Thursday, November 22, 2007

Landscape: Dead Branches, Tops, and Dead Trees - The Effect of Water Stress

We have seen considerable drought damage to trees and shrubs this year. Effects of this year's drought will extend well into next year. The following is an article on the subject.


The sudden appearance of dead branches, dead tops, or dead trees can alarm anyone, especially tree growers. In Delaware, these symptoms can be unusually dramatic and are often widespread. The following information describes the damage that is often observed and discusses the usual causes and treatment strategies.


Damage occurs throughout the state, but is most severe in urban areas, on the fringe of forested areas, and on shallow or droughty soil types. Trees growing near roads, ditches, pastures, or in areas of soil disturbance or abundant competing vegetation are most frequently affected. Trees growing beyond their natural range or from non-local seed sources generally are at an increased risk of water stress compared to locally adapted trees or drought-tolerant species.


The most common cause of the damage described above is water stress inside the tree. Water stress results whenever water loss exceeds uptake long enough to cause plant damage or disturb its physiological processes. It usually results from a lack of available soil moisture due to drought, which in turn depends on the water storage capacity of the soil and on the rate at which plants take up water through their roots and evaporate it through the foliage. Water stress often affects groups of trees because they share common soil and environmental conditions that can affect their rate of uptake and the degree of water stress.

Because water storage capacity varies among soil types, water stress will develop in trees on some sites sooner than others under the same weather conditions. Trees exposed to full sunlight and greater air movements tend to lose water faster than trees growing in a closed canopy. Sudden changes in the stand that expose the crown can also increase the rate of water loss or competing vegetation can intercept water from tree roots during periods of low rainfall. Soil compaction can also stress trees by damaging roots, reducing aeration, and preventing water infiltration.

Trees respond to water stress in a number of ways. Moderate levels of stress reduce stem and root growth, while under more severe drought conditions, water content may drop to a critical level where trees are irreversibly damaged and branches, portions of the crown, or entire trees may suddenly die. It is often difficult to gauge the level of stress, as many of the tree's internal responses to water deficits occur without visible outward indicators.


In Delaware, water stress injury usually occurs in mid-summer through fall after trees have formed buds. Periods of greater moisture and cooler temperatures may initially improve tree water balance; however, the warmer conditions of late winter and spring often cause previously stressed trees to rapidly decline. Examinations of discolored branches or tops in early spring usually show little evidence of insects or disease, and are more often the result of water stress the previous year. If damage is due to these other agents, it will usually become more obvious as their activity and development increases in late spring and summer.

Water stress from winter events is also common. Low temperatures, especially following a warm period, can damage sapwood and impair water transport to branches and foliage. Severe needle desiccation and drop, occurs as the result of slower water movement in the soil, along with dry winds and sunny weather that cause increased water loss.

Water stress can also induce the loss of older foliage in the fall. This occurs as the tree mounts a "drought-resistance" response by reducing the total surface area of the foliage and subsequently lowering the rate of water loss.


Healthy, vigorous trees usually resist attacks by insect and disease agents by producing defensive chemical compounds. Tree stress, especially water stress, can reduce the production of these compounds and decrease the tree's ability to withstand attacks.


Populations of insects that attack weakened trees build during or following extended periods of drought or other events that cause water stress.


Several canker-causing fungi also infect and kill branches or stems, giving them an appearance that is often similar to water stress. Cankers are most visible in spring and summer, 1-2 years following water stress, and are distinguished by portions of bark becoming sunken and discolored. Removing a portion of the bark in affected areas reveals dead tissue underneath.


Most tree damage occurs on disturbed sites and is due to a combination of factors including soil conditions, tree species, and weather patterns. It is unlikely that stress will be alleviated by simply altering a single factor. Rather, improvement will come from an accumulation of many moderate changes to relieve stress and increase vigor.

• Prevent soil compaction caused by vehicle or foot traffic near trees. Excess traffic can compact surface soils and damage fine roots, most of which lie within a foot of the soil surface. Clay soils are especially vulnerable.
• Avoid direct damage to trees and roots by machinery.
• Reduce competing vegetation and apply mulch to maintain soil moisture (1-3 inches is usually sufficient).
• Irrigate landscape trees during dry weather. Apply water slowly over many hours so it penetrates to tree roots or use drip irrigation lines.
• Do not alter drainage patterns (ditches, ponds, etc.) near established trees.
• Plant trees that are well suited for the site; use local seed sources and species that are adapted to your soil types.
• If insect larvae or branch/stem cankers are evident, prune and destroy affected branches to reduce the spread of these agents.
• Do not fertilize during drought conditions. Fertilization stimulates foliage production and can increase a tree's water requirements.

Modified from "Dead Branches, Dead Tops & Dead Trees: The Interaction of Water Stress, Insects and Disease", Forest Health & Monitoring Unit, Oregon Department of Forestry

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