade Tree Decline

Stress kills trees, however stress is part of the environment of all trees, and a stress-free condition for a tree rarely exists in nature.  Rather, it is the degree or the intensity and the duration of stress that determines the degree of decline.  For the most part our native forest trees have evolved to tolerate the low levels of stress found in the natural forest ecosystem. However, even in the forest and especially in our urban environments where extremes of environmental stress are more common, rapid tree decline occurs when extreme weather conditions persist.  Each additional level of stress beyond the initial stress, for example shallow rooting in compacted soils, has been shown to deplete the tree's energy reserves and thereby decrease the tree's ability to protect and repair itself.

Lack of adequate water and drought is arguably the main contributing stress factor to urban shade tree decline and most of Maryland is currently in an extended period of drought that has lasted for the last three years.  Extended drought can influence the health of shade trees by causing the loss of the majority of small absorbing roots found in the top 8 to 12 inches of soil. Once this soil area dries, many of these absorbing roots die. Many of our urban trees are planted on marginal soils or in sub soils that have been further deteriorated by compaction.  Trees planted into areas with restricted root zones such as planting islands in roadways or parking lots have additional drought and heat stress.  Leaves and stems can also be damaged by drought conditions, especially when there is not enough water available for evaporative cooling and food production.   

Some types of trees will be inherently more susceptible to drought damage.  A season-long drought period with high temperatures can adversely effect all trees even if supplemental water is added. Trees may not readily show initial decline symptoms because of stored carbohydrates in the woody tissues. As these stored foods are near depletion, however the trees begin to prematurely defoliate and show die back. Other drought symptoms can be delayed two or more years making it hard for many to believe that drought was actually the problem.

Compacted soil can also lead to tree decline. Soil compaction may be the result of foot traffic, heavy construction equipment, or any activity that applies pressure to the soil. Tree roots grow in the pore space between soil particles. In compacted soils overall pore space in the soil is reduced, resulting in decreased oxygen and moisture in the soil. Fine roots have difficulty penetrating the compacted soil. Poor root growth reduces the plant's ability to take up nutrients and water, results in less vigorous growth and makes the tree less resistant to environmental stress.

Tree/shrub decline may be the result of severed or damaged roots. If roots are severed some buds and twigs may immediately die, weakening the tree. The weakened tree may begin to show decline symptoms such as lack of vigor and reduced growth. Other stress symptoms can include wilt, scorch, and early fall color. In addition, trees/shrubs with severed roots may lean or show symptoms on the side of the plant with root damage. These symptoms usually progress over a number of years. Most trees/shrubs will survive and recover from damage that does not exceed 1/4 of the total root zone. Roots may be severed during nearby grading and construction, especially during excavation for streets, sidewalks, footings, foundations and walls. Trenching for utilities may also sever tree roots.

Actions That May Alleviate Decline

Although irrigating trees during periods of drought is recommended, frequent and shallow watering contributes to shallow root development. This increases the chances for drought injury as well as the potential for winter injury during periods of extremely cold weather. When watering, be sure the moisture reaches depths of at least 5 to 7 inches. Water once every three to four days during periods of severe drought. Watering everyday may contribute to decline because the potential of pathogenic organisms stimulated by too much water that can cause root rot.  The amount of water to apply depends upon soil texture and potential size of the tree rooting area. Clay soils can hold more water than sandy soils and therefore can become more easily waterlogged which destroys tree roots.

Correction of soil compaction is difficult. Cultivating the soil to incorporate organic matter is not recommended in areas with existing trees. The process of vertical mulching is the best method of aerating soils adjacent to tree roots. Recently the use of air spades and augers have been used to loosen the soil within the root zone area followed by the addition of ground bark, compost, or peat moss. These products increase pore space in the soil and allow greater root growth and moisture retention in the area.

Severed roots may be reduced with careful planning to avoid root damage. Coordinate and consolidate utility trench locations. It is advisable to tunnel under tree roots larger than 2". Other strategies to avoid damage include, use of discontinuous footings for walls and structures, use of paving materials which require a minimum amount of excavation, and location of structures away from mature trees. Avoid stripping topsoil around trees. When clearing land, cut woody vegetation to ground level rather than pulling out large root systems, which may damage nearby tree roots. Management of trees and shrubs with severed or damaged roots involves careful maintenance. Provide adequate water during periods of drought. Prune out any branches that die as a result of the reduced root system.

David Clement, Regional Specialist, Plant Pathology